The success of the personal computer is due largely to its ability to expand to meet the changing needs and economic requirements of the user. In this chapter, we describe the array of expansion buses that help to expand the system and work with an ever-growing number of enhancements, including modems, video cards, and portable drives. We also discuss conflicts within the computer—how they are created and reconciled.
A competent technician has to know how to attach new devices to a computer. Knowledge of the different methods of doing so and the various cables used to link devices is critical in day-to-day operations. This chapter explores the various options and how to employ them properly and safely.
Understanding Expansion Buses
Expansion buses connect devices to the motherboard using the motherboard's data bus. They allow the flow of data between internal and external devices that make up the computer system. Early computers moved data between devices and the processor at about the same rate as the processor. As processor speeds increased, the movement of data through the bus became a bottleneck. Therefore, the design capability of the buses needed to evolve, too. This lesson discusses that evolution.- Identify the different types of expansion buses in a computer
- Understand the difference between the system bus and the expansion bus
Development of the Expansion Bus
As discussed earlier in Chapter 4, "The Central Processing Unit," every device in the computer—random access memory (RAM), the keyboard, network interface card (NIC), sound card, and so forth—is connected to the external data bus. Expansion slots on the motherboard are standardized connections that allow the installation of devices not soldered to the motherboard. The function of an expansion slot is to provide configuration flexibility when devices are added to a computer.Whether a device is soldered to the motherboard or connected through an expansion slot, all integrated circuits (ICs) are regulated by a quartz crystal. The crystal sets the timing for the system, giving all parts access to a common reference point for performing actions. Most central processing units (CPUs) divide the crystal speed by two. (If the CPU has a 33-MHz speed, a 66-MHz crystal is required.) Every device soldered to the motherboard—keyboard chip, memory controller chip, and so on—is designed to run at the speed (or at half the speed) of the system crystal.
Although CPU speeds have continually increased as technology has improved, the speeds of expansion cards has remained relatively constant. It was not practical to redesign and replace every expansion card each time a new processor was released—this would have been complicated and expensive for manufacturers. (And, of course, the additional expense would have been passed along to the consumer.) The commitment of the industry to maintain backward compatibility further complicated design tasks because any new technology would have to be compatible with the older, slower devices.
To resolve this dilemma, designers divided the external data bus into two parts:
- System bus. This supports the CPU, RAM, and other motherboard components and runs at speeds that support the CPU.
- Expansion bus. This supports any add-on devices by means of the expansion slots and runs at a steady rate, based on the specific bus design.
Industry Standard Architecture
The first-generation IBM XT (with the 8088 processor) had an 8-bit external data bus and ran at a speed of 4.77 MHz. These machines were sold with an 8-bit expansion bus (PC bus) that ran at 8.33 MHz (see Figure 8.1).
When IBM designed the first PC back in 1981, it took steps that fueled the rapid development of the PC market. IBM's engineers designed the PC as an open system, capable of using standard, off-the-shelf components. This allowed third-party developers to manufacture cards that could snap into the PC bus. IBM also allowed competitors to copy the PC bus.
With this decision, IBM established the Industry Standard Architecture (ISA) interface, thus generating the market for "clones." A host of third-party companies developed products that enhanced the basic PC design and kept prices much lower for add-ons than those for competing proprietary systems such as the Apple II. Without this push, the PC market would not have developed as rapidly as it did.
In 1984, when IBM released the AT (Advanced Technology) PC, featuring Intel's 80286 16-bit processor, it wanted to include a new expansion bus—one that would be compatible with previously released devices. To accomplish this, the designers added a bus that allowed insertion of either an 8-bit card or a 16-bit card. This change resulted in the standard 16-bit ISA slot. This new 16-bit bus officially ran at a top speed of 8.33 MHz, but on some Peripheral Component Interconnect (PCI)-based systems, the actual rate for ISA slots proved to be as high as about 10 MHz. (PCI is discussed later in this lesson.)
NOTE
The term "ISA" did not become official until 1990. Therefore, the 8-bit slot is called the XT, and the 16-bit slot is called the AT. When we refer to an ISA slot or an ISA card, we generally mean the 16-bit AT-style interface. The speed of the slots remained at about 7 MHz.
