unibus to modern disk interface?

* On Sun, Oct 14, 2012 at 07:47:45PM +0100, Rob Jarratt <robert.jarratt at ntlworld.com> wrote: It could be, yes, at the expense of a lot of board space. One of the biggest issues with the DEC busses nowadays is that they use open collector drivers. What that means is that the signal lines in the bus are normally held at something like 3.4V (a logical "on") by resistors at both ends of the bus that terminate the bus and form a voltage divider. The chips are all TTL, so if you were to just directly connect a TTL-level driver to the bus and assert a "high" logic level, it would try to pull the bus to +5V and you would be in a world of trouble. The open collector drivers don't ever do that, they can either pull the line down to ground, or they can act like an open circuit, but that's it. That's how signaling on the DEC bus works - a bunch of open collector drivers share the same line, and any of them can pull the line down to ground, safely. The old chips that were designed for this purpose also had other electrical requirements, especially the amount of current they could sink when pulling the bus to ground. There are some open collector drivers still made, but not as many as you would think. And certainly nothing I know of that is in a single convenient package like the DEC or National or Signetics chips were. There are some off-the-shelf things that are close but not close enough. Maybe the best bet would be to use a CPLD with open collector outputs and +5V tolerant inputs on the other side? Though you'd have to be careful about current sinking requirements. Anyway, it's not as easy as I wish it were :) Believe me, I'd love a QBus card with a CompactFlash or SDCard that knew how to speak MSCP. But if anyone wants to actually build it, it'll be a lot of work. Also hard! Differential signaling on the disk data lines, MFM encoding/decoding, wicked tight timing requirements. But it has been done commercially, so it's certainly not impossible! -Seth