Expansion buses are provided to connect devices to the motherboard using the motherboard"s data bus. They allow the flow of data in between that an equipment and other devices in and also connected to the computer. Early computer systems moved data between devices and the processor at about the same rate as the processor. Together processor speeds increased, the motion of data v the bus came to be a bottleneck. Therefore, the design ability of the buses necessary to evolve, too. This class discusses the evolution.

You are watching: Which of the following expansion buses

After this lesson, you will certainly be maybe to: determine the different varieties of growth buses in a computer.

Identify the difference in between the mechanism bus and the growth bus.

Estimated great time: 30 minutes advance of the growth Bus

As discussed earlier in chapter 4, "The main Processing Unit," every machine in the computer—RAM, the keyboard, network card (NIC), sound card, and also so forth—is connected to the external data bus. Development slots ~ above the motherboard room standardized relationships that permit the environment of tools not soldered to the motherboard. The duty of an development slot is to provide configuration flexibility when gadgets are included to a computer.

Whether a maker is soldered come the motherboard or associated through an growth slot, all incorporated circuits room regulated by a quartz crystal. The crystal sets the timing because that the system, offering all parts access to a usual reference suggest for performing actions. Most CPUs divide the crystal rate by two. (If the CPU has actually a 33-MHz speed, a 66-MHz decision is required.) Every maker soldered to the motherboard—keyboard chip, memory controller chip, and also so on—is draft to operation at the rate (or at half the speed) the the device crystal.

CPU speeds boosted as modern technology improved, if the speed of development cards remained fairly constant. It to be not practical to redesign and also replace every development card each time a new processor to be released—this would have actually been complex and expensive for manufacturers. (And, the course, the additional expense would have actually been passed follow me to the consumer.) The commitment of the sector to preserve backward compatibility further complicated design tasks, since any new technology would need to run the older, slow devices.

To resolve this dilemma, designers have divided the external data bus right into two parts:

System bus: This supports the CPU, RAM, and also other motherboard components. The device bus runs at speeds that assistance the CPU.

Expansion bus: This supports any type of add-on devices by way of the development slots and also runs at a secure rate, based upon the certain bus design.

Dividing the bus enhances all at once system efficiency. Because the CPU runs off the system clock, upgrading a CPU requires an altering only the time of the device bus, when the existing growth cards proceed to run as before. Over there is usually a jumper setting that changes the mechanism clock rate to complement the CPU. The capability of the motherboard to do this readjust sets the border for the processor speed. Next, we take a look at the evolving types of development buses.

market Standard design (ISA)

The first-generation IBM XT (with the 8088 processor) had actually an 8-bit exterior data bus and also ran in ~ a rate of 4.77 MHz. These machines were sold with one 8-bit expansion bus (PC bus) the ran at 8.33 MHz (see figure 10.1).


Figure 10.1 8-bit pc bus slot

IBM took steps that sustained the rapid development of the personal-computer market. Their designers framed the PC"s architecture as an open system, utilizing standard, off-the-shelf components. That permitted third-party developers to manufacture cards that might snap right into the pc bus. IBM also permitted its competitors to copy the computer bus.

With this move, IBM created the industry Standard style (ISA) interface, thus generating the sector for clones. A hold of third-party developers operated to create commodities that enhanced the simple machine"s features and also to save prices much reduced for add-ons than competing proprietary systems such as those native Apple. Without this push, the PC sector would have grown more slowly and also probably would have been limited to businesses through the money come pay because that the much more expensive products.

IBM wanted to include a brand-new expansion bus—one that would be compatible with previously released devices—with the relax of its in ~ (Advanced Technology) PC, special Intel"s 80286 16-bit processor. To attain this, the designers added a bus that permitted insertion of either an 8-bit map or a 16-bit card. This readjust resulted in the typical 16-bit ISA slot. This brand-new 16-bit bus officially ran at a height speed that 8.33 MHz, however on part Peripheral component Interconnect (PCI)-based systems the actual rate for ISA slots verified to be together high as about 10 MHz. (PCI is questioned later in this lesson.)

