PDP-11/70 progress (and a cry for help)

[Jon Elson]

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