3 ms·

I have a good example - Piotr Grzesik's prototype of 486 SBC recently covered on Hackaday https://hackaday.com/2026/01/08/m8sbc-86-is-an-fpga-based-kinda-pc-com

by rasz 9mo ago

I have a good example - Piotr Grzesik's prototype of 486 SBC recently covered on Hackaday https://hackaday.com/2026/01/08/m8sbc-86-is-an-fpga-based-kinda-pc-compatible-486/ https://hackaday.com/2026/01/08/m8sbc-86-is-an-fpga-based-ki... and HN https://news.ycombinator.com/item?id=46578601 https://news.ycombinator.com/item?id=46578601 https://imgur.com/gallery/486-homebrew-computer-lsUiWdw#dIBtgXv https://imgur.com/gallery/486-homebrew-computer-lsUiWdw#dIBt... Looks like something that shouldnt work at all :)

jacquesm 9mo ago
Indeed, by rights that shouldn't work. But it does and he threw in an ISA bus just for the heck of it and that works too. And all of this at a very respectable clock speed. Mad props.
- varjag 9mo ago
  I mean Cray-1 backplane was wire-wrapped, it definitely should have worked for 486.
  - jacquesm 9mo ago
    Sure, it's easy ;) Try it. Anything with latches above 10 MHz is pretty hard to get to run stable when using wirewrap. The Cray-1 was at its time absolute state-of-the-art. Replicating something like a 486 motherboard, which already had a whole raft of timing tricks to make sure that it all stayed synchronized is really not that easy. I've spent more than one evening baffled by stuff that should have been trivial at clock speeds a lot lower than that. A 33 MHz clock has a cycle time of 1/33000000 = 33 nano seconds. But the rise time of that clock is much, much shorter, on the order of a few ns. At that sort of slew rate anything becomes an antenna. The backplane of the Cray-1 was set up as transmission lines with two spiral wound wires for each signal, cut exactly to size to make sure the signal arrived at the right moment, and without the bulk of the signal leaking away. On this circuit board things are - let's put it friendly - a bit less organized than that. So by rights this really shouldn't have worked, the fact that it does absolutely amazes and inspires me.

by rasz 9mo ago

jacquesm 9mo ago

Indeed, by rights that shouldn't work. But it does and he threw in an ISA bus just for the heck of it and that works too. And all of this at a very respectable clock speed. Mad props.

varjag 9mo ago
I mean Cray-1 backplane was wire-wrapped, it definitely should have worked for 486.
- jacquesm 9mo ago
  Sure, it's easy ;) Try it. Anything with latches above 10 MHz is pretty hard to get to run stable when using wirewrap. The Cray-1 was at its time absolute state-of-the-art. Replicating something like a 486 motherboard, which already had a whole raft of timing tricks to make sure that it all stayed synchronized is really not that easy. I've spent more than one evening baffled by stuff that should have been trivial at clock speeds a lot lower than that. A 33 MHz clock has a cycle time of 1/33000000 = 33 nano seconds. But the rise time of that clock is much, much shorter, on the order of a few ns. At that sort of slew rate anything becomes an antenna. The backplane of the Cray-1 was set up as transmission lines with two spiral wound wires for each signal, cut exactly to size to make sure the signal arrived at the right moment, and without the bulk of the signal leaking away. On this circuit board things are - let's put it friendly - a bit less organized than that. So by rights this really shouldn't have worked, the fact that it does absolutely amazes and inspires me.