PDP-11/84 SLU problem?

Noel Chiappa jnc at mercury.lcs.mit.edu
Mon Dec 15 12:47:46 CST 2014

    > From: John Rollins

    > it doesn't appear to transmit data from the console port on the back. I
    > have tried ... every conceivable switch setting on the PDP with nothing
    > ...
    > So when I power up, the LED display on the front counts down through
    > the numbers and stops at 4. As near as I can tell, this is something of a
    > good sign.

Should be; my User's+Maint Guide says that means "Dialog Mode: Waiting for

    > Short of spending an entire day disassembling half the machine and
    > tracing every wire hoping to find something out of place, does anyone
    > have any ideas on what may be happening here, or what to look for?

I asssume you've tried setting the baud rate with the switches on the CPU
card (in case the cable to the back panel is damaged - or, more likely, the
rotary baud select switch has a bad contact due to corrosion over time)?

If you have the parts, fabricating a substitute serial interface cable would
also be something to try.

The next step would normally be to disable the on-board serial interface, and
provide a substitute console interface, out on the bus. _Alas_, as far as I
can tell from reading the KDB11-B User's Guide (EK-KDJ1B-UG-001), there is no
way to disable the on-board serial interface.

AFAIK, the prints for the 11/84 (MP-02536/MP-01955) don't seem to be online.
I can't locate prints for the 11/83 (uses the same CPU board, I think)
either. So I can't check to make sure that it is in fact impossible to
disable the on-board serial interface.

And the other next step, to look at the serial output and see if it looks OK,
will be a little tricky without prints, although the KDB11-B User's Guide
does contain something about it, on pp. 5-38 to 5-41 (165-168 of the PDF).


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