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Upgrading & Repairing PCs Eighth Edition

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Bus Slots and I/O Cards

At the heart of every system is the motherboard; you learned about various motherboards in Chapter 4, "Motherboards." A motherboard is made up of components. The major component that determines how the motherboard actually works is called the bus. In this chapter, you learn about system buses.

What Is a Bus?

A bus is nothing but a common pathway across which data can travel within a computer. This pathway is used for communication and can be established between two or more computer elements. A PC has many kinds of buses, including the following:

If you hear someone talking about the bus in a PC, chances are good that he or she is referring to the I/O bus, which also is called the expansion slot bus. Whatever name it goes by, this bus is the main system bus and the one over which most data flows. The I/O bus is the highway for most data in your system. Anything that goes to or from any device--including your video system, disk drives, and printer--travels over this bus. The busiest I/O pathway typically is to and from your video card.

Because the I/O bus is the primary bus in your computer system, it is the main focus of discussion in this chapter. The other buses deserve some attention, however, and they are covered in the following sections.

The Processor Bus

The processor bus is the communication pathway between the CPU and immediate support chips. These support chips are usually called the chipset in modern systems. This bus is used to transfer data between the CPU and the main system bus, for example, or between the CPU and an external memory cache. Figure 5.1 shows how this bus fits into a typical PC system.

FIG. 5.1  The processor bus.

Most systems have an external cache for the CPU; these caches have typically been employed in all systems that use the Pentium, Pentium MMX, Pentium Pro, and Pentium II chips.

Because the purpose of the processor bus is to get information to and from the CPU at the fastest possible speed, this bus operates at a much faster rate than any other bus in your system; no bottleneck exists here. The bus consists of electrical circuits for data, for addresses (the address bus, which is discussed in the following section), and for control purposes. In a Pentium-based system, the processor bus has 64 data lines, 32 address lines, and associated control lines. The Pentium Pro and Pentium II have 36 address lines, but otherwise are the same as the Pentium and Pentium MMX.

The processor bus operates at the same base clock rate as the CPU does externally. This can be misleading as most CPUs these days run internally at a higher clock rate than they do externally. For example, a Pentium 100 system has a Pentium CPU running at 100MHz internally, but only 66.6MHz externally. A Pentium 133, Pentium 166, and even a Pentium Pro 200 also run the processor external bus at 66.6MHz. In most newer systems, the actual processor speed is some multiple (1.5x, 2x, 2.5x, 3x, and so on) of the processor bus. For more information on this, see "Processor Speed Ratings" in Chapter 6, "Microprocessor Types and Specifications."

The processor bus is tied to the external processor pin connections and can transfer one bit of data per data line every one or two clock cycles. Thus, a Pentium, Pentium Pro, or Pentium II can transfer 64 bits of data at a time.

To determine the transfer rate for the processor bus, you multiply the data width (64 bits for a Pentium, Pentium Pro, or Pentium II) by the clock speed of the bus (the same as the base or unmultiplied clock speed of the CPU). If you are using a Pentium, Pentium MMX, Pentium Pro, or Pentium II chip that runs at a 66MHz motherboard speed, and it can transfer a bit of data each clock cycle on each data line, you have a maximum instantaneous transfer rate of 528M/sec. You get this result by using the following formula: 66MHz x 64 bits = 4,224Mbit/sec 4,224Mbit/sec ÷ 8 = 528M/sec This transfer rate, often called the bandwidth of the bus, represents a maximum. Like all maximums, this rate does not represent the normal operating bandwidth; you should expect much lower average throughput. Other limiting factors such as chipset design, memory design and speed, and so on, conspire to lower the effective average throughput.

The Memory Bus

The memory bus is used to transfer information between the CPU and main memory--the RAM in your system. This bus is either a part of the processor bus itself, or in most cases is implemented separately by a dedicated chipset that is responsible for transferring information between the processor bus and the memory bus. Systems that run at mother-board clock speeds of 16MHz or faster cycle at rates that exceed the capabilities of standard Dynamic RAM chips. In virtually all systems that are 16MHz or faster, there will be a special memory controller chipset that controls the interface between the faster processor bus and the slower main memory. This chipset typically is the same chipset that is responsible for managing the I/O bus. Figure 5.2 shows how the memory bus fits into your PC.

FIG. 5.2  The memory bus.

The information that travels over the memory bus is transferred at a much slower rate than the information on the processor bus. The chip sockets or the slots for memory SIMMs/DIMMs (Dual Inline Memory Modules) are connected to the memory bus in much the same way that expansion slots are connected to the I/O bus.

CAUTION: Notice that the main memory bus is always the same width as the processor bus. This means that in a 64-bit system, such as the various Pentium CPUs, you will have a 64-bit memory bus. This will define the size of what is called a "bank" of memory. For example, a 486DX4 processor has a 32-bit bus, so the memory in that system must be added 32 bits at a time for each bank. If you are using 30-pin (8-bit) SIMMs, then four will be required per bank; if the system uses 72-pin (32-bit) SIMMs, then only one has to be added at a time to make up a bank. Pentium systems are 64-bit and always require two 72-pin (32 bits each) SIMMs to be added at a time. Some newer systems use 168-pin DIMMs, which are 64 bits each. These compose a single bank in a 64-bit system.

The Address Bus

The address bus actually is a subset of the processor and memory buses. In our discussion of the processor bus, you learned that a Pentium system bus consists of 64 data lines, 32 address lines (36 in a Pentium Pro or Pentium II), and a few control lines. These address lines constitute the address bus; in most block diagrams, this bus is actually considered a part of the processor and memory buses.

The address bus is used to indicate what address in memory or what address on the system bus are to be used in a data transfer operation. The address bus indicates precisely where the next bus transfer or memory transfer will occur. The size of the memory bus also controls the amount of memory that the CPU can address directly.

The Need for Expansion Slots

The I/O bus or expansion slots are what enables your CPU to communicate with peripheral devices. The bus and its associated expansion slots are needed because basic systems cannot possibly satisfy all the needs of all the people who buy them. The I/O bus enables you to add devices to your computer to expand its capabilities. The most basic computer components, such as sound cards and video cards, can be plugged into expansion slots; you also can plug in more specialized devices, such as network interface cards, SCSI host adapters, and others.

NOTE: In most modern PC systems, a variety of basic peripheral devices are built into the motherboard. Most systems today have at least dual (primary and secondary) IDE controllers, a floppy controller, two serial ports, and a parallel port directly built into the motherboard. This is normally contained on a single chip called a Super I/O chip. Many will even add more items such as an integrated mouse port, video adapter, SCSI host adapter, or network interface also built into the mother-board; in such a system, an expansion slot on the I/O bus is probably not even needed. Never-theless, these built-in controllers and ports still use the I/O bus to communicate with the CPU. In essence, even though they are built in, they act as if they are cards plugged into the system's bus slots.

Although some PC systems provide only a single expansion slot, most provide up to eight slots on the motherboard. This slot typically is called a riser card slot. The riser card that plugs into it in turn has expansion slots on its sides. Standard adapter cards are installed in the riser card, meaning that the adapter cards end up being parallel to the motherboard rather than perpendicular to it.

Riser cards are used when a vendor wants to produce a computer that is shorter in height than normal. These computers usually are called Slimline, Low Profile, or sometimes even pizza-box systems. Even though this type of configuration may seem to be odd, the actual bus used in these systems is the same kind used in normal computer systems; the only difference is the use of the riser card.

I usually recommend avoiding the Low Profile or Slimline systems that have what is called an LPX form factor. This refers to the shape of the board. Replacement LPX motherboards with riser cards that match the case for a given system are difficult to find. However, a newer form factor recently developed by Intel called NLX incorporates a Low Profile design, while at the same time enjoying growing industry support.

Bus Mastering

Newer bus types use a technology called bus mastering to speed up the system. In essence, a bus master is an adapter with its own processor that can execute operations independently of the CPU. To work properly, bus-mastering technology relies on an arbitration unit, most often called an integrated system peripheral (ISP) chip. The ISP enables a bus-mastered board to temporarily take exclusive control of the system, as though the board were the entire system. Because the board has exclusive control of the system, it can perform operations very quickly. A bus-mastering hard drive controller, for example, achieves much greater data throughput with a fast drive than can controller cards that are not bus-mastered.

The ISP determines which device gains control by using a four-level order of priority:

A bus-mastering adapter board notifies the ISP when it wants control of the system. At the earliest possible time (after the higher priorities have been satisfied), the ISP hands control over to the bus-mastered board. The board, in turn, has built-in circuitry to keep it from taking over the system for periods of time that would interfere with first-priority operations, such as memory refresh.

Types of I/O Buses

Since the introduction of the first PC, many I/O buses have been introduced. The reason is quite simple: Faster I/O speeds are necessary for better system performance. This need for higher performance involves three main areas:

Each of these areas requires the I/O bus to be as fast as possible. Surprisingly, virtually all PC systems shipped today still incorporate the same basic bus architecture as the 1984 vintage IBM PC/AT. However, most of these systems now also include a second high-speed local I/O bus such as VL-Bus or PCI, which offer much greater performance for adapters that need it.

One of the primary reasons why new I/O-bus structures have been slow in coming is compatibility--that old Catch-22 that anchors much of the PC industry to the past. One of the hallmarks of the PC's success is its standardization. This standardization spawned thousands of third-party I/O cards, each originally built for the early bus specifications of the PC. If a new high-performance bus system is introduced, it often has to be compatible with the older bus systems so that the older I/O cards do not become obsolete. Therefore, bus technologies seem to evolve rather than make quantum leaps forward.

You can identify different types of I/O buses by their architecture. The main types of I/O architecture are:

The differences among these buses consist primarily of the amount of data that they can transfer at one time and the speed at which they can do it. Each bus architecture is implemented by a chipset that is connected to the processor bus. Typically, this chipset also controls the memory bus (refer to Figure 5.2). The following sections describe the different types of PC buses.

The ISA Bus

ISA, which is an acronym for Industry Standard Architecture, is the bus architecture that was introduced as an 8-bit bus with the original IBM PC in 1981 and later expanded to 16 bits with the IBM PC/AT in 1984. ISA is the basis of the modern personal computer and the primary architecture used in the vast majority of PC systems on the market today. It may seem amazing that such a seemingly antiquated architecture is used in today's high-performance systems, but this is true for reasons of reliability, affordability, and compatibility, plus this old bus is still faster than many of the peripherals that we connect to it!

