Another vintage computer enthusiast was showing me some interesting items when the topic of expansion slot buses came up. If you think PCI is confusing, wait until you hear about the things that came before PCI. Let’s dive into the slow and confusing world of ISA vs EISA vs VLB.
The PC bus, aka ISA
When IBM was developing the original IBM PC, aka the IBM 5150, they knew they wanted it to have expansion slots. But rather than use an existing design, they came up with their own connector and bus. They simply called it the PC bus. And they documented it extensively, because they wanted third parties to make things that plugged into it. They saw the ecosystem that built up around the Apple II and knew they needed something similar.
The IBM 5150 used an Intel 8088 CPU, which had an 8-bit data bus. The original PC bus, therefore, was also 8 bits wide. And it ran at the same 4.77 megahertz clock speed as the 8088.
When IBM released the 5170, which had a 16-bit Intel 80286 CPU running at 6 MHz, they added a second connector to the bus to extend it to 16 bits, ran it at the higher rate, and called it the AT Bus. When IBM sped up the 5170 to 8 MHz, the bus speed followed.
The clock speed problem
The IBM PC line offered CPUs running at one of three clock speeds, all of them eight megahertz or slower. Intel sold faster versions of most of these chips. Part of the plot line of the television series Halt and Catch Fire, which is a dramatization of the early PC industry, talks about trying to use these faster chips in a computer that would be compatible with IBM, but faster. This caused some problems. Some expansion boards were happy to run at higher speeds, but others malfunctioned if you ran them much faster than 8 megahertz.
IBM mostly abandoned the PC bus when it released the PS/2 line of computers. They used it in the entry level Model 25 and Model 30, but used the faster and more capable and more expensive Microchannel bus on the more expensive models.
Ironically this resulted in solving the problem with overclocking the original PC bus. IBM didn’t charge any royalty to use the original PC bus. They did charge royalties for Microchannel. This created a crisis for the clone industry. A group of rival PC makers, informally called the Gang of Nine met to figure out what to do.
ISA: the new AT bus, same as the old AT bus
The Gang of Nine met to consider their options. The Gang of Nine was a consortium of nine PC makers from around the world: Compaq, HP, Seiko Epson, AST, NEC, Olivetti, Tandy, Wyse, and Zenith. Some of them are gone now but in 1988, they were just about the biggest non-IBM names out there. Their goal was to develop an open alternative to Microchannel, ideally one with better backward compatibility than Micro Channel. But before they could do that, they needed to define the standard they were trying to be compatible with.
So the first thing they did was redefine what IBM called the PC bus and the AT bus as something called ISA. This avoided infringing on the AT trademark. “AT” was a questionable trademark, but IBM was in a litigious mood and the Gang of Nine didn’t want to poke the bear. ISA was an acronym that stood for industry standard architecture. It was a brilliant move, because it allowed them to paint IBM as trying to be expensive and proprietary and predatory. In the computer industry, the word proprietary is almost guaranteed to be a kiss of death.
While IBM didn’t define the speed of its PC bus, the Gang of Nine standardized ISA at a speed of 8.33 megahertz. You could run the computer CPU at a higher rate, but it communicated with the bus at 8.33 megahertz for compatibility. This introduced a bottleneck, but it brought stability to higher speed PCs.
The ISA slot is pretty familiar to most vintage computer enthusiasts. It consists of the initial 62 pin 8-bit connector, plus an optional additional 36 pin connector to extend it to 16 bits, for a total of 98 pins. The connector was made of black plastic. IBM had used black, and keeping the color consistent made it easy to tell what kind of connectors a motherboard had.
ISA was showing its age even in 1989 when it was defined and named. But momentum is a powerful thing, and it had a lot of momentum behind it.
If you’re wondering, ISA is generally pronounced EYE-sa.
EISA: extending the standard
The new, open EISA standard extended ISA to 32 bits, to accommodate the new 32-bit processors like the Intel 386DX and 486. Like Micro Channel, it was 32 bits wide, and had the ability to configure expansion cards via software rather than using jumpers, which one can consider an early form of plug and play. EISA, in case you’re wondering, was pronounced EE-sa.