NOTE The term "ISA" go not become official until 1990. Therefore, the 8-bit slot is dubbed the XT, and also the 16-bit slot is called the AT. As soon as we refer to an ISA slot or one ISA card, we normally mean the 16-bit AT-style interface. The rate of the slots remained at about 7 MHz.

troubles with the ISA style

The ISA style is among the many enduring elements of the PC. It can be discovered on practically all systems, indigenous the second-generation IBM computer to machines developed today. But it suffers from two major shortcomings: lack of speed and compatibility difficulties stemming from map design.

As CPU power increased and also applications became much more powerful, card designers sought an interface that would permit add-on cards to store up with the need for improved difficult drives, display adapters, and comparable products.

Expansion cards must make use of system resources in one orderly way, so that they do not conflict with other devices. When demands because that these system resources space not coordinated, the system can behave erratically or even fail to boot up. Formerly, ISA cards frequently used a bewildering range of jumpers and switches to collection addresses because that memory use or the IRQ places they would certainly use.

The need to overcome the growth card"s slowness and compatibility problems led to a find for a new, standard growth card interface—one that everyone might agree on and that would acquire user acceptance.

Micro Channel design (MCA)

In 1986, the market happened dominated through the brand-new 386 equipments with your 32-bit architecture. Most PC manufacturers stuck to the same an easy ISA design and also MS-DOS. Growth devices based on ISA modern technology for the 286 AT course machines could be placed in a brand-new 386 clone without problems.

IBM, however, was feeling the pinch of competition native cheaper clones, and sought to maintain its prominence in the pc market. IBM designers developed a new version the the PC, the PS/2 (Personal System/2) and also created a proprietary expansion bus dubbed Micro Channel design (MCA) as component of the design. To run at 10 MHz, it offered more performance and detailed a 32-bit data path. That was additionally totally incompatible through older ISA cards.

A feature of MCA is its ability to "self-configure" devices. Unlike tools that use modern technology in i m sorry the pc configures itself automatically to work-related with peripherals such as monitors, modems, and printers, an MCA device always comes v a construction disk. As soon as installing a new an equipment in an MCA computer, insert the construction disk (when prompted), and the IRQs, I/O addresses, and DMA channels will it is in configured automatically. (IRQs, I/O addresses, and DMA networks are debated in information in the following lesson.) an MCA bus is presented in number 10.2.

The PS/2 never gained enough market share to complete with the 386. MCA cards were few and much between, and much more expensive than completing interface designs.


Figure 10.2 MCA bus

MCA is now a lost technology. As a computer technician, you will certainly not conference MCA on new computers. However, the is still uncovered in part older machines, and you will must know just how to identify it. If a customer brings in a PS/2 an equipment for service, be sure to acquire the construction disks for the computer and also any MCA cards the go through it.

enhanced ISA (EISA)

In 1988, an industry team answered the difficulty of MCA and also released a new open standard called amplified ISA (EISA—pronounced "ee-suh"). It"s a 32-bit, 8-MHz standard.

Unlike MCA, EISA supplies a variation of the ISA slot the accepts enlarge ISA cards, with a two-step style that provides a shallow collection of pins to affix to ISA cards and also a deeper link for attaching to EISA cards. In other words, ISA cards on slide part-way down right into the socket; EISA cards chair farther down.

CAUTION Be an extremely careful to line up cards being put in one EISA slot precisely and also push directly down! If you shot to edge the card in, it have the right to be very difficult to seat and you could damage one of two people the connector or the slot.

Although EISA is faster and cheaper than MCA, that never gained much more acceptance than MCA.

Confusion in between MCA and EISA technology—along through a restricted need because that cards that ran at the faster rate and the truth that just a couple of display, drive controller, and also network cards were made available—led come the early demise of both bus technologies. Numbers 10.3 and also 10.4 show how the slot design of the two technologies differs.


Figure 10.3 peak view that ISA and also EISA bus


Figure 10.4 Cross ar of ISA and EISA bus

VESA neighborhood Bus (VLB)

The troubles posed through MCA and also EISA designs supposed that developers required an improved bus style to rate up graphics adapter performance and also keep up with the evolving innovations as brand-new 32-bit operating equipment such together Microsoft windows 3.1 got popularity. The latter"s graphical user interface (GUI) compelled a much faster display screen adapter, since every pixel (not just lines of character data) had actually to it is in represented and refreshed. Around the same time, laser printers and graphics programs like PageMaker and also CorelDRAW promoted the "desktop posting revolution." The hardware industry arisen the VESA neighborhood bus (VLB) to meet the require for a faster development interface. (VESA, the video clip Electronics criter Association, to be the driving pressure behind the brand-new bus technology.) discovered only in 386 and also 486 machines, the VLB had a short lifespan. The cards based on this design are linked directly come the system-bus next of the PC"s exterior data bus (see figure 10.5).


