Extended Industry Standard Architecture

The original IBM PC included five 8-bit slots, running at the system clock speed of 4.77 MHz. The PC/AT, introduced in 1984, had three 8-bit slots and five 16-bit slots, all running at the system clock speed of 6 MHz in the earlier models and 8 MHz in the last version of the computer. The 16-bit slots were a superset of the 8-bit configuration, so most 8-bit cards were able to plug into a 16-bit slot (some cards used a "skirt" design that physically interfered with the extended portion of the slot) and continue to run in 8-bit mode. One of the key reasons for the success of the IBM PC (and the PC clones that followed it) was the active ecosystem of third-party expansion cards available for the machines. IBM was restricted from patenting the bus and widely published the bus specifications.

As the PC-clone industry continued to build momentum in the mid- to late-1980s, several problems with the bus began to be apparent. First, because the "AT slot" (as it was known at the time) was not managed by any central standards group, there was nothing to prevent a manufacturer from "pushing" the standard. One of the most common issues was that as PC clones became more common, PC manufacturers began increasing the processor speed to maintain a competitive advantage. Unfortunately, because the ISA bus was originally locked to the processor clock, this meant that some 286 machines had ISA buses that ran at 10, 12, or even 16 MHz. In fact, the first system to clock the ISA bus at 8 MHz was the turbo 8088 clones that clocked the processors at 8 MHz. This caused many issues with incompatibility, where a true IBM-compatible third-party card (designed for an 8 MHz or 4.77 MHz bus) might not work in a higher-speed system (or even worse, would work unreliably). Most PC makers eventually decoupled the slot clock from the system clock, but there was still no standards body to "police" the industry.

As companies like Dell modified the AT bus design,[4] the architecture was so well entrenched that no single clone manufacturer had the leverage to create a standardized alternative, and there was no compelling reason for them to cooperate on a new standard. Because of this, when the first 386-based system (the Compaq Deskpro 386) hit the market in 1986, it still supported 16-bit slots. Other 386 PCs followed suit, and the AT (later ISA) bus remained a part of most systems even into the late 1990s.

Meanwhile, IBM began to worry that it was losing control of the industry it had created. In 1987, IBM released the PS/2 line of computers, which included the MCA bus. MCA included numerous enhancements over the 16-bit AT bus, including bus mastering, burst mode, software-configurable resources, and 32-bit capabilities. However, in an effort to reassert its dominant role, IBM patented the bus and placed stringent licensing and royalty policies on its use. A few manufacturers did produce licensed MCA machines (most notably, NCR), but overall the industry balked at IBM's restrictions.

Steve Gibson proposed that clone makers adopt NuBus.[5] Instead, a group (the "Gang of Nine"), led by Compaq, created a new bus, which was named the Extended (or Enhanced) Industry Standard Architecture, or "EISA" (and the 16-bit bus became known as Industry Standard Architecture, or "ISA").[6] This provided virtually all of the technical advantages of MCA, while remaining compatible with existing 8-bit and 16-bit cards, and (most enticing to system and card makers) minimal licensing cost.

The EISA bus slot is a two-level staggered pin system, with the upper part of the slot corresponding to the standard ISA bus pin layout. The additional features of the EISA bus are implemented on the lower part of the slot connector, using thin traces inserted into the insulating gap of the upper / ISA card card edge connector. Additionally, the lower part of the bus has five keying notches, so an ISA card with unusually long traces cannot accidentally extend down into the lower part of the slot.

Intel introduced their first EISA chipset (and also their first chipset in the modern sense of the word) as the 82350 in September 1989.[7][8] Intel introduced a lower-cost variant as the 82350DT, announced in April 1991; it began shipping in June of that year.[9]

The first EISA computer announced was the HP Vectra 486 in October 1989.[10] The first EISA computers to hit the market were the Compaq Deskpro 486 and the SystemPro. The SystemPro, being one of the first PC-style systems designed as a network server, was built from the ground up to take full advantage of the EISA bus. It included such features as multiprocessing, hardware RAID, and bus-mastering network cards.

One of the benefits to come out of the EISA standard was a final codification of the standard to which ISA slots and cards should be held (in particular, clock speed was fixed at an industry standard of 8.33 MHz). Thus, even systems that didn't use the EISA bus gained the advantage of having the ISA standardized, which contributed to its longevity.

The Gang of Nine was the informal name given to the consortium of personal computer manufacturing companies that together created the EISA bus. Rival members generally acknowledged Compaq's leadership, with one stating in 1989 that within the Gang of Nine "when you have 10 people sit down before a table to write a letter to the president, someone has to write the letter. Compaq is sitting down at the typewriter".[6] The members were:[2]

Although the MCA bus had a slight performance advantage over EISA (bus speed of 10 MHz, compared to 8.33 MHz), EISA contained almost all of the technological benefits that MCA boasted, including bus mastering, burst mode, software-configurable resources, and 32-bit data/address buses. These brought EISA nearly to par with MCA from a performance standpoint, and EISA easily defeated MCA in industry support.

EISA replaced the tedious jumper configuration common with ISA cards with software-based configuration. Every EISA system shipped with an EISA configuration utility; this was usually a slightly customized version of the standard utilities written by the EISA chipset makers. The user would boot into this utility, either from floppy disk or on a dedicated hard-drive partition. The utility software would detect all EISA cards in the system and could configure any hardware resources (interrupts, memory ports, etc.) on any EISA card (each EISA card would include a disk with information that described the available options on the card) or on the EISA system motherboard. The user could also enter information about ISA cards in the system, allowing the utility to automatically reconfigure EISA cards to avoid resource conflicts.

Similarly, Windows 95, with its Plug-and-Play capability, was not able to change the configuration of EISA cards, but it could detect the cards, read their configuration, and reconfigure Plug-and-Play hardware to avoid resource conflicts. Windows 95 would also automatically attempt to install appropriate drivers for detected EISA cards.

EISA's success was far from guaranteed. Many manufacturers, including those in the "Gang of Nine", researched the possibility of using MCA. For example, Compaq actually produced prototype DeskPro systems using the bus. However, these were never put into production, and when it was clear that MCA had lost, Compaq allowed its MCA license to expire (the license actually cost relatively little; the primary costs associated with MCA, and at which the industry revolted, were royalties to be paid per system shipped).

On the other hand, when it became clear to IBM that Micro Channel was dying, IBM licensed EISA for use in a few server systems.

• The Intel EISA chipset explained
• EISA bus technical summary
• Intel EISA Controllers
• Intel 82350 EISA chip family