The 8-bit expansion bus

The Industry Standard Architecture (ISA) bus

The Micro Channel Architecture (MCA) bus

The Extended ISA (EISA) bus

VESA Local Bus (VL-Bus)

Peripheral Component Interconnect (PCI)

Accelerated Graphics Port (AGP)

Personal Computer Memory Card International Association (PCMCIA)

What is a Bus?

The bus is a set of signal pathways that allow information and signals to travel between components inside or outside of a computer. There are three types of buses inside a computer :

•  The address bus

•  The data bus

•  The External bus

Address Bus

It allows the CPU to talk to a device. It will select the particular memory address that the device is using and use the address bus to write to that address.

Data Bus

It is used to send information back to the microprocessor.

External Bus (Expansion Bus)

It allows the CPU to talk to the other devices in the computer and vice versa. The Expansion bus allows the computer to be expanded using a modular approach. Whenever you need to add something to the computer, you plug specially made circuit boards into the connectors on the expansion bus. The devices on these circuit boards are then able to communicate with the CPU and are semi permanently part of the computer.

Bus Types

The 8-bit Expansion Bus

When the first PPC was developed with the Intel 8088, it only had eight data lines running from the processor to the expansion connectors. Each line carried 1 bit of data. Thus was born the 8-bit data. This was the most common name given to this particular bus.

The 8-bit bus is characterized by having a maximum bus clock speed of 4.77 (approximately 5) MHz, 8 interrupts (of which 6 could be used by expansion devices), 4 DMA channels, and 1 large connector with 62 tiny finger slots. It is rare to find as 8-biit slot. The expansion cards for this size bus are easily identifiable because they have only one connector.Also,when these cards came out, very Large Scale Integration(VLSI) had not taken off yet, so if you fond the 8-bit card, it usually be packed with resistors and other large electronics components.

 

Bus Configuration

Configuration your devices involves assigning system resources that aren't being used by the other devices. Configuration of the 8-bit bus is relatively complex, primarily because there is only one IRQ and one DMA channel (DMA 3) available. With so few system resources available , you have to decide which components will use each of the limited resources. One way to free up resources is to disable a device that you won't be using at the same time you'll be using the device you need to work with.

Each card must be separately configured to operate with the computer according to the instructions that come with the card. You set the configuration on each card using jumpers and Dual Inline Package (DIP) switches so that the settings are the way you want them. The first step in the procedure is to take the case off the computer. Next, you configure the card using the aforementioned jumpers. After that, you install the card in a free slot (assuming you have one). Finally, you boo the computer and install the software drivers to activate the card. If all was successful, the drivers will load without incident. If a conflict exists, you must repeat the entire procedure, changing one setting at a time until you get the correct setting, one that doesn't conflict with any other devices. A tedious process, to be sure.

The Industry Standard Architecture (ISA) Bus

The biggest shortcoming of the 8-bit bus was that it was only 8 bits wide. The design of the IBM AT processor specified a processor with a 16-bit data path. A new bus was needed for this processor because the old bus would cause the “traffic jam” scenario described earlier. The new bus design was a 16-bit bus: it was given the same name as the computer it was designed for: the AT bus. It was also known as the Industry Standard Architecture (ISA) bus.

The ISA bus has eight more interrupts than the 8-bit bus and four additional DMA channels. Also, this bus can operate at nearly twice the speed of the older, 8-bit bus (ISA can run at 8MHz, and Turbo models can run as fast as 10MHz reliably). Finally, this bus can use one bus-mastering device, if necessary.

ISA Bus IRQ Defaults

 

Even though IBM developed the original 8-bit bus and had a hand in developing ISA, through the early part of the 1980s it steadily lost its domination of the PC market. But IBM had an ace up its sleeve. It was developing a new line of computers called the PS/2 (Personal System 2). Along with the new computer, it was developing a new bus that was supposed to be better than ISA in every way. Its higher-end models (PS/2s numbered more or less sequentially from 50 to 80) were going to incorporate this new bus. This bus used a smaller, high-density connector and was known as Micro Channel Architecture (MCA).

MCA was a major step forward in bus design. First, it was available in either 16-bit or 32-bit versions. Second, it could have multiple bus-mastering devices installed. Third, the bus clock speed was slightly faster (10 MHz instead of 8 MHz). And finally, it offered the ability to change configurations with software rather than with jumpers and DIP switches.

The Extended ISA (EISA) Bus

Because MCA was rather expensive and ISA was slowing down their systems, a few companies (mainly Olivetti, Compaq, AST, Tandy, WYSE, Hewlett-Packard, Zenith, NEC, and Epson, a.k.a. the “gang of nine”) got together and came up with the Extended ISA, or EISA. This bus took the best parts of the other buses and combined them into a 32-bit software-configurable bus. The best part was that 16-bit ISA cards were compatible with this new bus and the standard was open.

There were several new, desirable features introduced with EISA. Its creators took the best of MCA's features and added to them. As we have already mentioned, EISA has a 32-bit data path. Additionally, it has more I/O addresses, it allows expansion cards to be set up using software, there is no need for interrupts or DMA channels, and it allows for multiple bus-mastering devices. However, despite all these advances, it still uses the 8MHz clock speed of ISA (to ensure backward compatibility with ISA cards).

