ISA Bus

Industry Standard Architecture (ISA) is the bus architecture that was introduced as an 8-bit bus with the original IBM PC in 1981; it was 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 might seem amazing that such a presumably 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 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, and 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 8/16-bit versions of the ISA bus for backward-compatibility. Some systems have the capability 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 8MBps, as the following formula shows:

8.33MHz x 2 bytes (16 bits) ÷ 2 cycles per transfer = 8.33MBps

The bandwidth of the 8-bit bus would be half this figure (4.17MBps). 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 about 8MBps, the ISA bus is still faster than many of the peripherals connected to it, such as serial ports, parallel ports, floppy controllers, keyboard controllers, and so on.

8-Bit ISA Bus

This bus architecture is used in the original IBM PC computers and was retained for several years in later systems. 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 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 8 data lines and 20 addressing lines, enabling the slot to handle 1MB of memory. Figure 1 describes the pinouts for the 8-bit ISA bus; Figure 2 shows how these pins are oriented in the expansion slot.

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'' (106.68mm) high

13.13'' (333.5mm) long

0.5'' (12.7mm) wide

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 communications between the processor and motherboard as well as memory would now be 16 bits wide instead of only 8.

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.

Rather than create a new I/O bus, at that time IBM instead came up with a system that could support both 8- and 16-bit cards by retaining the same basic 8-bit connector layout but adding an optional 16-bit extension connector. This first debuted on the PC/AT in August 1984, which is why we also refer to the ISA bus as the AT-bus.

The extension connector in each 16-bit expansion slot adds 36 connector pins (for a total of 96 signals) 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 did not alter the function of 8-bit cards.

Figure 3 describes the pinouts for the full 16-bit ISA expansion slot, and Figure 4 shows how the additional pins are oriented in the expansion slot.

Because of physical interference with some ancient 8-bit card designs, IBM left 16-bit extension connectors off two of the slots in the AT. This was not a problem in newer systems, so any system with ISA slots would have all of them as full 16-bit versions.

The dimensions of a typical AT expansion board are as follows:

4.8'' (121.92mm) high

13.13'' (333.5mm) long

0.5'' (12.7mm) wide

Two heights actually are available for cards commonly used in AT systems: 4.8'' and 4.2'' (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'' maximum height.

Most board makers trimmed the height of their boards; most manufacturers who still make ISA cards now make only 4.2'' tall (or less) boards so 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. Fortunately, these proprietary buses were few and far between.

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.