Two versions of the ISA bus exist, based on the number of data bits that can be transferred on the bus at a time. The older version is an 8-bit bus; the newer version is a 16-bit bus. The original 8-bit version ran at 4.77MHz in the PC and XT. The 16-bit version used in the AT ran at 6MHz and then 8MHz. Later, the industry as a whole agreed on an 8.33MHz maximum standard speed for 8- and 16-bit versions of the ISA bus for backward compatibility. Some systems have the ability to run the ISA bus faster than this, but some adapter cards will not function properly at higher speeds. ISA data transfers require anywhere from two to eight cycles. Therefore, the theoretical maximum data rate of the ISA bus is about 8M/sec, as the following formula shows: 8MHz x 16 bits = 128Mbit/sec

128Mbit/sec ÷ 2 cycles = 64Mbit/sec 64Mbit/sec ÷ 8 = 8M/sec The bandwidth of the 8-bit bus would be half this figure (4M/sec). Remember, however, that these figures are theoretical maximums; because of I/O bus protocols, the effective bandwidth is much lower--typically by almost half. Even so, at 8M/sec, the ISA bus is still faster than many of the peripherals we connect to it.

The 8-Bit ISA Bus

This bus architecture is used in the original IBM PC computers. Although virtually nonexistent in new systems today, this architecture still exists in hundreds of thousands of PC systems in the field. Physically, the 8-bit ISA expansion slot resembles the tongue-and-groove system that furniture makers once used to hold two pieces of wood together. It is specifically called a Card/Edge connector. An adapter card with 62 contacts on its bottom edge plugs into a slot on the motherboard that has 62 matching contacts. Electronically, this slot provides eight data lines and 20 addressing lines, enabling the slot to handle 1M of memory.

Figure 5.3 describes the pinouts for the 8-bit ISA bus.

FIG. 5.3  Pinouts for the 8-bit ISA bus.

Figure 5.4 shows how these pins are oriented in the expansion slot.

FIG. 5.4  The 8-bit ISA bus connector.

Although the design of the bus is simple, IBM waited until 1987 to publish full specifications for the timings of the data and address lines, so in the early days of PC compatibles, manufacturers had to do their best to figure out how to make adapter boards. This problem was solved, however, as PC-compatible personal computers became more widely accepted as the industry standard and manufacturers had more time and incentive to build adapter boards that worked correctly with the bus.

The dimensions of 8-bit ISA adapter cards are as follows: 4.2 inches (106.68mm) high

13.13 inches (333.5mm) long 0.5 inch (12.7mm) wide

The 16-Bit ISA Bus

IBM threw a bombshell on the PC world when it introduced the AT with the 286 processor in 1984. This processor had a 16-bit data bus, which meant that communications between the processor and the motherboard as well as memory would now be 16 bits wide instead of only 8 bits wide.

Although this processor could have been installed on a motherboard with only an 8-bit I/O bus, that would have meant a huge sacrifice in the performance of any adapter cards or other devices installed on the bus. The introduction of the 286 chip posed a problem for IBM in relation to its next generation of PCs. Should the company create a new I/O bus and associated expansion slots, or should it try to come up with a system that could support both 8- and 16-bit cards? IBM opted for the latter solution, and the PC/AT was introduced with a set of expansion slots with 16-bit extension connectors. You can plug an 8-bit card into the forward part of the slot or a 16-bit card into both parts of the slot.

NOTE: The expansion slots for the 16-bit ISA bus also introduced access keys to the PC environment. An access key is a cutout or notch in an adapter card that fits over a corresponding tab in the connector into which the adapter card is inserted. Access keys typically are used to keep adapter cards from being inserted into a connector improperly.

The extension connector in each 16-bit expansion slot adds 36 connector pins to carry the extra signals necessary to implement the wider data path. In addition, two of the pins in the 8-bit portion of the connector were changed. These two minor changes do not alter the function of 8-bit cards.

Figure 5.5 describes the pinouts for the full 16-bit ISA expansion slot.

FIG. 5.5  Pinouts for the 16-bit ISA bus.

Figure 5.6 shows the orientation and relation of 8-bit and 16-bit ISA bus slots.

FIG. 5.6  The 8-bit and 16-bit ISA bus connectors.

The extended 16-bit slots physically interfere with some 8-bit adapter cards that have a skirt--an extended area of the card that drops down toward the motherboard just after the connector. To handle these cards, IBM left two expansion ports in the PC/AT without the 16-bit extensions. These slots, which are identical to the expansion slots in earlier systems, can handle any skirted PC or XT expansion card. This is not a problem today, as no skirted 8-bit cards have been manufactured for years.

NOTE: 16-bit ISA expansion slots were introduced in 1984. Since then, virtually every manufacturer of 8-bit expansion cards have designed them without drop-down skirts so that they fit properly in 16-bit slots. Most new systems do not have any 8-bit only slots, because a properly designed 8-bit card will work in any 16-bit slot.

The dimensions of a typical AT expansion board are as follows: 4.8 inches (121.92mm) high

13.13 inches (333.5mm) long 0.5 inch (12.7mm) wide Two heights actually are available for cards that are commonly used in AT systems: 4.8 inches and 4.2 inches (the height of older PC-XT cards). The shorter cards became an issue when IBM introduced the XT Model 286. Because this model has an AT motherboard in an XT case, it needs AT-type boards with the 4.2-inch maximum height. Most board makers trimmed the height of their boards; many manufacturers now make only 4.2-inch tall (or less) boards so that they will work in systems with either profile.

32-Bit Buses

After 32-bit CPUs became available, it was some time before 32-bit bus standards became available. Before MCA and EISA specs were released, some vendors began creating their own proprietary 32-bit buses, which were extensions of the ISA bus. Although the proprietary buses were few and far between, they do still exist. The expanded portions of the bus typically are used for proprietary memory expansion or video cards. Because the systems are proprietary (meaning that they are nonstandard), pinouts and specifications are not available.

The Micro Channel Bus

The introduction of 32-bit chips meant that the ISA bus could not handle the power of another new generation of CPUs. The 386DX chips can transfer 32 bits of data at a time, but the ISA bus can handle a maximum 16 bits. Rather than extend the ISA bus again, IBM decided to build a new bus; the result was the MCA bus. MCA (an acronym for Micro Channel Architecture) is completely different from the ISA bus and is technically superior in every way.

IBM not only wanted to replace the old ISA standard but also to receive royalties on it; the company required vendors that licensed the new MCA bus to pay IBM royalties for using the ISA bus in all previous systems. This requirement led to the development of the competing EISA bus (see the next section on the EISA Bus) and hindered acceptance of the MCA bus. Another reason why MCA has not been adopted universally for systems with 32-bit slots is that adapter cards designed for ISA systems do not work in MCA systems.

NOTE: The MCA bus is not compatible with the older ISA bus, so cards designed for the ISA bus do not work in an MCA system.

MCA runs asynchronously with the main processor, meaning that fewer possibilities exist for timing problems among adapter cards plugged into the bus.

MCA systems produced a new level of ease of use, as anyone who has set up one of these systems can tell you. An MCA system has no jumpers and switches--neither on the motherboard nor on any expansion adapter. You don't need an electrical engineering degree to plug a card into a PC.

The MCA bus also supports bus mastering. Through implementing bus mastering, the MCA bus provides significant performance improvements over the older ISA buses. (Bus mastering is also implemented in the EISA bus. In the MCA bus mastering implementation, any bus mastering devices can request unobstructed use of the bus in order to communicate with another device on the bus. The request is made through a device known as the Central Arbitration Control Point (CACP). This device arbitrates the competition for the bus, making sure all devices have access and that no single device monopolizes the bus.

Each device is given a priority code to ensure that order is preserved within the system. The main CPU is given the lowest priority code. Memory refresh has the highest priority, followed by the DMA channels, and then the bus masters installed in the I/O slots. One exception to this is when an NMI (non-maskable interrupt) occurs. In this instance, control returns to the CPU immediately.

The MCA specification provides for four adapter sizes, which are described in Table 5.1.

Table 5.1  Physical Sizes of MCA Adapter Cards

Adapter Type Height (in Inches) Length (in Inches)
Type 3 3.475 12.3
Type 3 half 3.475 6.35
Type 5 4.825 13.1
Type 9 9.0 13.1

Four types of slots are involved in the MCA design:

The sixth edition of this book, included on the CD-ROM, has detailed information about each of these cards and what systems they can be found in. IBM still has all of the technical reference manuals for MCA available; however, development has stopped for MCA devices due to the other faster and more feature-rich buses available today.

The EISA Bus

EISA is an acronym for Extended Industry Standard Architecture. This standard was announced in September 1988 as a response to IBM's introduction of the MCA bus--more specifically, to the way that IBM wanted to handle licensing of the MCA bus. Vendors did not feel obligated to pay retroactive royalties on the ISA bus, so they turned their backs on IBM and created their own buses.

The EISA standard was developed primarily by Compaq, and was intended as being their way of taking over future development of the PC bus away from IBM. Compaq knew that nobody would clone their bus if they were the only company that had it, so they essentially gave the design away to other leading manufacturers. They formed the EISA Committee, a non-profit organization designed specifically to control development of the EISA bus. Very few EISA adapters were ever developed. Those that were developed centered mainly around disk array controllers and server type network cards.

The EISA bus provides 32-bit slots for use with 386DX or higher systems. The EISA slot enables manufacturers to design adapter cards that have many of the capabilities of MCA adapters, but the bus also supports adapter cards created for the older ISA standard. EISA provides markedly faster hard drive throughput when used with devices such as SCSI bus-mastering hard drive controllers. Compared with 16-bit ISA system architecture, EISA permits greater system expansion with fewer adapter conflicts.

The EISA bus adds 90 new connections (55 new signals) without increasing the physical connector size of the 16-bit ISA bus. At first glance, the 32-bit EISA slot looks much like the 16-bit ISA slot. The EISA adapter, however, has two rows of connectors. The first row is the same kind used in 16-bit ISA cards; the other, thinner row extends from the 16-bit connectors. This means that ISA cards can still be used in EISA bus slots. Although this compatability was not enough to ensure the popularity of EISA buses, it is a feature that was carried over into the newer VL-bus standard. The physical specifications of an EISA card are as follows: 5 inches (127mm) high

13.13 inches (333.5mm) long 0.5 inches (12.7mm) wide The EISA bus can handle up to 32 bits of data at an 8.33MHz cycle rate. Most data transfers require a minimum of two cycles, although faster cycle rates are possible if an adapter card provides tight timing specifications. The maximum bandwidth on the bus is 33M/sec, as the following formula shows: 8.33MHz x 32 bits = 266.56Mbit/sec 266.56Mbit/sec ÷ 8 = 33.32M/sec Data transfers through an 8- or 16-bit expansion card across the bus would be reduced appropriately. Remember, however, that these figures represent theoretical maximums. Wait states, interrupts, and other protocol factors can reduce the effective bandwidth--typically, by half.

