The Restoration of the PDP-8/I Minicomputer (PAGE 12)
Looks like a gate has failed during operation. The symptom is that both the IOT and the JMS
lights turn on during an IOT operation. The JMS light turns on for a genuine JMS as well, and is off for all other opcodes.
Tracing this in the circuit, it looks like the gate at pins N2/P2/R2/S2 is ignoring one of its inputs.
Replaced M115@E21 S/N 0004 with
M115 S/N 0013 from spares, and the problem went away. Total repair and
diagnosis time 5 minutes. I wish all problems were this simple! :-)
My next trick is to diagnose the EAE (KE8I) on the 8/I.
The schematics are available from this PDF file on bitsavers.org
in the Options Schematics -- they are the first three diagrams.
I managed to find a set of blueprints, which are identical except for the EAE instruction register logic, which, in my blueprint
version has an "-EAE IR CLEAR" signal going to the left-hand side of each EAE IR flip flop near the bottom right of the
schematic (in the PDF mentioned above, these signals are tied high).
I'm unable to find the origin of the "-EAE IR CLEAR" signal though...
In any event, there are many issues with the EAE (KE8I) on my PDP-8/I. Here's a summary:
- SCL (7403) -- damages memory in subsequent locations (wrong)
- MUY (7405) -- attempting to multiply by one causes the PC to go from 0001 -> 0026 (wrong)
- DVI (7407) -- attempting to divide by one causes the PC to go from 0001 -> 0164 (wrong)
- NMI (7411) -- SC only goes as high as 2, and AC contents are lost (wrong)
- SHL (7413) -- AC=0001/MQ=0000 afterwards AC=10000/MQ=0000 (wrong)
- ASR (7415) -- AC=0001/MQ=0000 afterwards both AC/MQ are 0000 (wrong)
- LSR (7417) -- AC=0001/MQ=0000 afterwards both AC/MQ are 0000 (wrong)
- MQL (7421) -- works
- SCA (7441) -- AC gets zeroed (wrong)
- MQA (7501) -- no change in AC (wrong)
- CLA (7601) -- works
- CAM (7621) -- works
This deficiency list was generated by entering tiny one, two, or three instruction programs at location 0000 and
using the single-step mode of the 8/I to see what happens.
So, there are a few things to fix. Given that MQL works, I'll try to get the other non-operand instructions like
MQA, NMI, and SCA working next.
Meanwhile, you want AC and MQ blinkenlights? See the AC/MQ Blinker.