17 Feb
2021
17 Feb
'21
8:05 a.m.
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