Figure 5.7 describes the pinouts for the EISA bus.

FIG. 5.7  Pinouts for the EISA bus. Figure 5.8 shows the locations of the pins.

FIG. 5.8  The card connector for the EISA bus.

Automated Setup

EISA systems also use an automated setup to deal with adapter-board interrupts and addressing issues. These issues often cause problems when several different adapter boards are installed in an ISA system. EISA setup software recognizes potential conflicts and automatically configures the system to avoid them. EISA does, however, enable you to do your own troubleshooting, as well as to configure the boards through jumpers and switches. This concept was not new to EISA; IBM's MCA bus also supported configuration via software. Another new feature of EISA systems is IRQ sharing, meaning that multiple bus cards can share a single interrupt. This feature has also been implemented in PCI bus cards.

NOTE: Although automated setup traditionally has not been available in ISA systems, it is now available with Plug and Play (PnP) systems and components. PnP systems are discussed toward the end of this chapter in the section "Plug and Play Systems."

Local Buses

The I/O buses discussed so far (ISA, MCA, and EISA) have one thing in common: relatively slow speed. This speed limitation is a carryover from the days of the original PC, when the I/O bus operated at the same speed as the processor bus. As the speed of the processor bus increased, the I/O bus realized only nominal speed improvements, primarily from an increase in the bandwidth of the bus. The I/O bus had to remain at a slower speed, because the huge installed base of adapter cards could operate only at slower speeds.

Figure 5.9 shows a conceptual block diagram of the buses in a computer system.

FIG. 5.9  Bus layout in a traditional PC.

The thought of a computer system running slower than it could is very bothersome to some computer users. Even so, the slow speed of the I/O bus is nothing more than a nuisance in most cases. You don't need blazing speed to communicate with a keyboard or a mouse, for example; you gain nothing in performance. The real problem occurs in subsystems in which you need the speed, such as video and disk controllers.

The speed problem became acute when graphical user interfaces (such as Windows) became prevalent. These systems required the processing of so much video data that the I/O bus became a literal bottleneck for the entire computer system. In other words, it did little good to have a CPU that was capable of 66MHz speed if you could put data through the I/O bus at a rate of only 8MHz.

An obvious solution to this problem is to move some of the slotted I/O to an area where it could access the faster speeds of the processor bus--much the same way as the external cache. Figure 5.10 shows this arrangement.

FIG. 5.10  How a local bus works.

This arrangement became known as local bus, because external devices (adapter cards) now could access the part of the bus that was local to the CPU--the processor bus. Physically, the slots provided to tap this new configuration would need to be different from existing bus slots, to prevent adapter cards designed for slower buses from being plugged into the higher bus speeds that this design made accessible.

It is interesting to note that the very first 8-bit and 16-bit ISA buses were a form of Local Bus architecture. These systems had the processor bus as the main bus, and everything ran at full processor speeds. When ISA systems ran faster than 8MHz, the main ISA bus had to be decoupled from the processor bus since expansion cards, memory, and so on could not keep up. In 1992, an extension to the ISA bus called the VESA Local Bus started showing up on PC systems, indicating a return to Local Bus architecture.

NOTE: A system does not have to have a local-bus expansion slot to incorporate local-bus technology; instead, the local-bus device can be built directly into the motherboard. (In such a case, the local-bus-slotted I/O shown in Figure 5.11 would in fact be built-in I/O.) This built-in approach to local bus is the way the first local-bus systems were designed.

Local-bus solutions do not replace earlier standards, such as ISA; they are designed as an extension to existing standards. Therefore, a typical system is based on ISA or EISA and has one or more local-bus slots available as well. Older cards still are compatible with the system, but high-speed adapter cards can also take advantage of the local-bus slots.

Local-bus systems are especially popular with users of Windows and OS/2, because these slots are used for special 32-bit video accelerator cards that greatly speed the repainting of the graphics screens used in those operating systems. The performance of Windows and OS/2 suffers greatly from bottlenecks in even the best VGA cards connected to an ISA or EISA bus.

VESA Local Bus

The VESA Local Bus was the most popular local bus design from its debut in August 1992 through 1994. It was created by the VESA committee, a non-profit organization founded by NEC to further develop video display and bus standards. In a similar fashion to how EISA evolved, NEC had done most of the work on the VL-bus (as it would be called) and, after founding the non-profit VESA committee, they turned over future development to VESA. At first, the local-bus slot seemed primarily designed to be used for video cards. Improving video performance was a top priority at NEC to help sell their high-end displays as well as their own PC systems. By 1991, video performance had become a real bottleneck in most PC systems.

The Video Electronics Standards Association (VESA) developed a standardized local-bus specification known as VESA Local Bus or simply VL-Bus. As in earlier local-bus implementations, the VL-Bus slot offers direct access to system memory at the speed of the processor itself. The VL-Bus can move data 32 bits at a time, enabling data to flow between the CPU and a compatible video subsystem or hard drive at the full 32-bit data width of the 486 chip. The maximum rated throughput of the VL-Bus is 128M to 132M/sec. In other words, local bus went a long way toward removing the major bottlenecks that existed in earlier bus configurations.

Additionally, VL-Bus offers manufacturers of hard-drive interface cards an opportunity to overcome another traditional bottleneck: the rate at which data can flow between the hard drive and the CPU. The average 16-bit IDE drive and interface can achieve throughput of up to 5M/sec, whereas VL-Bus hard drive adapters for IDE drives are touted as providing throughput of as much as 8M/sec. In real-world situations, the true throughput of VL-Bus hard drive adapters is somewhat less than 8M/sec, but VL-Bus still provides a substantial boost in hard-drive performance.

Despite all the benefits of the VL-Bus (and, by extension, of all local buses), this tech-nology has a few drawbacks, which are described in the following list:

The VL-Bus did not seem to be a well-engineered concept. The design was simple indeed--just take the pins from the 486 processor and run them out to a card connector socket. In other words, the VL-Bus is essentially the raw 486 processor bus. This allowed a very inexpensive design, since no additional chipsets or interface chips were required. A motherboard designer could add VL-Bus slots to their 486 motherboards very easily and at a very low cost. This is why these slots appeared on virtually all 486 system designs overnight.

Unfortunately, the 486 processor bus was not designed to have multiple devices (called loads) plugged into it at one time. Problems arose with timing glitches caused by the capacitance introduced into the circuit by different cards. Since the VL-Bus ran at the same speed as the processor bus, different processor speeds meant different bus speeds, and full compatibility was difficult to achieve. Although the VL-Bus could be adapted to other processors, including the 386 or even the Pentium, it was designed for the 486, and worked best as a 486 solution only. Despite the low cost, after a new bus called PCI (Peripheral Component Interconnect) appeared, VL-Bus fell into disfavor very quickly. It never did catch on with Pentium systems, and there is little or no further development of the VL-Bus in the PC industry. I would not recommend purchasing VL-Bus cards or systems today.

For a used system, or as an inexpensive upgrade for an older system, VL-Bus might be appropriate and can provide an acceptable solution for high-speed computing.

Physically, the VL-Bus slot is an extension of the slots used for whatever type of base system you have. If you have an ISA system, the VL-Bus is positioned as an extension of your existing 16-bit ISA slots. Likewise, if you have an EISA system or MCA system, the VL-Bus slots are extensions of those existing slots. Figure 5.11 shows how the VL-Bus slots could be situated in an ISA system. The VESA extension has 112 contacts and uses the same physical connector as the MCA bus.

The VL-Bus adds a total 116 pin locations to the bus connectors that your system already has. Table 5.2 lists the pinouts for only the VL-Bus connector portion of the total connector. (For pins for which two purposes are listed, the second purpose applies when the card is in 64-bit transfer mode.)

Table 5.2  Pinouts for the VL-Bus

Pin Signal Name Pin Signal Name
B1 Data 0 A1 Data 1
B2 Data 2 A2 Data 3
B3 Data 4 A3 Ground
B4 Data 6 A4 Data 5
B5 Data 8 A5 Data 7
B6 Ground A6 Data 9
B7 Data 10 A7 Data 11
B8 Data 12 A8 Data 13
B9 VCC A9 Data 15
B10 Data 14 A10 Ground
B11 Data 16 A11 Data 17
B12 Data 18 A12 VCC
B13 Data 20 A13 Data 19
B14 Ground A14 Data 21
B15 Data 22 A15 Data 23
B16 Data 24 A16 Data 25
B17 Data 26 A17 Ground
B18 Data 28 A18 Data 27
B19 Data 30 A19 Data 29
B20 VCC A20 Data 31
B21 Address 31 or Data 63 A21 Address 30 or Data 62
B22 Ground A22 Address 28 or Data 60
B23 Address 29 or Data 61 A23 Address 26 or Data 58
B24 Address 27 or Data 59 A24 Ground
B25 Address 25 or Data 57 A25 Address 24 or Data 56
B26 Address 23 or Data 55 A26 Address 22 or Data 54
B27 Address 21 or Data 53 A27 VCC
B28 Address 19 or Data 51 A28 Address 20 or Data 52
B29 Ground A29 Address 18 or Data 50
B30 Address 17 or Data 49 A30 Address 16 or Data 48
B31 Address 15 or Data 47 A31 Address 14 or Data 46
B32 VCC A32 Address 12 or Data 44
B33 Address 13 or Data 45 A33 Address 10 or Data 42
B34 Address 11 or Data 43 A34 Address 8 or Data 40
B35 Address 9 or Data 41 A35 Ground
B36 Address 7 or Data 39 A36 Address 6 or Data 38
B37 Address 5 or Data 37 A37 Address 4 or Data 36
B38 Ground A38 Write Back
B39 Address 3 or Data 35 A39 Byte Enable 0 or 4
B40 Address 2 or Data 34 A40 VCC
B41 Unused or LBS64# A41 Byte Enable 1 or 5
B42 Reset A42 Byte Enable 2 or 6
B43 Data/Code Status A43 Ground
B44 Memory-I/O Status or Data 33 A44 Byte Enable 3 or 7
B45 Write/Read Status or Data 32 A45 Address Data Strobe
B46 Access key A46 Access key
B47 Access key A47 Access key
B48 Ready Return A48 Local Ready
B49 Ground A49 Local Device
B50 IRQ 9 A50 Local Request
B51 Burst Ready A51 Ground
B52 Burst Last A52 Local Bus Grant
B53 ID0 A53 VCC
B54 ID1 A54 ID2
B55 Ground A55 ID3
B56 Local Clock A56 ID4 or ACK64#
B57 VCC A57 Unused
B58 Local Bus Size 16 A58 Loc/Ext Address Data Strobe

FIG. 5.11  An example of VL-Bus slots in an ISA system.

