[TUHS] PDP 11/34a Serial Communications Problem

Mark Hare markhare at buffalo.edu
Mon Feb 6 06:03:10 AEST 2017

Hello all,

This is my first time emailing the list, so please let me know if this
doesn't belong here of if I'm breaking any rules.

A few months ago, I rescued a PDP 11/34a with 2 RL01 drives from the scrap
heap. The unit appears to work fine based on my limited front-panel
testing. I haven't gotten the drives running yet since someone cut the
power cords when the cabinet was being removed.

There is a DL11-W serial line unit/realtime clock (M7856) installed in the
11/34 that I want to use for serial input/output. I have configured the
card for 9600 baud, 8N1. Using some jumper wires, I carefully connected the
card to a serial cable and a computer running a terminal and I was able to
send some characters back and forth successfully.

For a more permanent solution, I designed a simple adapter board that
connects to the BERG 40 connector on the DL11-W and converts it to a DB9
serial port (In restrospect, this product was already available at
https://oshpark.com/shared_projects/uTMf3v08 but I didn't know about that
at the time). I also ordered a 40-pin (non-IDE) ribbon cable to connect the
DL11-W to my adapter.

When I connected everything, the 11/34 would start but no lights would
appear on the front panel. I tried disconnecting the adapter but leaving
the ribbon cable plugged into the BERG connector, but the problem
persisted. When I removed the ribbon cable entirely, the unit powered on
with no problems.

Since this is a straight-through ribbon cable, I don't see what could be
causing this problem. I have checked the continuity of each wire in the
cable, and there doesn't appear to be a problem. I'd appreciate any advice
that anyone has to offer.


Mark D. Hare

markhare at buffalo.edu
University at Buffalo
B. S. Civil Engineering '16
M. S. Structural/Earthquake Engineering Student
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