Problems with the ISA Design
The ISA design is one of the most enduring elements of the PC. It can be found on virtually all systems, from the second-generation IBM PC to machines built today. However, it suffers from two major shortcomings: lack of speed and compatibility problems stemming from card design.As CPU performance increased and applications became more powerful, card designers sought an interface that would allow add-on cards to keep up with the need for improved hard drives, display adapters, and similar products.
Expansion cards must make use of system resources in an orderly way, so that they do not conflict with other devices. When demands for these system resources are not coordinated, the system could behave erratically or even fail to boot up. Formerly, ISA cards often used a bewildering array of jumpers and switches to set addresses for memory use or the IRQ (interrupt request) locations they would use.
The need to overcome the expansion card's lack of speed and compatibility problems led to a search for a new, standard expansion card interface—one that everyone could agree on and that would gain user acceptance.
Micro Channel Architecture
In 1986 the market came to be dominated by the new 386 machines with their 32-bit architecture. Most PC manufacturers stuck to the same basic ISA design and MS-DOS. Expansion devices based on ISA technology for the 286 AT class machines could be placed in a new 386 clone without problems.IBM, however, was feeling the pinch of competition from cheaper clones, and sought to retain its dominance in the PC market. IBM designers produced a new version of the PC, the PS/2 (Personal System/2), and created a proprietary expansion bus called Micro Channel Architecture (MCA) as part of the design. Running at 10 MHz, it offered more performance and provided a 32-bit data path, but was also totally incompatible with older ISA cards.
A feature of MCA was its ability to "self-configure" devices. Unlike devices that use technology in which the PC configures itself automatically to work with peripherals such as monitors, modems, and printers, an MCA device always came with a configuration disk. When installing a new device in an MCA computer, the technician inserted the configuration disk (when prompted), and the IRQs, input/output (I/O) addresses, and direct memory access (DMA) channels were configured automatically. (IRQs, I/O addresses, and DMA channels are discussed in detail in the next lesson.) An MCA bus is shown in Figure 8.2.
The PS/2 and its Micro Channel Architecture expansion bus never gained enough market share to compete with the 386. MCA cards were few and far between, and more expensive than competing interface designs.
MCA is now a lost technology. As a computer technician, you will not encounter MCA on new computers. However, it is still found in some older machines, and you will need to know how to identify it. If (by some rare chance) a customer brings in a PS/2 machine for service, be sure to obtain the configuration disks for the computer as well as any MCA cards that go with it.
Extended ISA
In 1988 an industry group answered the challenge of MCA and released a new 32-bit, 8-MHz open standard called Extended ISA (EISA—pronounced ee-suh).EISA is an improved variation of the ISA slot that accepts older ISA cards, with a two-step design that uses a shallow set of pins to attach to ISA cards and a deeper connection for attaching to EISA cards. In other words, ISA cards slip partway down into the socket; EISA cards seat farther down.
CAUTION
Be very careful to line up cards being placed in an EISA slot precisely and push straight down! If you try to angle the card in, it can be very difficult to seat and you might damage either the connector or the slot.
Although EISA is faster and cheaper than MCA, it never gained much more acceptance than MCA.Confusion between MCA and EISA technology—along with a limited need for cards that ran at the faster rate and the fact that only a few display, drive controller, and network cards were made available—led to the early demise of both bus technologies. Figures 8.3 and 8.4 show how the slot design of the two technologies differs.
VESA Local Bus
The problems posed by MCA and EISA designs meant that developers needed an improved bus architecture to speed up graphics adapter performance and keep up with the evolving operating system technologies, such as Microsoft Windows. The Windows graphical user interface (GUI) required a much faster display adapter, because every pixel (not just lines of character data) had to be represented and refreshed. About the same time, laser printers and graphics programs like PageMaker and CorelDRAW entered the mainstream market, extending the desktop publishing revolution. The hardware industry developed the VESA local bus (VLB) to meet the need for a faster expansion interface. (VESA, the Video Electronics Standards Association, was the driving force behind the new bus technology.) Found only in 386 and 486 machines, the VLB had a short life span. The cards based on this design are connected directly to the system-bus side of the PC's external data bus (see Figure 8.5).Figure 8.5 VESA local bus design
The speed of the system data bus is based on the clock rate of the motherboard's crystal. During the heyday of the VLB, this was usually 33 MHz, and VLB cards usually ran at half that rate, far outpacing the ISA bus. Some cards ran as fast as 50 MHz, using the full speed of the souped-up system bus. That often caused system crashes, because 50 MHz was outside the VLB specification.
