ISA bus

When it appeared on the first PC the 8-bit ISA bus ran at a modest 4.77MHz - the same speed as the processor. It was improved over the years, eventually becoming the Industry Standard Architecture (ISA) bus in 1982 with the advent of the IBM PC/AT using the Intel 80286 processor and 16-bit data bus. At this stage it kept up with the speed of the system bus, first at 6MHz and later at 8MHz.

The ISA bus specifies a 16-bit connection driven by an 8MHz clock, which seems primitive compared with the speed of today's processors. It has a theoretical data transfer rate of up to 16 MBps. Functionally, this rate would reduce by a half to 8 MBps since one bus cycle is required for addressing and a further bus cycle for the 16-bits of data. In the real world it is capable of more like 5 MBps - still sufficient for many peripherals - and the huge number of ISA expansion cards ensured its continued presence into the late 1990s.

As processors became faster and gained wider data paths, the basic ISA design wasn't able to change to keep pace. As recently as the late 1990s most ISA cards remained as 8-bit technology. The few types with 16-bit data paths - hard disk controllers, graphics adapters and some network adapters - are constrained by the low throughput levels of the ISA bus, and these processes can be better handled by expansion cards in faster bus slots. ISA's death-knell was sounded in the PC99 System Design Guide, co-written by the omnipotent Intel and Microsoft. This categorically required the removal of ISA slots, making its survival into the next millennium highly unlikely.

Indeed, there are areas where a higher transfer rate than ISA could support was essential. High resolution graphic displays need massive amounts of data, particularly to display animation or full-motion video. Modern hard disks and network interfaces are certainly capable of higher rates.

The first attempt to establish a new standard was the Micro Channel Architecture (MCA), introduced by IBM. This was closely followed by Extended ISA (EISA), developed by a consortium made up of IBM's major competitors. Although these systems both operate at clock rates of 10MHz and 8MHz respectively, they are both 32-bit and capable of transfer rates well over 20 MBps. As its name suggests, an EISA slot can also take a conventional ISA card. However, MCA is not compatible with ISA at all.

Neither system flourished, largely because they were too expensive to merit support on all but the most powerful file servers.