PDP 11/35 troubleshooting

Guy Sotomayor ggs at shiresoft.com
Tue Mar 8 01:35:23 CST 2005

On Mon, 2005-03-07 at 22:18 -0500, Ashley Carder wrote:
> Ok, since everyone seems to be in ancient PDP-11 
> troubleshooting mode, I decided to get out my 11/35
> and see what I can do with it.
> I've double checked all the processor options and
> the CPU boards all seem to have the jumpers set 
> correctly.  The only questionable jumpers are the
> two jumpers that indicate parity memory is present,
> but since the system seems to have some problems,
> I've disconnected the M981 from the MF11-LP core
> backplane and replaced it temporarily with an
> M9302.  There is also an M7800 console card in
> the SPC slot.

Noooo!!!  The M981 contains the CPU side terminators.  Repeat after me,
I will not plug stuff into special slots that isn't what's supposed to
be there!

The CPU will just flat not work if the M981 isn't plugged in.  It
doesn't matter what system unit it is also plugged into (just so long as
it is powered).

I only hope you haven't fried something.

Here's what you should do:
     1. Put the M981 back in
     2. Put a terminator in the next system unit (M930...keep it as
        simple as possible) after the CPU
     3. Remove all peripheral cards and replace them with bus grant
        cards (G727)
     4. If the next system unit is a memory backplane, depopulate it (ie
        remove all cards)
     5. put the halt/enable switch in "halt" (ie down)
     6. power the system up
     7. depress the "start" switch a couple of times (what happens?)
     8. put an address in the switch register (say 1000)
     9. depress the "load address" switch (does the address load?)
    10. depress the "examine" switch repeatedly (does the address

Until you can get to #10, you don't have a healthy CPU.  Having a KM11
will allow you to single step the micro-code but until you've studied
the micro-code flows it'll be hard to figure out what is really wrong.

> I've checked all the voltages and they look ok.
> When I power the system on, it comes up with the
> following lights lit:
> Address: 000026 (octal)
> Data   : 000002 (octal)
> The PROC, BUS, and CONSOLE lights are also lit.
> If I press HALT and then press START, all the
> DATA lights come on.  The RUN light also comes
> on, along with the PROC, BUS, and CONSOLE lights.
> The load address, deposit, and examine switches
> don't seem to do anything, although I know they
> are functional.
> Ideas?  Does anyone know what the ADDR 000026 and
> DATA 000002 lights indicate when the system is 
> powered on?
> Thanks,
> Ashley

TTFN - Guy

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