PDP 8/e Problem

Rick Bensene rickb at bensene.com
Sat Oct 24 21:59:07 CDT 2009

Hello, everyone,

This weekend I decided to spend some time and fire up my PDP 8/e system.
It's been a while, and figured it was overdue for a few hours of

The machine has EAE, 2 8K DEC Core memory units, an 8K solid state
memory board (12x8 1K static RAMs) the extended memory/timeshare option,
a PC8E paper tape interface with a PC04 reader/punch, an RX8E controller
and dual RX01 drives, and (not installed) an RK8E controller (currently
busted) with two RK05 drives, and two M8650 serial I/O  interfaces.

It powered up just fine, and I toggled in a few little test programs.  I
couldn't get it to talk to the terminal (a Heathkit H19).  Then, I tried
a test program that runs the punch...it too would not work.  Then, I
toggled in a little program that uses the OSR (OR Switch Register)
instruction, and it too wouldn't work...the accumulator remained
untouched by this instruction no matter what the state of the switch
register was.

I did some other testing, AND and TAD work, as does ISZ, and as many of
the group 7 operate instructions that I bothered to test.  DCA also
works fine, as do JMP and JSR.

I could load extended address from the front panel, and address memory
beyond 4K with no problems, so at least the extended memory part of the
ext. mem./timeshare option was working.

I also noted that the HLT instruction doesn't work.  So, IOP
instructions, OSR, and HLT are on the list of instructions that don't
work. It appears that these instructions just act as a No-Op.
I tried stripping the system down to just the CPU, EAE, 8K of Core, and
the console async. interface, and it still behaves the same.

It seems like the instructions that aren't working are those that would
be trapped by the timeshare option, but with that board removed from the
system, the fact that the behavior didn't change is of concern.

Anyone have any ideas what might be going on?  

Thanks in advance for any help.

Rick Bensene

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