The early days of home computing were quite a jungle of different standards and convoluted solutions to make one piece of hardware work on as many different platforms as possible. IBM’s PC was an unexpected shift here, as with its expansion card-based system (retroactively called the ISA bus) it inspired a new evolution in computers. Of course, by the early 1990s the ISA bus couldn’t keep up with hardware demands, and a successor was needed. Many expected this to be VESA’s VLB, but as [Ernie Smith] regales us in a recent article in Tedium, Intel came out of left field with its PCI standard after initially backing VLB.
IBM, of course, wanted to see its own proprietary MCA standard used, while VLB was an open standard. One big issue with VLB is that it isn’t a new bus as such, but rather an additional slot tacked onto the existing ISA bus, as it was then called. While the reasoning for PCI was sound, with it being a compact, 32-bit (also 64-bit) design with plug and play and more complex but also more powerful PCI controller, its announcement came right before VLB was supposed to be announced.
Although there was some worry that having both VLB and PCI in the market competing would be bad, ultimately few mainboards ended up supporting VLB, and VLB quietly vanished. Later on PCI was extended into the Accelerated Graphics Port (AGP) that enabled the GPU revolution of the late 90s and still coexists with its PCIe successor. We covered making your own ISA and PCI cards a while ago, which shows that although PCI is more complex than ISA, it’s still well within the reach of today’s hobbyist, unlike PCIe which ramps up the hardware requirements.
Top image: PC AT mainboard with both 16-bit ISA and 32-bit PCI slots. (Credit: htomari, Flickr)
You might have heard the term “bifurcation” if you’ve been around PCIe, especially in mining or PC tinkering communities. This is splitting a PCIe slot into multiple PCIe links, and as you can imagine, it’s quite tasty of a feature for hackers; you don’t need any extra hardware, really, all you need is to add a buffer for REFCLK. See, it’s still needed by every single extra port you get – but you can’t physically just pull the same clock diffpair to all the slots at once, since that will result in stubs and, consequently, signal reflections; a REFCLK buffer chip takes the clock from the host and produces a number of identical copies of the REFCLK signal that you then pull standalone. You might have seen x16 to four NVMe slot cards online – invariably, somewhere in the corner of the card, you can spot the REFCLK buffer chip. In a perfect scenario, this is all you need to get more PCIe out of your PCIe.
