Bus Connections

ISA

Short for "Industry Standard Architecture", the ISA made its debut with the original IBM PC computer. In its first incarnation, ISA was an 8-bit bus that ran on a 4.77Mhz clock. Later on ISA was expanded to 16-bits, and ran first at 6Mhz, then 8Mhz. Later the industry settled on a 8.33Mhz clock speed for the ISA bus. ISA bus is highly compatible, but slow. Running on an 8.33Mhz clock gave it a low 8MB/s (rounded) of bandwidth. It wasn't long before the bandwidth requirements of higher resolution and color depth video displays were outstripping the bandwidth provided by ISA. For all other devices being sold, the ISA was still more than fast enough to support them. The industry began to look for an improved system bus, driven by the need for higher quality video, and a bus fast enough to support it.

MCA

Short for "Micro Channel Architecture", MCA was a 32-bit system bus with plug and play bus master architecture. It provided for a high bandwidth allowance, which gave newer video systems the room they needed to do their work. IBM intended the MCA bus to replace the ISA, but also wanted to charge a royalty for the use of it. Before IBM would be willing to license the MCA bus, they also required that vendors pay a retroactive royalty for the use of the ISA bus as well. Manufacturers were unwilling to pay the royalty for the MCA bus, and nearly outraged over being told that they now had to pay for a technology that they had been freely (and legally) using for several years. As such, the MCA bus saw duty only in IBM computer systems, and never gained any acceptance in the industry. This was a turning point that signaled IBM's loss of dominance in the consumer computer industry. IBM persists to this day, and are still one of the biggest and strongest companies in the industry, offering a wide variety of both hardware and software products. They would no longer be a guiding force in the PC industry that they had created though.

EISA

Short for "Extended ISA", the EISA bus was announced at the end of 1988 as a competitor for IBM's new MCA bus. Primarily developed by Compaq, they basically gave the design for EISA away since Compaq knew that no one was going to use it if they had to pay for it. Compaq was one of the vendors that scoffed at paying both royalties for the MCA bus

The new EISA bus ran on the same 8.33Mhz clock as the original ISA, but expanded the bus from 16-bits to 32-bits, while maintaining backwards compatibility with all ISA devices. EISA also featured Automated device setup, and IRQ sharing (a sort of pseudo Plug and Play). All in all, EISA contained a great feature set, but was plagued with problems and never took off as a result of these technical difficulties.