Figure 10.5 VESA neighborhood bus style

The rate of the device data bus is based on the clock price of the motherboard"s crystal. During the heyday that the VLB, this was generally 33 MHz, and VLB cards commonly ran at half that rate, far outpacing the ISA bus. Part cards ran as quick as 50 MHz, utilizing the complete speed that the souped-up system bus. That often caused system crashes, since 50 MHz was outside the VLB specification.

The chip design for the VLB controller to be relativity simple, since many the the main point instructions were organized by the ISA circuits already on the motherboard, yet the yes, really data passes were on the same regional bus as the one used by the CPU.

The style specification provides two other performance-boosting features: explode mode and bus mastering. In explode mode, VLB devices gain finish control of the external data bus for up to four bus cycles, passing as much as 16 bytes (128 bits) that data in a solitary burst. Bus mastering allows the VLB controller come arbitrate data transfers in between the outside data bus and also up to three VLB gadgets without aid from the CPU. This border of three devices also limited the maximum variety of VLB slots to three and called for the usage of a coprocessor. Display-system design is covered in more detail in thing 11, "The display System: Monitors and Adapters."

The yes, really connectors top top the motherboard resemble an ISA slot with an additional short slot aligned through it. On equipment that support this interface, one to 3 slots are located on the next of the motherboard closest to the keyboard connection.

Peripheral ingredient Interconnect (PCI)

Peripheral ingredient Interconnect (PCI) allows developers to architecture cards the will job-related in any type of PCI-compatible machine. That overcomes the constraints of ISA, EISA, MCA, and VLB, and it offers the performance essential for today"s fast systems.

At very first glance, there are many similarities in between PCI and the enlarge VLB specifications. Both are neighborhood bus equipment with 32-bit data paths and burst modes. Also, the original PCI design operates in ~ 33 MHz—roughly the exact same speed together the VLB. But the essential differences in between them obtained PCI that dominant role in expansion-bus technology. These distinctions stem from the following features:

The PCI design"s unique bus and chip collection are design for advanced bus mastering techniques and also full arbitration the the PCI neighborhood bus. This permits support of much more than three slots.

The PCI bus has its own collection of 4 interrupts, which room mapped to constant IRQs on the system. If a pc has more than four PCI slots, some will certainly be share interrupts and IRQs.

NOTE In home windows 95 or v poorly draft PCI cards (both are becoming rarities), the shared addresses can lead to system conflicts and source problems. Download PCI cards one at a time to minimize problems. Also, be aware that on numerous systems not all PCI slots offer complete bus mastering. Inspect the owner"s hands-on for details, especially on devices with an ext than four slots. In general, the PCI slot closest come the key-board connector room the finest choices for complete bus mastering.

The PCI bus allows multiple bus-mastering devices. Advanced controllers such as SCSI (Small computer System Interface) cards have the right to incorporate your own internal bus mastering and also directly manage attached devices, climate arbitrate v the PCI bus because that data transfers throughout the system bus.

Autoconfiguration lets the PC"s BIOS entrust the IRQ linking the map to the mechanism bus. Many PCI cards have no switches or jumpers to set, speeding installation and preventing plenty of hardware conflicts.

Most computers on the sector today have one or more ISA slot for backward compatibility; however, most growth cards space now developed using the PCI interface. Back Intel was the initial driving force behind PCI development, a PCI requirements committee maintains the specification, and also it is an open design—anyone can style hardware utilizing PCI there is no being required to salary royalties.

variations on a Theme: differences in PCI execution

The earlier discussion makes PCI sound choose a technician"s dream interface: fast, reliable, and also doing many of the job-related itself. In most cases, that"s true; still, over there is always a "but." PCI has actually gone through numerous changes, and there room some features to be aware of once you job-related with one:

The beforehand PCI motherboards often have jumpers and also BIOS setups that should be set to permit proper PCI operation. These are most often discovered on Pentium 60-MHz and also 66-MHz machines.

The PCI bus rate is not fixed. More recent chip sets deserve to drive it—and the cards top top it—at 66 MHz. At complete performance, the PCI bus can deliver data move at up to 132 MB per second.

