[TUHS] PDP 11/34a Serial Communications Problem

Mark Hare markhare at buffalo.edu
Mon Feb 6 06:58:49 AEST 2017

I believe I have found the problem. Upon closer inspection, I noticed
that the ribbon cable that I am using is terminated in such a way that
the conductors are exposed on the outside of the top of the female
connector. Looking at the DL11-W, I saw that there is an uncovered
trace on the circuit board just below the pins of the BERG connector.
It appears that the exposed conductor ends of the cable were making
contact with this trace, which shorted the entire cable together. I
covered the exposed ends with a piece of electrical tape and the whole
setup now works precisely as intended.

Frankly, I'm just glad that there doesn't appear to be any lasting damage.

Thank you Michael and Noel for your help; I'm sure I'll be back with
more questions as I make progress with this machine.


Mark D. Hare

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

On Sun, Feb 5, 2017 at 3:38 PM, Noel Chiappa <jnc at mercury.lcs.mit.edu> wrote:
>     > From: Mark Hare
>     > 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 ...  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.
> That's extremely odd. There isn't a pin on the DL11-W Berg connector which
> should be able to cause anything like that kind of behaviour. The only
> _possible_ thing I can think of, looking at the list of signals on the Berg,
> is that you are grounding +5 (TT). Either that, or your DL11-W has some
> serious issue?
>     > Since this is a straight-through ribbon cable, I don't see what could be
>     > causing this problem.
> Me either. But clearly it's not just a straight-through ribbon cable....
> I myself wouldn't have gone that route; one can obtain 40-pin IDE/DuPont (they
> are all .1" spacing pins, and basically interchangeable, module keying)
> connector shells, and female pins for same; I would have made a custom cable
> to plug into the Berg with that, to a DE-9S or DB-9P connector (depending on
> whether one wanted one wired as a DCE or DTE). (I myself make such cables, but
> to a DB-25S connector, and then use a commercial DB-25P to DE-9S adapter when
> needed.) Oh well...
> Does the DL11-W still work, using the jumper cables kludge?
>     Noel

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