Figure 5.12 shows the locations of the pins.

FIG. 5.12  The card connector for the VL-Bus.

The PCI Bus.

In early 1992, Intel spearheaded the creation of another industry group. It was formed with the same goals as the VESA group in relation to the PC bus. Recognizing the need to overcome weaknesses in the ISA and EISA buses, the PCI Special Interest Group was formed.

PCI is an acronym for Peripheral Component Interconnect. The PCI bus specification, released in June 1992 and updated in April 1993, redesigned the traditional PC bus by inserting another bus between the CPU and the native I/O bus by means of bridges. Rather than tap directly into the processor bus, with its delicate electrical timing (as was done in the VL-Bus), a new set of controller chips was developed to extend the bus, as shown in Figure 5.13.

The PCI bus often is called a mezzanine bus because it adds another layer to the traditional bus configuration. PCI bypasses the standard I/O bus; it uses the system bus to increase the bus clock speed and take full advantage of the CPU's data path. Systems that integrate the PCI bus became available in mid 1993 and have since become the mainstay high-end systems.

Information is transferred across the PCI bus at 33MHz, at the full data width of the CPU. When the bus is used in conjunction with a 32-bit CPU, the bandwidth is 132M per second, as the following formula shows: 33MHz x 32 bits = 1,056Mbit/sec 1,056Mbit/sec ÷ 8 = 132M/sec When the bus is used in future 64-bit implementations, the bandwidth doubles, meaning that you can transfer data at speeds up to 264M/sec. Real-life data transfer speeds necessarily will be lower, but still much faster than anything else that is currently available. Part of the reason for this faster real-life throughput is the fact that the PCI bus can operate concurrently with the processor bus; it does not supplant it. The CPU can be processing data in an external cache while the PCI bus is busy transferring information between other parts of the system--a major design benefit of the PCI bus.

A PCI adapter card uses its own unique connector. This connector can be identified within a computer system because it typically is offset from the normal ISA, MCA, or EISA connectors. See Figure 5.14 for an example. The size of a PCI card can be the same as that of the cards used in the system's normal I/O bus.

FIG. 5.13  Conceptual diagram of the PCI bus.

FIG. 5.14  Possible configuration of PCI slots in relation to ISA or EISA slots.

The PCI specification identifies three board configurations, each designed for a specific type of system with specific power requirements. The 5v specification is for stationary computer systems, the 3.3v specification is for portable machines, and the universal specification is for motherboards and cards that work in either type of system.

Table 5.3 shows the 5v PCI pinouts, and Figure 5.15 shows the pin locations. Table 5.4 shows the 3.3v PCI pinouts; the pin locations are indicated in Figure 5.16. Finally, Table 5.5 shows the pinouts, and Figure 5.17 shows the pin locations for a universal PCI slot and card. Notice that each figure shows both the 32-bit and 64-bit variations on the respective specifications.

NOTE: If the PCI card supports only 32 data bits, it needs only pins B1/A1 through B62/A62. Pins B63/A63 through B94/A94 are used only if the card supports 64 data bits.

Table 5.3  Pinouts for a 5v PCI Bus

Pin Signal Name Pin Signal Name
B1 -12v A1 Test Reset
B2 Test Clock A2 +12v
B3 Ground A3 Test Mode Select
B4 Test Data Output A4 Test Data Input
B5 +5v A5 +5v
B6 +5v A6 Interrupt A
B7 Interrupt B A7 Interrupt C
B8 Interrupt D A8 +5v
B9 PRSNT1# A9 Reserved
B10 Reserved A10 +5v I/O
B11 PRSNT2# A11 Reserved
B12 Ground A12 Ground
B13 Ground A13 Ground
B14 Reserved A14 Reserved
B15 Ground A15 Reset
B16 Clock A16 +5v I/O
B17 Ground A17 Grant
B18 Request A18 Ground
B19 +5v I/O A19 Reserved
B20 Address 31 A20 Address 30
B21 Address 29 A21 +3.3v
B22 Ground A22 Address 28
B23 Address 27 A23 Address 26
B24 Address 25 A24 Ground
B25 +3.3v A25 Address 24
B26 C/BE 3 A26 Init Device Select
B27 Address 23 A27 +3.3v
B28 Ground A28 Address 22
B29 Address 21 A29 Address 20
B30 Address 19 A30 Ground
B31 +3.3v A31 Address 18
B32 Address 17 A32 Address 16
B33 C/BE 2 A33 +3.3v
B34 Ground A34 Cycle Frame
B35 Initiator Ready A35 Ground
B36 +3.3v A36 Target Ready
B37 Device Select A37 Ground
B38 Ground A38 Stop
B39 Lock A39 +3.3v
B40 Parity Error A40 Snoop Done
B41 +3.3v A41 Snoop Backoff
B42 System Error A42 Ground
B43 +3.3v A43 PAR
B44 C/BE 1 A44 Address 15
B45 Address 14 A45 +3.3v
B46 Ground A46 Address 13
B47 Address 12 A47 Address 11
B48 Address 10 A48 Ground
B49 Ground A49 Address 9
B50 Access key A50 Access key
B51 Access key A51 Access key
B52 Address 8 A52 C/BE 0
B53 Address 7 A53 +3.3v
B54 +3.3v A54 Address 6
B55 Address 5 A55 Address 4
B56 Address 3 A56 Ground
B57 Ground A57 Address 2
B58 Address 1 A58 Address 0
B59 +5v I/O A59 +5v I/O
B60 Acknowledge 64-bit A60 Request 64-bit
B61 +5v A61 +5v
B62 +5v A62 +5v
Access key Access key
B63 Reserved A63 Ground
B64 Ground A64 C/BE 7
B65 C/BE 6 A65 C/BE 5
B66 C/BE 4 A66 +5v I/O
B67 Ground A67 Parity 64-bit
B68 Address 63 A68 Address 62
B69 Address 61 A69 Ground
B70 +5v I/O A70 Address 60
B71 Address 59 A71 Address 58
B72 Address 57 A72 Ground
B73 Ground A73 Address 56
B74 Address 55 A74 Address 54
B75 Address 53 A75 +5v I/O
B76 Ground A76 Address 52
B77 Address 51 A77 Address 50
B78 Address 49 A78 Ground
B79 +5v I/O A79 Address 48
B80 Address 47 A80 Address 46
B81 Address 45 A81 Ground
B82 Ground A82 Address 44
B83 Address 43 A83 Address 42
B84 Address 41 A84 +5v I/O
B85 Ground A85 Address 40
B86 Address 39 A86 Address 38
B87 Address 37 A87 Ground
B88 +5v I/O A88 Address 36
B89 Address 35 A89 Address 34
B90 Address 33 A90 Ground
B91 Ground A91 Address 32
B92 Reserved A92 Reserved
B93 Reserved A93 Ground
B94 Ground A94 Reserved

FIG. 5.15  The 5v PCI slot and card configuration.

Table 5.4  Pinouts for a 3.3v PCI Bus

Pin Signal Name Pin Signal Name
B1 -12v A1 Test Reset
B2 Test Clock A2 +12v
B3 Ground A3 Test Mode Select
B4 Test Data Output A4 Test Data Input
B5 +5v A5 +5v
B6 +5v A6 Interrupt A
B7 Interrupt B A7 Interrupt C
B8 Interrupt D A8 +5v
B9 PRSNT1# A9 Reserved
B10 Reserved A10 +3.3v
B11 PRSNT2# A11 Reserved
B12 Access key A12 Access key
B13 Access key A13 Access key
B14 Reserved A14 Reserved
B15 Ground A15 Reset
B16 Clock A16 +3.3v
B17 Ground A17 Grant
B18 Request A18 Ground
B19 +3.3v A19 Reserved
B20 Address 31 A20 Address 30
B21 Address 29 A21 +3.3v
B22 Ground A22 Address 28
B23 Address 27 A23 Address 26
B24 Address 25 A24 Ground
B25 +3.3v A25 Address 24
B26 C/BE 3 A26 Init Device Select
B27 Address 23 A27 +3.3v
B28 Ground A28 Address 22
B29 Address 21 A29 Address 20
B30 Address 19 A30 Ground
B31 +3.3v A31 Address 18
B32 Address 17 A32 Address 16
B33 C/BE 2 A33 +3.3v
B34 Ground A34 Cycle Frame
B35 Initiator Ready A35 Ground
B36 +3.3v A36 Target Ready
B37 Device Select A37 Ground
B38 Ground A38 Stop
B39 Lock A39 +3.3v
B40 Parity Error A40 Snoop Done
B41 +3.3v A41 Snoop Backoff
B42 System Error A42 Ground
B43 +3.3v A43 PAR
B44 C/BE 1 A44 Address 15
B45 Address 14 A45 +3.3v
B46 Ground A46 Address 13
B47 Address 12 A47 Address 11
B48 Address 10 A48 Ground
B49 Ground A49 Address 9
B50 Ground A50 Ground
B51 Ground A51 Ground
B52 Address 8 A52 C/BE 0
B53 Address 7 A53 +3.3v
B54 +3.3v A54 Address 6
B55 Address 5 A55 Address 4
B56 Address 3 A56 Ground
B57 Ground A57 Address 2
B58 Address 1 A58 Address 0
B59 +3.3v A59 +3.3v
B60 Acknowledge 64-bit A60 Request 64-bit
B61 +5v A61 +5v
B62 +5v A62 +5v
Access key Access key
B63 Reserved A63 Ground
B64 Ground A64 C/BE 7
B65 C/BE 6 A65 C/BE 5
B66 C/BE 4 A66 +3.3v
B67 Ground A67 Parity 64-bit
B68 Address 63 A68 Address 62
B69 Address 61 A69 Ground
B70 +3.3v A70 Address 60
B71 Address 59 A71 Address 58
B72 Address 57 A72 Ground
B73 Ground A73 Address 56
B74 Address 55 A74 Address 54
B75 Address 53 A75 +3.3v
B76 Ground A76 Address 52
B77 Address 51 A77 Address 50
B78 Address 49 A78 Ground
B79 +3.3v A79 Address 48
B80 Address 47 A80 Address 46
B81 Address 45 A81 Ground
B82 Ground A82 Address 44
B83 Address 43 A83 Address 42
B84 Address 41 A84 +3.3v
B85 Ground A85 Address 40
B86 Address 39 A86 Address 38
B87 Address 37 A87 Ground
B88 +3.3v A88 Address 36
B89 Address 35 A89 Address 34
B90 Address 33 A90 Ground
B91 Ground A91 Address 32
B92 Reserved A92 Reserved
B93 Reserved A93 Ground
B94 Ground A94 Reserved

FIG. 5.16  The 3.3v PCI slot and card configuration.

