On 02/17/2021 02:45 AM, Josh Dersch via cctalk wrote:
"0" here selects DR (Destination Register) input to the mux and is incorrect; it should be 1 (PCB). During a single-instruction-step run, this value reads out OK on the analyzer. I noted a few other discrepancies in the capture (all of which match the ucode listing during a single-instruction step) which makes me think that the high outputs of the PROM are right on the bleeding-edge of acceptable TTL. I checked out the signal on the scope while running a BR .-1 instruction (which also doesn't execute correctly but at least doesn't halt... I don't have a storage scope to capture this during a single instruction execution) and it looks like the voltage swing is from about 0.15V to 1.7V or so.
Well, that sounds like the pull-up transistor on the PROM has gone open or extremely weak. You might try a resistor to +5V and see if that makes it work. It would explain the error only
happening at full speed. Jon