PCI is not offered only through PCs. Macintosh and also some various other non-PC-style computers incorporate PCI. Manufacturers evaluate this feature because it allows them to style core technology and harbor it to different models, with little effort and using the same production line. Return that"s good, you should be sure that a map is in reality designed for the an equipment you room working on, even if it fits.

Keep in mind the PCI is evolving. That fact will assist to keep it a viable interface for the foreseeable future, yet it might likewise lead to incompatibilities between brand-new cards and older machines.

accelerated Graphics harbor (AGP)

In the early on days the PCI, the major market because that that modern technology was the high-performance screen adapter. The popularity of PCI resulted in its dominance of the expansion-bus sector for map manufacturers. Today, the PCI market includes network cards, sound cards, SCSI adapters, UDMA controllers, and DVD interfaces. That posed a trouble for display-card designers: having much more cards on a solitary bus slowed under the performance, just when the increasing popularity that 24-bit graphics and 3D rendering referred to as for higher demands on the display system. The search was on for yet another interface; this time, the systems was a solitary slot—tuned for the screen adapter. Once again, Intel led the method and arisen the increased Graphics harbor (AGP).

The AGP removes all the display data website traffic from the PCI bus and gives that website traffic its own 525-MB per second pipe right into the system"s chip collection and, indigenous there, right to the CPU. It also provides a direct path come the mechanism memory for dealing with graphics. This procedure is described as direct Memory Execute (DIME). The AGP data course is shown in figure 10.6.


Figure 10.6 AGP straight Memory Execute supplies priority accessibility to screen data

The AGP slot, if present, is the only among its kind on the motherboard and is usually the slot closest come the key-board connector (see number 10.7). The is set forward of the earlier PC"s instance than the PCI slots. APG connectors are discovered only on Pentium II-based and later computers or on similar CPUs from non-Intel vendors.


Figure 10.7 one AGP slot top top the motherboard

universal Serial Bus (USB)

The newest enhancement to the pc bus collection, the universal serial bus (USB) connects outside peripherals such as computer mouse devices, keyboards, joysticks, scanners, and digital cameras come the computer. The USB harbor is a slim slot; most new motherboards market two, located near the keyboard. Lock can additionally be noted through an growth card.

USB support isochronous (time-dependent) and also asynchronous (intermittent) data transfers. Isochronous relations transfer data in ~ a guaranteed fixed price of delivery. This is compelled for an ext demanding multimedia applications and devices. Asynchronous data have the right to be moved whenever there is no isochronous website traffic on the bus. USB support the adhering to data transport rates, depending on the amount of bus bandwidth a peripheral device requires:

1.5 megabits per 2nd (Mbps) because that devices, such as a mouse or keyboard, that carry out not call for a huge amount of bandwidth.

12 Mbps isochronous transfer rate for high-bandwidth tools such as modems, speakers, scanners, and also monitors. The guarantee data-delivery rate listed by isochronous data transfer is forced to support the need of multimedia applications and also devices.

USB tools can it is in attached v the computer system running. A new maker will usually be well-known by the operating system, and the user will certainly be prompted for drivers, if chauffeurs are required. Be affected by each other in mind the USB is a new standard, and also some beforehand USB ports and chip sets perform not correctly support some newer devices. Difficulties with installed USB ports space not generally worth repairing. It is usually better to download a brand-new USB user interface card.

Lesson an overview

The following points summary the main elements of this lesson:

Expansion buses administer a way of connecting devices to the motherboard.

ISA design could accommodate both 8-bit and also 16-bit growth cards.

MCA was a proprietary architecture for IBM"s PS/2 computers.

EISA 32-bit design could accommodate enlarge ISA expansion cards.

VLB employed explode mode and bus mastering to rise performance.

PCI style makes use of autoconfiguration to let the PC"s BIOS entrust the IRQ linking the map to the device bus.

AGP design removes screen data traffic from the PCI bus.

USB architecture supports both isochronous (time-dependent) and also asynchronous (intermittent) data transfers.

Expansion buses have readjusted to store up with increases in processor speed.

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A computer technician must know just how to identify the various development buses (ISA, MCA, EISA, PCI, AGP, and USB) come ensure compatibility and know just how to maximize performance once upgrading a computer.