Table 5.5  Pinouts for a Universal PCI Bus

Pin Signal Name Pin Signal Name
B1 -12v A1 Test Reset
B2 Test Clock A2 +12v
B3 Ground A3 Test Mode Select
B4 Test Data Output A4 Test Data Input
B5 +5v A5 +5v
B6 +5v A6 Interrupt A
B7 Interrupt B A7 Interrupt C
B8 Interrupt D A8 +5v
B9 PRSNT1# A9 Reserved
B10 Reserved A10 +v I/O
B11 PRSNT2# A11 Reserved
B12 Access key A12 Access key
B13 Access key A13 Access key
B14 Reserved A14 Reserved
B15 Ground A15 Reset
B16 Clock A16 +v I/O
B17 Ground A17 Grant
B18 Request A18 Ground
B19 +v I/O A19 Reserved
B20 Address 31 A20 Address 30
B21 Address 29 A21 +3.3v
B22 Ground A22 Address 28
B23 Address 27 A23 Address 26
B24 Address 25 A24 Ground
B25 +3.3v A25 Address 24
B26 C/BE 3 A26 Init Device Select
B27 Address 23 A27 +3.3v
B28 Ground A28 Address 22
B29 Address 21 A29 Address 20
B30 Address 19 A30 Ground
B31 +3.3v A31 Address 18
B32 Address 17 A32 Address 16
B33 C/BE 2 A33 +3.3v
B34 Ground A34 Cycle Frame
B35 Initiator Ready A35 Ground
B36 +3.3v A36 Target Ready
B37 Device Select A37 Ground
B38 Ground A38 Stop
B39 Lock A39 +3.3v
B40 Parity Error A40 Snoop Done
B41 +3.3v A41 Snoop Backoff
B42 System Error A42 Ground
B43 +3.3v A43 PAR
B44 C/BE 1 A44 Address 15
B45 Address 14 A45 +3.3v
B46 Ground A46 Address 13
B47 Address 12 A47 Address 11
B48 Address 10 A48 Ground
B49 Ground A49 Address 9
B50 Access key A50 Access key
B51 Access key A51 Access key
B52 Address 8 A52 C/BE 0
B53 Address 7 A53 +3.3v
B54 +3.3v A54 Address 6
B55 Address 5 A55 Address 4
B56 Address 3 A56 Ground
B57 Ground A57 Address 2
B58 Address 1 A58 Address 0
B59 +5 I/O A59 +v I/O
B60 Acknowledge 64-bit A60 Request 64-bit
B61 +5v A61 +5v
B62 +5v A62 +5v
Access key Access key
B63 Reserved A63 Ground
B64 Ground A64 C/BE 7
B65 C/BE 6 A65 C/BE 5
B66 C/BE 4 A66 +v I/O
B67 Ground A67 Parity 64-bit
B68 Address 63 A68 Address 62
B69 Address 61 A69 Ground
B70 +v I/O A70 Address 60
B71 Address 59 A71 Address 58
B72 Address 57 A72 Ground
B73 Ground A73 Address 56
B74 Address 55 A74 Address 54
B75 Address 53 A75 +v I/O
B76 Ground A76 Address 52
B77 Address 51 A77 Address 50
B78 Address 49 A78 Ground
B79 +v I/O A79 Address 48
B80 Address 47 A80 Address 46
B81 Address 45 A81 Ground
B82 Ground A82 Address 44
B83 Address 43 A83 Address 42
B84 Address 41 A84 +v I/O
B85 Ground A85 Address 40
B86 Address 39 A86 Address 38
B87 Address 37 A87 Ground
B88 +v I/O A88 Address 36
B89 Address 35 A89 Address 34
B90 Address 33 A90 Ground
B91 Ground A91 Address 32
B92 Reserved A92 Reserved
B93 Reserved A93 Ground
B94 Ground A94 Reserved

Notice that the universal PCI board specifications effectively combine the 5v and 3.3v specifications. For pins for which the voltage is different, the universal specification labels the pin simply V I/O. This type of pin represents a special power pin for defining and driving the PCI signaling rail.

FIG. 5.17  The universal PCI slot and card configuration.

Another important feature of PCI is the fact that it was the model for the Intel PnP specification. This means that PCI cards do not have jumpers and switches, and are instead configured through software. True PnP systems are able to automatically configure the adapters, while non-PnP systems with ISA slots have to configure the adapters through a program that is usually a part of the system CMOS configuration. Starting in late 1995, most PC-compatible systems have included a PnP BIOS that allows the automatic PnP configuration.

For online information about PCI bus technology, try

http://www.us.pc.ibm.com/infobrf/ibpci.html

FireWire (IEEE-1394)

FireWire is a relatively new bus technology; it is the result of the large data-moving demands of today's audio and video multimedia devices. It is extremely fast, with data transfer rates up to an incredible 400M/sec, and even faster speeds are being developed. The IEEE-1394 (as it is officially known) specification was published by the IEEE Standards Board in late 1995.

The IEEE-1394 standard currently exists with three different signaling rates: 100M/sec, 200M/sec, and 400M/sec. Most PC adapter cards support the 200M/sec rate, even though current devices generally only operate at 100M/sec. A maximum of 63 devices can be connected to a single IEEE-1394 adapter card by daisy chaining. Cables for IEEE-1394 devices use GameBoy-derived connectors and consist of six conductors; four wires are used for data transmission, and two conduct power. Connection with the motherboard is made either by a dedicated IEEE-1394 interface or by a PCI adapter card.

As of this writing, devices which conform to the IEEE-1394 standard are limited primarily to camcorders and VCRs with digital video (DV) capability. Sony was among the first to release such devices, although their products have a unique four-wire connector that requires an adapter cord to be used with IEEE-1394 PC cards. DV products are also available from Panasonic and Matsushita, and future IEEE-1394 applications should include DV conferencing devices, satellite audio and video data streams, audio synthesizers, DVD, and other high-speed disc drives.

Because of the current DV emphasis for IEEE-1394 peripherals, most PC cards currently offered by Adaptec, FAST Multimedia, Matrox, and others involve DV capturing and editing. If you're willing to spend $1,000 or more on DV equipment, these items should provide substantial video editing and dubbing capabilities on your PC. Of course, you will need IEEE-1394 I/O connectivity, which is still a rarity on current motherboards. Fortunately, Adaptec and Texas Instruments both offer PCI adapter cards which support IEEE-1394.

IEEE-1394 stands to offer unprecedented multimedia capabilities to current and future PC users. Current peripherals--particularly DV devices--are expensive, but as with any emerging technology, prices should come down in the future, opening the door wide for new PC uses both in the home and office. A great number of people would gain the ability to do advanced audio and video editing. If you anticipate having multimedia needs on your PC in the future, IEEE-1394 connectivity is a must.

Get more information about IEEE-1394 at

http://www.adaptec.com/firewire/1394wire.html

Universal Serial Bus (USB)

Like IEEE-1394, the Universal Serial Bus (USB) is a new and promising bus technology that is rapidly gaining popularity among high-end manufacturers. Essentially, USB is a cable that allows for connection of up to 127 devices through the use of daisy chaining. While it is not as fast at data transfer as FireWire, at 12M/sec it is still more than adequate for most peripherals. The USB specification was published in 1996 by a consortium comprised of representatives from Compaq, Digital, IBM, Intel, Microsoft, NEC, and Northern Telecom.

Another benefit of the USB specification is self-identifying peripherals, a feature that should greatly ease installations. This feature is fully compatible with PnP systems and provides an industry standard for future connectivity. Also, USB devices can be "hot" plugged or unplugged, meaning that you should not have to turn off your computer every time you want to connect or disconnect a peripheral. As of this writing, USB-compatible devices are still hard to find, although most newer motherboards are being made to support them. One thing to keep in mind before purchasing USB peri-pherals is that your operating system must offer USB support. Whereas the original Windows 95 upgrade and NT 4.0 do not support USB, the later OSR-2 (OEM Service Release 2) release of Windows 95 does. Future versions of Windows and NT 5.0 will fully support USB. Because the USB standard shows promise, it should become an important bus technology in the years to come.

Get more information about USB online at

http://www.usb.org

System Resources

System resources are the communications channels, addresses, and other signals used by hardware devices to communicate on the bus. At their lowest level, these resources typically include the following:

I have listed these roughly in the order you would experience problems with them. Memory conflicts are perhaps the most troublesome of these, certainly the most difficult to fully explain and overcome. These are discussed in Chapter 7, "Memory," which focuses on the others listed here in the order you will likely have problems with them. IRQs cause more problems than DMA because they are in much higher demand; therefore, virtually all cards will use IRQ channels. There are fewer problems with DMA channels because few cards use them, and there are usually more than enough channels to go around. I/O ports are used by all hardware devices on the bus, but there are technically 64K of them, which means plenty to go around. With all of these resources, you have to make sure that a unique card or hardware function uses each resource; they cannot or should not be shared.

These resources are required and used by many different components of your system. Adapter cards need these resources to communicate with your system and to accomplish their purposes. Not all adapter cards have the same resource requirements. A serial communications port, for example, needs an IRQ channel and I/O port address, whereas a sound board needs these resources and at least one DMA channel as well. Most network cards use a 16K block of memory addresses, an IRQ channel, and an I/O port address.

As your system increases in complexity, the chance for resource conflicts increases dramatically. Modern systems with sound cards and network cards can really push the envelope and can become a configuration nightmare for the uninitiated. So that you can resolve conflicts, most adapter cards allow you to modify resource assignments by setting jumpers or switches on the cards. Fortunately, in almost all cases there is a logical way to configure the system--once you know the rules.

Interrupts (IRQs)

Interrupt request channels (IRQs), or hardware interrupts, are used by various hardware devices to signal the motherboard that a request must be fulfilled. This procedure is the same as a student raising his hand to indicate that he needs attention.