The chip design for the VLB controller was relatively simple because many of the core instructions were hosted by the ISA circuits already on the motherboard, but the actual data passes were on the same local bus as the one used by the CPU.
The design specification provides two other performance-boosting features: burst mode and bus mastering. In burst mode, VLB devices gain complete control of the external data bus for up to four bus cycles, passing up to 16 bytes (128 bits) of data in a single burst. Bus mastering allows the VLB controller to arbitrate data transfers between the external data bus and up to three VLB devices without assistance from the CPU. This limit of three devices also limited the maximum number of VLB slots to three and called for the use of a coprocessor. Display-system design is covered in more detail in Chapter 11, "The Display System."
The actual connectors on the motherboard resemble an ISA slot with an additional short slot aligned with it. On systems that support this interface, one to three slots are located on the side of the motherboard closest to the keyboard connection.
Peripheral Component Interconnect
PCI allows developers to design cards that will work in any PCI-compatible machine. It overcomes the limitations of ISA, EISA, MCA, and VLB, and it offers the performance needed for today's fast systems.At first glance, there are many similarities between PCI and the older VLB specifications. Both are local bus systems with 32-bit data paths and burst modes. Also, the original PCI design operates at 33 MHz—roughly the same speed as the VLB. However, the important differences between them allowed PCI to dominate in expansion bus technology. These differences stem from the following features:
- The PCI design's special bus and chip set are designed for advanced bus-mastering techniques and full arbitration of the PCI local bus. This allows support of more than three slots.
- The PCI bus has its own set of four interrupts, which are mapped to regular IRQs on the system. If a PC has more than four PCI slots, some will be sharing interrupts and IRQs.
NOTE
Under Windows 95 or with poorly designed PCI cards (both are becoming rarities), the shared addresses could lead to system conflicts and resource problems. Installing PCI cards one at a time minimizes these problems. Also, be aware that on many systems not all PCI slots offer full bus mastering. Check the owner's manual for details, especially on machines with more than four slots. In general, the PCI slots closest to the keyboard connector are the best choices for full bus mastering.
- The PCI bus allows multiple bus-mastering devices. Advanced controllers such as SCSI (Small Computer System Interface) cards can incorporate their own internal bus mastering and directly control attached devices, then arbitrate with the PCI bus for data transfers across the system bus.
- Autoconfiguration lets the PC's BIOS assign the IRQ linking the card to the system bus. Most PCI cards have no switches or jumpers to set, speeding installation and preventing many hardware conflicts.
Variations on a Theme: Differences in PCI Versions
The earlier discussion makes PCI sound like a technician's dream interface: fast, reliable, and doing most of the work itself. In most cases, that's true; still, there is always a "but." PCI has gone through many changes, and there are some features to be aware of when you work with a PCI card:- The early PCI motherboards often have jumpers and BIOS settings that must be set to enable proper PCI operation. These are most often found on Pentium 60-MHz and 66-MHz machines.
- The PCI bus speed is not fixed. Newer chip sets can drive it—and the cards on it—at 66 MHz. At full performance, the PCI bus can deliver data transfers at up to 132 MB per second.
- PCI is not used only by PCs. Macintosh and some other non-PC-style computers incorporate PCI. Manufacturers appreciate this feature because it allows them to design core technology and port it to different models using the same production line. You still need to be certain, however, that a particular card is actually designed for the machine you are working on, even if it fits.