VESA Local Bus (VL-Bus)

The EISA bus had several major advantages, but it had one glaring problem: It had a maximum clock speed of 8 MHz. As processors got faster and faster, the 8 MHz limit was a major obstacle. What was needed was a bus that would run at the same clock speed as the processor. This type of bus is known as a local bus.

There are several different types of local buses. One type is a bus slot, usually called something like a processor direct slot. It is used for adding higher-speed expansion cards, like memory or cache cards. And because it is a local bus slot, it runs at the processor's rated speed. It is usually a single connector, and it's highly proprietary. Usually, only cards made by the computer's manufacturer will work in this slot.

Some companies took this idea and thought that they could speed up some components in the computer by putting them onto the local bus. In addition to the memory and cache cards, one of the first components to be put on the local bus was the video circuitry, because it could benefit from direct communication with the processor. Some manufacturers designed a special local-bus video card slot and designed special, high-performance cards for these slots. This approach became very popular and most companies adopted it. The problem with this approach is that the local-bus video card from one vendor would not work in a local bus slot from another vendor.

The Video Electronics Standards Association (VESA) was formed for this reason. This group made sure that cards made sure that cards made for one vendor's slot would work in another vendor's computer. As time passed, the slot design changed and was given a new name, the VESA local Bus slot(and was also known variously as VL-Bus, VLB, or just VESA, after the group that came up with the standard). This slot was a 32-bit addition to the ISA bus and was therefore backward compatible with it.

VLB has one major drawback in that it really is just a bigger ISA bus; namely, it still has the same limitations as ISA. Configuration is still done through jumpers and DIP switches instead of through special bus-configuration programs. It's been called the “big ISA” bus because that's what it is: just a 32 –bit version of ISA.

Peripheral Component Interconnect (PCI)

With the introduction of the Pentium-generation processors, all existing buses instantly became obsolete. Because the Pentiums were 64-bit processors and most buses were of the 16-bit or 32-bit variety, using existing buses would severely limit the performance of the new technology. It was primarily for this reason that the peripheral Component Interconnect (PCI) bus was developed.

PCI has many benefits over other bus types. First, it supports both 64-bit and 32-bit data paths, so it can be used in both 482 and Pentium-based systems. In addition , it is processor independent. The bus communicates with a special “bridge circuit” that communicates with both the CPU and the bus. This has the benefit of making the bus an almost universal one. PCI buses can be found in PCs, Mac OS-based computers, and RISC computers. The same expansion card will work for all of them; you just need a different configuration program for each.

Another advantage to PCI over other buses is a higher clock speed. PCI in its current revision) can run up to 66 MHz. Also, the bus can support multiple bus-mastering expansion cards. These two features give PCI a maximum bus throughput of up to 265 Mbps (with 64-bit cards).

The final two features of PCI that we should discuss are its backward compatibility and software setup features. The PCI bus uses a chipset that works with PCI, ISA, and EISA. It is possible to have a PC that contains all these buses on the same motherboard. Also, the PCI cards are mostly Plug and Play. The cards will automatically configure themselves for IRQ, DMA, and I/O port addresses.

Bus Configuration for PCI

When you need to configure a PCI expansion card, you don't move jumpers or dip switches; you simply install the card. The computer's BIOS takes care of configuring IRQ, I/O, and DMA addresses. Then you install the appropriate software so that the computer can use the device.

Accelerated Graphics Port (AGP)

As Pentium systems got faster, PC game players got games that had better graphics, more realism, and more speed. However, as the computers got faster, the video technology just couldn't seem to keep up. VL-Bus could only run at 33 MHz, and with 100 and 200 MHz processors, there was a need for a faster, processor-direct, video expansion bus. The bus that was developed to meet this need was the Accelerated Graphics Port (AGP) bus.

Bus Configuration for AGP

Configuration of an AGP expansion card is simplicity itself. Motherboards that support AGP have Plug-and-Play BIOS's that will automatically configure the card. To add an AGP card, simply power down the system, install the card in the AGP slot, and power the system back up. Once the system comes back up, the bios will configure the card automatically. Finally, you can install the drivers for your operating system. This step is unnecessary if you have a Plug-and-Play operating system.

PCMCIA

The PCMCIA stands for Personal Computer Memory Card International Association. The bus was originally designed to provide a way of expanding the memory in a small, handheld computer,. The PCMCIA was organized to provide a standard way of expanding portable computers. The PCMCIA bus has been recently renamed PC card to make it easier to pronounce. The PC card bus uses a small expansion card (about the size of a credit card). Although it is primarily used in portable computers, there are PC card bus adapters for desktop PCs. It was designed to be a universal expansion bus that could accommodate any device.

The first release of the PCMCIA standard (PCMCIA 1, the same used in that original handheld computer) defined only the bus to be used for memory expansion. The second release (PCMCIA 2) is the most common; it is in use throughout the computer industry and has remained relatively unchanged. PCMCIA 2 was designed to be backward compatible with version 1, so memory cards can be used in the version 2 specification.

Bus configuration for PCMCIA

The process for installing a PC card is different than that for any of the other bus types, mainly because this type was designed to allow the cards to be “hot swapped” –inserted or removed while the computer is powered up . This is the only bus that allows this.

The process for installing a PC card is very straightforward. Just slip the card into an available slot, making sure the card type matches the slot type. Once the card is installed, you must install the software to use the card (Windows 9x will do this automatically).You must have the Card and Socket Services software installed before you try to physically install the card so that the computer can manage the card's resources.