These interrupt channels are represented by wires on the motherboard and in the slot connectors. When a particular interrupt is invoked, a special routine takes over the system, which first saves all the CPU register contents in a stack and then directs the system to the interrupt vector table. This vector table contains a list of memory addresses that correspond to the interrupt channels. Depending on which interrupt was invoked, the program corresponding to that channel is run.

The pointers in the vector table point to the address of whatever software driver is used to service the card that generated the interrupt. For a network card, for example, the vector may point to the address of the network drivers that have been loaded to operate the card; for a hard disk controller, the vector may point to the BIOS code that operates the controller.

After the particular software routine finishes performing whatever function the card needed, the interrupt-control software returns the stack contents to the CPU registers, and the system then resumes whatever it was doing before the interrupt occurred.

Through the use of interrupts, your system can respond to external events in a timely fashion. Each time that a serial port presents a byte to your system, an interrupt is generated to ensure that the system reads that byte before another comes in. Keep in mind that in some cases a port device--in particular, a modem with a 16550 or higher UART chip--may incorporate a byte buffer that allows multiple characters to be stored before an interrupt is generated.

Hardware interrupts are generally prioritized by their numbers; with some exceptions, the highest-priority interrupts have the lowest numbers. Higher-priority interrupts take precedence over lower-priority interrupts by interrupting them. As a result, several interrupts can occur in your system concurrently, each interrupt nesting within another.

If you overload the system--in this case, by running out of stack resources (too many interrupts were generated too quickly)--an internal stack overflow error occurs and your system halts. If you experience this type of system error and run DOS, you can compensate for it by using the STACKS parameter in your CONFIG.SYS file to increase the available stack resources. Most people will not see this error in Windows 95 or Windows NT.

The ISA bus uses edge-triggered interrupt sensing, in which an interrupt is sensed by a signal sent on a particular wire located in the slot connector. A different wire corresponds to each possible hardware interrupt. Because the motherboard cannot recognize which slot contains the card that used an interrupt line and therefore generated the interrupt, confusion would result if more than one card were set to use a particular interrupt. Each interrupt, therefore, usually is designated for a single hardware device, and most of the time, interrupts cannot be shared.

A device can be designed to share interrupts, and a few devices allow this; most cannot, however, because of the way interrupts are signaled in the ISA bus. Systems with the MCA bus use level-sensitive interrupts, which allow complete interrupt sharing to occur. In fact, in an MCA system, all boards can be set to the same inter- rupt with no conflicts or problems. The EISA bus can optionally use level-sensitive interrupts which allow sharing, but only for true EISA cards. For maximum performance, however, interrupts should be staggered as much as possible.

External hardware interrupts often are referred to as maskable interrupts, which simply means that the interrupts can be masked or turned off for a short time while the CPU is used for other critical operations. It is up to the programmer to manage interrupts properly and efficiently for the best system performance.

Because interrupts usually cannot be shared in an ISA bus system, you often run into conflicts and can even run out of interrupts when you are adding boards to a system. If two boards use the same IRQ to signal the system, the resulting conflict prevents either board from operating properly. The following sections discuss the IRQs that any standard devices use, as well as what may be free in your system.

8-Bit ISA Bus Interrupts

The PC and XT (the systems based on the 8-bit 8086 CPU) provide for eight different external hardware interrupts. Table 5.6 shows the typical uses for these interrupts, which are numbered 0 through 7.

Table 5.6  8-Bit ISA Bus Default Interrupt Assignments

IRQ Function Bus Slot
0 System Timer No
1 Keyboard Controller No
2 Available Yes (8-bit)
3 Serial Port 2 (COM2:) Yes (8-bit)
4 Serial Port 1 (COM1:) Yes (8-bit)
5 Hard Disk Controller Yes (8-bit)
6 Floppy Disk Controller Yes (8-bit)
7 Parallel Port 1 (LPT1:) Yes (8-bit)

If you have a system that has one of the original 8-bit ISA buses, you will find that the IRQ resources provided by the system present a severe limitation. Installing several devices that need the services of system IRQs in a PC/XT-type system can be a study in frustration, because the only way to resolve the interrupt-shortage problem is to remove the adapter board that you need the least.

16-Bit ISA, EISA, and MCA Bus Interrupts

The introduction of the AT, based on the 286 processor, was accompanied by an increase in the number of external hardware interrupts that the bus would support. The number of interrupts was doubled to 16 by using two Intel 8259 interrupt controllers, piping the interrupts generated by the second one through the unused IRQ 2 in the first controller. This arrangement effectively means that only 15 IRQ assignments are available, and IRQ 2 effectively became inaccessible.

By routing all of the interrupts from the second IRQ controller through IRQ 2 on the first, all of these new interrupts are assigned a nested priority level between IRQ 1 and IRQ 3. Thus, IRQ 15 ends up having a higher priority than IRQ 3. Figure 5.18 shows how the two 8259 chips were wired to create the cascade through IRQ 2 on the first chip.

FIG. 5.18  Interrupt controller cascade wiring.

To prevent problems with boards set to use IRQ 2, the AT system designers routed one of the new interrupts (IRQ 9) to fill the slot position left open after removing IRQ 2. This means that any card you install in a modern system that claims to use IRQ 2 is really using IRQ 9 instead. Some cards now label this selection as IRQ 2/9, while others may only call it IRQ 2 or IRQ 9. No matter what the labeling says, you must never set two cards to use that interrupt!

Table 5.7 shows the typical uses for interrupts in the 16-bit ISA, EISA, and MCA buses, and lists them in priority order from highest to lowest.

Table 5.7  16-Bit ISA, EISA, and MCA Default Interrupt Assignments

IRQ Standard Function Bus Slot Card Type
0 System Timer No -
1 Keyboard Controller No -
2 2nd IRQ Controller Cascade No -
8 Real-Time Clock No -
9 Network/Available (appears as IRQ 2) Yes 8/16-bit
10 Available Yes 16-bit
11 SCSI/Available Yes 16-bit
12 Motherboard Mouse Port/Available Yes 16-bit
13 Math Coprocessor No -
14 Primary IDE Yes 16-bit
15 Secondary IDE/Available Yes 16-bit
3 Serial Port 2 (COM2:) Yes 8/16-bit
4 Serial Port 1 (COM1:) Yes 8/16-bit
5 Sound/Parallel Port 2 (LPT2:) Yes 8/16-bit
6 Floppy Disk Controller Yes 8/16-bit
7 Parallel Port 1 (LPT1:) Yes 8/16-bit

Because IRQ 2 now is used directly by the motherboard, the wire for IRQ 9 has been re-routed to the same position in the slot that IRQ 2 normally would occupy. Therefore, any board you install that is set to IRQ 2 actually is using IRQ 9. The interrupt vector table has been adjusted accordingly to enable this deception to work. This adjustment to the system provides greater compatibility with the PC interrupt structure and enables cards that are set to IRQ 2 to work properly.

Notice that interrupts 0, 1, 2, 8, and 13 are not on the bus connectors and are not accessible to adapter cards. Interrupts 8, 10, 11, 12, 13, 14, and 15 are from the second interrupt controller and are accessible only by boards that use the 16-bit extension connector, because this is where these wires are located. IRQ 9 is rewired to the 8-bit slot connector in place of IRQ 2, which means that IRQ 9 replaces IRQ 2 and therefore is available to 8-bit cards, which treat it as though it were IRQ 2.

NOTE: Although the 16-bit ISA bus has twice as many interrupts as systems that have the 8-bit ISA bus, you still may run out of available interrupts, because only 16-bit adapters can use most of the newly available interrupts.

The extra IRQ lines in a 16-bit ISA system are of little help unless the adapter boards that you plan to use enable you to configure them for one of the unused IRQs. Some devices are hard-wired so that they can use only a particular IRQ. If you have a device that already uses that IRQ, you must resolve the conflict before installing the second adapter. If neither adapter enables you to reconfigure its IRQ use, chances are that you cannot use the two devices in the same system.

IRQ Conflicts

One of the most common areas of IRQ conflict involves serial (COM) ports. You may have noticed in the preceding two sections that two IRQs are set aside for two COM ports. IRQ 3 is used for COM2:, and IRQ 4 is used for COM1:. The problem occurs when you have more than two serial ports in a system--a situation that is entirely possible, because a PC can support up to four COM ports.

The problems arise here because most people purchase poorly designed COM port boards that do not allow IRQ settings other than 3 or 4. What happens is that they end up setting COM3: to IRQ 4 (sharing it with COM1:), and COM4: to IRQ 3 (sharing it with COM2:). This is not acceptable, as it will prevent you from using the two COM ports on any one of the interrupt channels simultaneously. This was somewhat acceptable under plain DOS, because single-tasking (running only one program at a time) was the order of the day, but is totally unacceptable with Windows and OS/2. If you must share IRQs, you can usually get away with sharing devices on the same IRQ as long as they use different COM ports. For instance, a scanner and an internal modem could share an IRQ, although if the two devices are used simultaneously a conflict will result.

The best solution is to purchase a multiport serial I/O card that will allow interrupt sharing among COM ports. As a side note, also make sure that the COM board you purchase uses a buffered 16550A or higher type UART (Universal Asynchronous Receiver Transmitter) chip rather than the slow, unbuffered 16450 types. One company providing specialized high quality COM boards is Byte Runner Technologies (see the vendor list in Appendix A).

For more information on COM boards and ports, see

http://www.byterunner.com http://comminfo.com

If a device listed in the table is not present, such as the motherboard mouse port (IRQ 12) or parallel port 2 (IRQ 5), then you can consider those interrupts as available. For example, a second parallel port is a rarity, and most systems will have a sound card installed and set for IRQ 5. Also, on most systems IRQ 15 is assigned to a secondary IDE controller. If you do not have a second IDE hard drive, you could disable the secondary IDE controller to free up that IRQ for another device.

DMA Channels

DMA (Direct Memory Access) channels are used by high-speed communications devices that must send and receive information at high speed. A serial or parallel port does not use a DMA channel, but a sound card or SCSI adapter often does. DMA channels sometimes can be shared if the devices are not of the type that would need them simultaneously. For example, you can have a network adapter and a tape backup adapter sharing DMA channel 1, but you cannot back up while the network is running. To back up during network operation, you must ensure that each adapter uses a unique DMA channel.

8-Bit ISA Bus DMA Channels

In the 8-bit ISA bus, four DMA channels support high-speed data transfers between I/O devices and memory. Three of the channels are available to the expansion slots. Table 5.8 shows the typical uses of these DMA channels.