Accelerated Graphics Port
In the early days of PCI, the major market for that technology was the high-performance display adapter. The popularity of PCI led to its dominance of the expansion bus market. Today, the PCI market includes NICs, sound cards, SCSI adapters, Ultra Direct Memory Access (UDMA) controllers, and DVD (digital video disc) interfaces. The variety of devices posed a problem for display-card designers: Having more cards on a single bus slowed down the performance, just when the increasing popularity of 24-bit graphics and 3D rendering called for greater demands on the display system. The search was on for yet another interface; this time, the solution was a single slot—tuned for the display adapter. Once again, Intel led the way and developed the Accelerated Graphics Port (AGP).The AGP removes all the display data traffic from the PCI bus and gives that traffic its own 525-MB-per-second pipe into the system's chip set and, from there, straight to the CPU. It also provides a direct path to the system memory for handling graphics. This procedure is referred to as Direct Memory Execute (DIME). The AGP data path is shown in Figure 8.6.
The AGP slot, if present, is the only one of its kind on the motherboard and is usually the slot closest to the keyboard connector (see Figure 8.7). It is set farther from the back of the PC's case than the PCI slots. AGP connectors are found only on Pentium II-based and later computers or on similar CPUs from non-Intel vendors.
IEEE 1394 FireWire High-Performance Serial Interface
One contender that has been touted as a possible replacement for SCSI (see Chapter 9, "Basic Disk Drives," for details on the SCSI interface) in connecting external peripherals is IEEE 1394, known also by its Apple trade name of FireWire. We'll use the short form and call it 1394. This high-speed serial interface allows up to 62 devices on a chain, at data transfer rates of up to 50 MB per second.This new interface offers several advantages: a hot swap capability (the ability to add and remove components while the machine is running), small and inexpensive connectors, and a simple cable design. Right now, few devices support 1394, but it is seen as a viable method for connecting multimedia devices like camcorders and other consumer electronic devices to PCs. Its isochronous transfer method (sending data at a constant rate) makes it a natural for video products. Currently, many 1394 PC products are expensive and there is no provision for connecting internal devices. Both 1394 and SCSI will coexist much like SCSI and universal serial bus (USB) for the foreseeable future. (USB is discussed in the next section.)
Although there are some standards defining how connections are made with 1394, several vendors are offering custom ways of linking products.
Universal Serial Bus
The newest addition to the general PC bus collection, the USB connects external peripherals such as mouse devices, printers, modems, keyboards, joysticks, scanners, and digital cameras to the computer. The USB port is a thin slot; most new motherboards offer two, located near the keyboard. They can also be provided through an expansion card.USB supports isochronous (time-dependent) and asynchronous (intermittent) data transfers. Isochronous connections transfer data at a guaranteed fixed rate of delivery. This is required for more demanding multimedia applications and devices. Asynchronous data can be transferred whenever there is no isochronous traffic on the bus. USB supports the following data transfer rates, depending on the amount of bus bandwidth a peripheral device requires:
- 1.5 megabits per second (Mbps) asynchronous transfer rate for devices, such as a mouse or keyboard, that do not require a large amount of bandwidth.
- 12 Mbps isochronous transfer rate for high-bandwidth devices such as modems, speakers, scanners, and monitors. The guaranteed data-delivery rate provided by isochronous data transfer is required to support the demand of multimedia applications and devices.
Attaching a new USB device is usually little more complicated than hooking up the appropriate cables and loading a driver disk if requested (see Figure 8.8). Keep in mind that older products may not be adequately supported, and that some devices will require an external power supply.
Lesson Summary
The following points summarize the main elements of this lesson:- Expansion buses provide a way of connecting devices to the motherboard.
- ISA could accommodate both 8-bit and 16-bit expansion cards.
- MCA was a proprietary architecture for IBM's PS/2 computers.
- EISA 32-bit architecture could accommodate older ISA expansion cards.
- VLB architecture employed burst mode and bus mastering to boost performance.
- PCI architecture makes use of autoconfiguration to let the PC's BIOS assign the IRQ linking the card to the system bus.
- AGP architecture removes display data traffic from the PCI bus.
- USB architecture supports both isochronous (time-dependent) and asynchronous (intermittent) data transfers.
- IEEE 1394 is mostly used for multimedia applications on desktop computers.
- Expansion buses have changed to keep up with increases in processor speed.
- A computer technician must know how to identify the various expansion buses (ISA, MCA, EISA, PCI, AGP, and USB) to ensure compatibility and know how to maximize performance when upgrading a computer.
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