Unlike Micro Channel, EISA was backward compatible with its predecessor. ISA cards would work in EISA slots. This limited EISA speed to 8.33 megahertz, so it wasn’t as fast as Micro Channel, but the Gang of Nine was betting that backward compatibility would win out over a 20% speed increase.
They were right, sort of. Arguably, EISA made it possible to win the war, but it wasn’t EISA itself that won. ISA was the real winner. 20 years after the release of the original IBM PC, most new computers still had at least a couple of ISA expansion slots.
EISA had the advantage of being 32 bits wide, so it was theoretically twice as fast as ISA. The processor still had to slow down to 8 megahertz to talk to it, but at least it didn’t have to act like a 286, or worse yet, an 8088, when doing it.
EISA slots look a lot like ISA slots. They are deeper than ISA, to accommodate a second row of pins below the ISA pins. EISA has 198 pins, in a staggered pattern. The slots are brown plastic to distinguish from ISA. Due to the cost, EISA was mostly used in server and workstation computers. You didn’t see it a lot in common desktop PCs.
In the end, ISA vs EISA vs VLB was about solving bottlenecks, and EISA was a step in the right direction.
VLB: extending ISA again
In the early 1990s, the industry was moving fast. Within a couple of years, even EISA was showing its limits. At the time EISA was defined, 25 megahertz was a fast computer. But within a couple of years, processor speeds increased to 33 and 40 megahertz, and Intel even tried a 50 megahertz 486.
Plus, the things attaching to the bus sped up. 8 megahertz was enough for an ST506 hard drive. It had a harder time keeping up with newer IDE or SCSI drives. Arguably, 8 megahertz was enough for VGA video, but Super VGA lagged at that speed.
So the Video Electronics Standards Association (VESA) stepped in with a standard for a faster bus they called VLB, for VESA Local Bus. They had already defined standard video modes for Super VGA, and we still use those today, but in the 486 era, the VLB was arguably VESA’s bigger claim to fame.
VLB added a third connector to the end of the ISA slot. Like EISA, it was a 32-bit bus, but unlike earlier standards, it ran at higher speeds. If your processor had a 33 megahertz bus, you could have three VLB slots running at 33 megahertz each. If your processor had a 40 megahertz bus, you could have two VLB slots. With a 50 megahertz bus, you were limited to one VLB slot.
VLB had its limits, but practically speaking, most people were happy with a fast disk controller and a fast video card. The most popular 486 processors ran on a 33 MHz bus, which meant you had three VLB slots, but most people I knew only ever populated two of them. Everyone talked about putting a SCSI card in that third slot, but far more people talked about doing that than actually did.
VLB did see extensive use in desktop PCs. For consumers, ISA vs EISA vs VLB meant you bought video cards and disk controllers in VLB form, skipped expensive EISA, and used ISA for less intensive uses like sound cards.
The end of ISA, EISA, and VLB
The Pentium era spelled the end of the line for EISA and VLB. Early Pentium systems had VLB slots, and I remember working on some Pentium servers that had EISA slots, but PCI supplanted both. It ran 33 megahertz, didn’t have the three slot limit, and was much more compact. VLB cards were a hassle to connect because of the three connectors on them. PCI was much easier.
ISA lived on a couple more generations, because certain peripherals were tricky to move to the new bus, especially sound cards. Plus, people had a tendency to recycle some of their old cards as they upgraded. There was more resistance to throwing out a 2-year-old sound card than a 10-year-old one. ISA slots became less common during the Pentium 4 era, and the 64-bit era pretty much relegated ISA to history. By then, even 33 megahertz PCI was a bottleneck. In fact, by then, PCI was a bigger bottleneck than ISA had been when PCI came out itself. That led to PCIe, but that’s another story, and not a retro story at that.
ISA vs EISA vs VLB is a thing of the past today, but it shows how an industry seized control of the PC standard from IBM.
David Farquhar is a computer security professional, entrepreneur, and author. He started his career as a part-time computer technician in 1994, worked his way up to system administrator by 1997, and has specialized in vulnerability management since 2013. He invests in real estate on the side and his hobbies include O gauge trains, baseball cards, and retro computers and video games. A University of Missouri graduate, he holds CISSP and Security+ certifications. He lives in St. Louis with his family.