Table 5.8  8-Bit ISA Default DMA-Channel Assignments

DMA Standard Function Bus Slot
0 Dynamic RAM Refresh No
1 Available Yes (8-bit)
2 Floppy disk controller Yes (8-bit)
3 Hard disk controller Yes (8-bit)

Because most systems typically have both a floppy and hard disk drive, only one DMA channel is available in 8-bit ISA systems.

16-Bit ISA DMA Channels

Since the introduction of the 286 CPU, the ISA bus has supported eight DMA channels, with seven channels available to the expansion slots. Like the expanded IRQ lines described earlier in this chapter, the added DMA channels were created by cascading a second DMA controller to the first one. DMA channel 4 is used to cascade channels 0 through 3 to the microprocessor. Channels 0 through 3 are available for 8-bit transfers, and channels 5 through 7 are for 16-bit transfers only. Table 5.9 shows the typical uses for the DMA channels.

Table 5.9  16-Bit ISA, EISA, and MCA Default DMA-Channel Assignments

DMA Standard Function Bus Slot Card Type Transfer
0 Available Yes 16-bit 8-bit
1 Sound/Available Yes 8/16-bit 8-bit
2 Floppy Disk Controller Yes 8/16-bit 8-bit
3 ECP Parallel/Available Yes 8/16-bit 8-bit
4 1st DMA Controller No - 16-bit Cascade
5 Sound/Available Yes 16-bit 16-bit
6 SCSI/Available Yes 16-bit 16-bit
7 Available Yes 16-bit 16-bit

The only standard DMA channel used in all systems is DMA 2, which is universally used by the floppy controller. DMA 4 is not usable, and does not appear in the bus slots. DMA channels 1 and 5 are most commonly used by sound cards such as the Sound Blaster 16. These cards use both an 8- and a 16-bit DMA channel for high-speed transfers.

NOTE: Although DMA channel 0 appears in a 16-bit slot connector extension and therefore can only be used by a 16-bit card, it only does 8-bit transfers! Because of this, you will generally not see DMA 0 as a choice on 16-bit cards. Most 16-bit cards (like SCSI host adapters) that use DMA channels have their choices limited to DMA 5 through 7.

EISA

Realizing the shortcomings inherent in the way DMA channels are implemented in the ISA bus, the creators of the EISA specification created a specific DMA controller for their new bus. They increased the number of address lines to include the entire address bus, thus allowing transfers anywhere within the address space of the system. Each DMA channel can be set to run either 8-, 16-, or 32-bit transfers. In addition, each DMA channel can be separately programmed to run any of four types of bus cycles when transferring data:

EISA DMA also allows for special reading and writing operations referred to as scatter write and gather read. Scattered writes are done by reading a contiguous block of data and writing it to more than one area of memory at the same time. Gathered reads involve reading from more than one place in memory and writing to a device. These functions are often referred to as Buffered Chaining, and they increase the throughput of DMA operations.

MCA

It might be assumed that because MCA is a complete rebuilding of the PC bus structure that DMA in an MCA environment would be better constructed. This is not so. Quite to the contrary, DMA in MCA systems were for the most part all designed around one DMA controller with the following issues:

The inability of the DMA controller to address more than 2 bytes per transfer severely cripples this otherwise powerful bus.

I/O Port Addresses

Your computer's I/O ports enable you to attach a large number of important devices to your system to expand its capabilities. A printer attached to one of your system's LPT (parallel) ports enables you to make a printout of the work on your system. A modem attached to one of your system's COM (serial) ports enables you to use telephone lines to communicate with computers thousands of miles away. A scanner attached to an LPT port or a SCSI host adapter enables you to convert graphics or text to images and type that you can use with the software installed on your computer.

Most systems come configured with at least two COM (serial) ports and one LPT (parallel printer) ports. The two serial ports are configured as COM1: and COM2:, and the parallel port as LPT1. The basic architecture of the PC provides for as many as four COM ports (1 through 4) and three LPT ports (1 through 3). If you use more than two COM ports, make sure that COM3: and COM4: have unique IRQ settings and do not share those with COM1: and COM2:. In general, on most machines both COM1: and COM3: use IRQ 4; and both COM2: and COM4: use IRQ 3.

CAUTION: Theoretically, each of the four COM ports in a system can be used to attach a device, such as a mouse or modem, but doing so may lead to resource conflicts. For more information, see the discussion of resolving IRQ conflicts in "IRQ Conflicts" earlier in this chapter.

Every I/O port in your system uses an I/O address for communication. This address, which is in the lower memory ranges, is reserved for communication between the I/O device and the operating system. If your system has multiple I/O cards, each card must use a different I/O address; if not, your system will not be able to communicate with the devices reliably.

The I/O addresses that your ports use depend on the type of ports. Table 5.10 shows the I/O addresses expected by the various standard ports in a PC system.

Table 5.10  Standard I/O Addresses for Serial and Parallel Ports

Port Base I/O Address
COM1 3F8h
COM2 2F8h
COM3 3E8h
COM4 2E8h
LPT1 378h
LPT2 278h

Besides your serial and parallel ports, other adapters in your system use I/O addresses. Quite truthfully, the I/O addresses for the serial and parallel ports are fairly standard; it is unlikely that you will run into problems with them. The I/O addresses used by other adapters are not standardized, however, and you may have problems finding a mix of port addresses that works reliably. In the next section, you learn some of the techniques that you can use to solve this problem.

Resolving Resource Conflicts

The resources in a system are limited. Unfortunately, the demands on those resources seem to be unlimited. As you add more and more adapter cards to your system, you will find that the potential for resource conflicts increases. If your system is fully PnP-compatible, then potential conflicts should be resolved automatically. If your system does not have a bus that resolves conflicts for you (such as an MCA or EISA bus), you need to resolve the conflicts manually.

How do you know whether you have a resource conflict? Typically, one of the devices in your system stops working. Resource conflicts can exhibit themselves in other ways, though. Any of the following events could be diagnosed as a resource conflict:

In the following sections, you learn some of the steps that you can take to head off resource conflicts or to track them down when they occur.

A good source of information online regarding resource conflicts can be found at:

http://www.atipa.com/InfoSheets/IRQs.shtml

CAUTION: Be careful in diagnosing resource conflicts; a problem may not be a resource conflict at all, but a computer virus. Many computer viruses are designed to exhibit themselves as glitches or as periodic problems. If you suspect a resource conflict, it may be worthwhile to run a virus check first to ensure that the system is clean. This procedure could save you hours of work and frustration.

Resolving Conflicts Manually

Unfortunately, the only way to resolve conflicts manually is to take the cover off your system and start changing switches or jumper settings on your adapter cards. Each of these changes then must be accompanied by a system reboot, which implies that they take a great deal of time. This situation brings us to the first rule of resolving conflicts: When you set about ridding your system of resource conflicts, make sure that you allow a good deal of uninterrupted time.

Also make sure that you write down your current system settings before you start making changes. That way, you will know where you began and can go back to the original configuration (if necessary).

Finally, dig out the manuals for all your adapter boards; you will need them. If you cannot find the manuals, contact the manufacturers to determine what the various jumper positions and switch settings mean. Additionally, you could look for more current information online at the manufacturers' Web sites.

Now you are ready to begin your detective work. As you try various switch settings and jumper positions, keep the following questions in mind; the answers will help you narrow down the conflict areas:

Whenever you make changes in your system, reboot and see whether the problem persists. When you believe that you have solved the problem, make sure that you test all your software. Fixing one problem often seems to causes another to crop up. The only way to make sure that all problems are resolved is to test everything in your system.

As you attempt to resolve your resource conflicts, you should work with and update a system-configuration template, as discussed in the following section.

Using a System-Configuration Template

A system-configuration template is helpful, simply because it is easier to remember something that is written down than it is to keep it in your head. To create a configuration template, all you need to do is start writing down what resources are used by which parts of your system. Then, when you need to make a change or add an adapter, you can quickly determine where conflicts may arise.

I like to use a worksheet split into three main areas--one for interrupts, another for DMA channels, and a middle area for devices that do not use interrupts. Each section lists the IRQ or DMA channel on the left and the I/O port device range on the right. This way, you get the clearest picture of what resources are used and which ones are available in a given system.

Here is the system-configuration template I have developed over the years and still use almost daily. This type of configuration sheet is resource-based instead of component-based. Each row in the template represents a different resource, and lists the component using the resource as well as the resources used. The chart has pre-entered all of the fixed items in a modern PC, whose configuration cannot be changed.

To fill out this type of chart, you would perform the following steps:

1. Enter the default resources used by standard components, such as serial and parallel ports, disk controllers, and video. You can use the filled out example I have provided to see how most standard devices are configured.

2. Enter the default resources used by additional add-on components such as sound cards, SCSI cards, network cards, proprietary cards, and so on.

3. Change any configuration items that are in conflict. Try to leave built-in devices at their default settings, as well as sound cards. Other installed adapters may have their settings changed, but be sure to document the changes.

Of course a template like this is best used when first installing components, not after. Once you have it completely filled out to match your system, you can label it and keep it with the system. When you add any more devices, the template will be your guide as to how any new devices should be configured.

The following example is the same template filled out for a typical modern PC system:

Form A:

Form B:

As you can see from this template, only two IRQs and two DMA channels remain available. In this example configuration, the primary and secondary IDE connectors were built into the motherboard:

Whether these devices are built into the motherboard or on a separate card makes no difference because the resource allocations are the same in either case. All default settings are normally used for these devices, and are indicated in the completed configuration. Next, the accessory cards were configured. In this example, the following cards were installed:

It helps to install the cards in this order. Start with the video card; next, add the sound card. Due to problems with software that must be configured to the sound card, it is best to install it early and make sure only default settings are used. It is better to change settings on other cards than the sound card.

After the sound card, the SCSI adapter was installed; however, the default I/O Port addresses (330-331) and DMA channel (DMA 5) used were in conflict with other cards (mainly the sound card). These settings were changed to their next logical settings which did not cause a conflict.

Finally, the network card was installed, which also had default settings that conflicted with other cards. In this case, the Ethernet card came pre-configured to IRQ 3, which was already in use by COM2:. The solution was to change the setting, and IRQ 9 was the next logical choice in the card's configuration settings.

Even though this is a fully loaded configuration, only three individual items among all of the cards had to be changed to achieve an optimum system configuration. As you can see, using a configuration template like the one shown can make what would otherwise be a jumble of settings lay out in an easy-to-follow manner. The only real problems you will run into once you work with these templates are cards that do not allow for enough adjustment in their settings, or cards which are lacking in documentation. As you can imagine, you will need the documentation for each adapter card, as well as the mother-board, in order to accurately complete a configuration table like the one shown.

