On 3/29/25 7:29 PM, Martin Bishop via cctalk wrote:
The why not use a UniBone comment has merit, what will your (FPGA) implementation add ?
I'm not shadoooo but I've also been working on a somewhat similar FPGA-based board called the USIC / QSIC. We started working on it before the UniBone but have been slower to come to completion. A lot slower. Maybe there's no point to it anymore but I keep poking along on the design anyway.
If you solve the (near) unobtanium OC driver / receiver problem - I for one will be all ears
Ah, yeah, this problem. At one point in the QSIC project I started doodling up circuits to deal with this so we wouldn't have to use up NOS bus transceivers, wouldn't have to deal with the 5V/3.3V conversion for the FPGA, and would be all surface-mount parts to make automated fabrication easier (I never found any DS3662s). A comparator for the receiver with just the right amount of hysteresis. Have to look around a little to find one that's fast enough to meet the 35 ns requirement but they're out there. And then the driver is just a transistor and a capacitor on the gate/base to limit the slew rate. Shouldn't be all that hard to design, right? Might want to go with a constant current source to charge/discharge that capacitor to make it a proper trapezoid waveform though I don't know that that's really needed.
This all needs testing and I was going to make up a little test board with both my circuit and a DS8641 that could be plugged into different busses to have a look at the waveforms that come out. I looked up the pinouts for SPC, MUD, and QBUS so I could design a board that would equally work in all three.
I was talked out of this idea though. We were doing enough new already with the whole rest of the board and from our prototype QSIC we knew that the DS8641s with level converters would work so it made sense to stick with that.
Still, I think about this idea from time to time. In the small chance that anyone is interested, I just now threw my circuit ideas up on GitHub. Remember, this is doodling. I can see three generations of ideas in there, as I thought through different possibilities. I also had this idea about switchable, active termination so that's in there too, though I'm now less sure that's a good idea.
https://github.com/dabridgham/DEC-Bus-Transceiver Dave
