What The System Bus Does, The ISA Bus, Microchannel Architecture (MCA) Bus, The EISA Bus

What The System Bus Does

Look on the bottom of the systemboard and you will see a maze of circuits that make up a

bus. These embedded wires are carrying four kinds of cargo:

Electrical power. Chips on the systemboard require power to function. These chips tap into the system bus’s power lines and draw what they need.

Control signals. Some of the wires on the system bus carry control signals that coordinate all the activity.

Memory addresses. Memory addresses are passed from one component to another as these components tell each other where to access data or instructions. The number of wires that make up the memory address lines of the bus determines how many bits can be used for a memory address. The number of wires thus limits the amount of memory the bus can address.

Data. Data is passed over a bus in a group of wires, just as the memory addresses are. The number of lines in the bus used to pass data determines how much data can be passed in parallel at one time. The number of lines depends on the type of CPU and determines the number of bits in the data path. (Remember that a data path is that part of the bus on which the data travels and can be 8, 16, 32, 64 or more bits wide.)

The ISA Bus

Used on the first IBM 8088 PCs in the early 1980s, the ISA bus had an 8-bit data path. Later, IBM revised the ISA bus to have a 16-bit path. The IBM AT personal computer used this bus and the 80286 chip, which is why the 16-bit bus is sometimes called the AT bus. IBM wanted this bus to be backward compatible with the older 8-bit ISA bus so that the older 8-bit circuit boards would fit into the newer AT computers.

To maintain compatibility, IBM kept the old 62-line slot connector and added another slot connector beside it to provide the extra 8 bits. Slots with both connectors are called 16-bit slots.

A new systemboard today usually has at least one 16-bit slot, which can be used by either an 8-bit or a 16-bit ISA card.

Microchannel Architecture (MCA) Bus

With the introduction of the line of PS/2 computers in 1987, IBM introduced the first 32-bit bus for personal computers, the Microchannel Architecture (MCA) bus. IBM did not intend the MCA bus to be compatible with ISA buses.

Circuit boards used in older IBM computers could not be used in the PS/2 line. (The PS/2 Models 25 and 30 still included the older ISA bus in order to support legacy cards.)

IBM chose to patent the bus so that other companies could not economically manufacture and market it. IBM intended to control a subset of the bus market with MCA.

In response, Compaq and eight other companies (called the “Gang of Nine”) joined to design and build a competing 32-bit bus, the EISA bus.

The EISA Bus

Designed to compete with the MCA bus, the EISA (Extended ISA) bus (pronounced “ease-sa”) has a 32-bit data path. The bus is compatible with older ISA buses so that expansion boards having 8-bit or 16-bit data paths work on the EISA bus. The speed of the EISA bus is about 20 MHz. To accommodate a 16-bit or 8-bit ISA circuit board, the 32-bit EISA has two slots that have the same width as 16- bit ISA slots.

However, the EISA bus slots are deeper than 16-bit slots. All 32-bit circuit boards have longer fingers on the edge connectors that go deep into the EISA slot connecting to the 32-bit pins. A 16-bit circuit board reaches only partway down the slot, connecting at a shallower level only to the 16-bit pins.