PDP-11/70 progress (and a cry for help)
elson at pico-systems.com
Wed Feb 17 10:05:56 CST 2021
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.
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