TIP: Do not rely too much on software diagnostics such as MSD.EXE that claim to be able to show hardware settings like IRQ and I/O port settings. While they can be helpful in certain situations, they are often wrong with respect to at least some of the information they are displaying about your system. One or two items shown incorrectly can be very troublesome if you believe the incorrect information and configure your system based on it!

Unless your system fully supports PnP, then there is simply no standard way for software to determine resource usage in a PC system. In a non-PnP system, these programs will instead guess at how things are configured, and display these guesses with confidence, even though they may be incorrect.

Heading Off Problems: Special Boards

A number of devices that you may want to install in a computer system require IRQ lines or DMA channels, which means that a world of conflict could be waiting in the box that the device comes in. As mentioned in the preceding section, you can save yourself problems if you use a system-configuration template to keep track of the way that your system is configured.

You also can save yourself trouble by carefully reading the documentation for a new adapter board before you attempt to install it. The documentation details the IRQ lines that the board can use, as well as its DMA-channel requirements. In addition, the documentation will detail the adapter's upper-memory needs for ROM and adapter.

The following sections describe some of the conflicts that you may encounter when you install today's most popular adapter boards. Although the list of adapter boards covered in these sections is far from comprehensive, the sections serve as a guide to installing complex hardware with minimum hassle. Included are tips on sound boards, SCSI host adapters, and network adapters.

Sound cards

Sound cards are probably the biggest single resource hog in your system. They usually use at least one IRQ, two DMA channels, and multiple I/O port address ranges. This is because a sound card is actually several different pieces of hardware all on one board. Most sound cards are similar to the Sound Blaster 16 from Creative Labs. Figure 5.19 shows the default resources used by the components on a typical Sound Blaster 16 card.

FIG. 5.19  Default resources for Sound Blaster 16 card.

As you can see, these cards use quite a few resources. If you take the time to read your sound board's documentation and determine its communications-channel needs, compare those needs to the IRQ lines and DMA channels that already are in use in your system, and then change the settings of the other adapters to avoid conflicts with the sound card, your installation will go quickly and smoothly.

TIP: The greatest single piece of advice I can give you for installing a sound card is to put the sound card in before all other cards except for video. In other words, let the sound card retain all of its default settings; never change a resource setting to prevent a conflict. Instead, always change the settings of other adapters when a conflict with the sound card arises. The problem here is that many educational and game programs that use sound are very poorly written with respect to supporting alternate resource settings on sound cards. Save yourself some grief and let the sound card have its way!

One example of a potential sound-board conflict is the combination of a Sound Blaster 16 and an Adaptec SCSI adapter. The Sound and SCSI adapters will conflict on DMA 5 as well as on I/O ports 330-331. Rather than changing the settings of the sound card, it is best to alter the SCSI adapter to the next available settings that will not conflict with the sound card or anything else. The final settings were shown in the previous configuration template.

The cards in question (Sound Blaster 16 and AHA-1542CF) are not singled out here because there is something wrong with them, but instead because they happen to be the most popular cards of their respective types, and as such will often be paired together.

SCSI Adapter Boards

SCSI adapter boards use more resources than just about any other type of add-in device except perhaps a sound card. They will often use resources that are in conflict with sound cards or network cards. A typical SCSI host adapter requires an IRQ line, a DMA channel, a range of I/O port addresses, plus a 16K range of unused upper memory for its ROM and possible scratch-pad RAM use. Fortunately, the typical SCSI adapter is also easy to reconfigure, and changing any of these settings should not affect performance or software operation.

Before installing a SCSI adapter, be sure to read the documentation for the card, and make sure that any IRQ lines, DMA channels, I/O ports, and upper memory that the card needs are available. If the system resources that the card needs are already in use, use your system-configuration template to determine how you can alter the settings on the SCSI card or other cards to prevent any resource conflicts before you attempt to plug in the adapter card.

Network Interface Cards (NICs)

Networks are becoming more and more popular all the time. A typical network adapter does not require as many resources as some of the other cards discussed here, but will require at least a range of I/O port addresses and an interrupt. Most NICs will also require a 16K range of free upper memory to be used for the RAM transfer buffer on the network card. As with any other cards, make sure that all of these resources are unique to the card, and are not shared with any other devices.

Multiple-COM-Port Adapters

A serial port adapter usually has two or more ports on-board. These COM ports require an interrupt and a range of I/O ports each. There aren't too many problems with the I/O port addresses, because the ranges used by up to four COM ports in a system are fairly well defined. The real problem is with the interrupts. Most older installations of more than two serial ports have any additional ones sharing the same interrupts as the first two. This is incorrect, and will cause nothing but problems with software that runs under Windows or OS/2. With these older boards, make sure that each serial port in your system has a unique I/O port address range, and more importantly, a unique interrupt setting.

Because COM ports are required for so many peripherals that connect to the modern PC, and because the number of COM ports that can be used is strictly limited by the IRQ setup in the basic IBM system design, special COM-port cards are available that enable you to assign a unique IRQ to each of the COM ports on the card. For example, you can use such a card to leave COM1: and COM2: configured for IRQ 4 and IRQ 3, respectively, but to configure COM3: for IRQ 10 and COM4: for IRQ 12 (provided you do not have a motherboard-based mouse port in your system).

Many newer multiport adapter cards--such as those offered by Byte Runner Technologies--allow "intelligent" interrupt sharing among ports. In some cases, you can have up to 12 COM port settings without conflict problems. Check with your adapter card's manufacturer to determine if it allows for automatic or "intelligent" interrupt sharing.

Although most people have problems incorrectly trying to share interrupts when installing more than two serial ports in a system, there is a fairly common problem with the I/O port addressing that should be mentioned. Many of the newer high-performance SVGA (Super VGA) chipsets, such as those from S3 Inc. and ATI, use some additional I/O port addresses that will conflict with the standard I/O port addresses used by COM4:.

In the example system-configuration just covered, you can see that the ATI video card uses some additional I/O port addresses, specifically 2EC-2EF. This is a problem as COM4: is normally configured as 2E8-2EF, which overlaps with the video card. The video cards that use these addresses are not normally adjustable for this setting, so you will either have to change the address of COM4: to a nonstandard setting, or simply disable COM4: and restrict yourself to using only three serial ports in the system. If you do have a serial adapter that supports nonstandard I/O address settings for the serial ports, you must ensure that those settings are not used by other cards, and you must inform any software or drivers, such as those in Windows, of your nonstandard settings.

With a multiple-COM-port adapter card installed and properly configured for your system, you can have devices hooked to numerous COM ports, and up to four devices can be functioning at the same time. For example, you can use a mouse, modem, plotter, and serial printer at the same time.

Plug and Play Systems

Plug and Play (PnP) represents a major revolution in recent interface technology. PnP first came on the market in 1995, and most new systems come ready to take advantage of it. In the past, PC users have been forced to muddle through a nightmare of DIP switches and jumpers every time they wanted to add new devices to their systems. The results, all too often, were system resource conflicts and non-functioning cards.

PnP is not an entirely new concept. It was a key design feature of MCA and EISA interfaces, but the limited appeal of MCA and EISA meant that they never became industry standards. Therefore, mainstream PC users still worry about I/O addresses, DMA channels, and IRQ settings. But now that PnP specifications are available for ISA-, PCI-, SCSI-, IDE-, and PCMCIA-based systems, worry-free hardware setup is within the grasp of all new computer buyers.

Of course, PnP may well be within your grasp, but that does not necessarily mean you are ready to take advantage of it. For PnP to work, the following components are required:

Each of these components needs to be PnP-compatible, meaning that it complies with the PnP specifications.

The Hardware Component

The hardware component refers to both computer systems and adapter cards. The term does not mean, however, that you cannot use your older ISA adapter cards (referred to as legacy cards) in a PnP system. You can use these cards; in fact, your PnP BIOS automatically re-assigns PnP-compatible cards around existing legacy components. PnP adapter cards communicate with the system BIOS and the operating system to convey information about what system resources are needed. The BIOS and operating system, in turn, resolve conflicts (wherever possible) and inform the adapter cards which specific resources it should use. The adapter card then can modify its configuration to use the specified resources.

The BIOS Component

The BIOS component means that most users of older PCs need to update their BIOSes or purchase new machines that have PnP BIOSes. For a BIOS to be compatible, it must support 13 additional system function calls, which can be used by the OS component of a PnP system. The PnP BIOS specification was developed jointly by Compaq, Intel, and Phoenix Technologies. The PnP features of the BIOS are implemented through an expanded POST. The BIOS is responsible for identification, isolation, and possible configuration of PnP adapter cards. The BIOS accomplishes these tasks by performing the following steps:

1. Disable any configurable devices on the motherboard or on adapter cards.

2. Identify any PnP PCI or ISA devices.

3. Compile an initial resource-allocation map for ports, IRQs, DMAs, and memory.

4. Enable I/O devices.

5. Scan the ROMs of ISA devices.

6. Configure initial-program-load (IPL) devices, which are used later to boot the system.

7. Enable configurable devices by informing them which resources have been assigned to them.

8. Start the bootstrap loader.

9. Transfer control to the operating system.

The Operating-System Component

The operating-system component can be implemented by most newer systems, such as OS/2, Windows 95, or DOS extensions. Extensions of this type should be familiar to most DOS users; extensions have been used for years to provide support for CD-ROM drives. Extension software is available now for existing operating systems, and you can expect all new PC operating systems to have PnP support built in. If you are using Windows NT 4.0, PnP drivers may or may not have been loaded automatically. If not, the driver can be found on the Windows NT 4.0 CD in the DRVLIB\PNPISA\ directory. Open the correct subdirectory for your chipset and install the file PNPISA.INF.

It is the responsibility of the operating system to inform users of conflicts that cannot be resolved by the BIOS. Depending on the sophistication of the operating system, the user then could configure the offending cards manually (on-screen) or turn the system off and set switches on the physical cards. When the system is restarted, the system is checked for remaining (or new) conflicts, any of which are brought to the user's attention. Through this repetitive process, all system conflicts are resolved.

NOTE: Plug and Play is still going through some revisions. Windows 95 requires at least version 1.0a of the ISA PnP BIOS. If your system does not have the most current BIOS, I suggest that you install a BIOS upgrade. With the Flash ROM used in most PnP systems, you can just download the new BIOS image from the system vendor or manufacturer and run the supplied BIOS update program.

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