18 Sep
2013
18 Sep
'13
7:09 a.m.
From: Josh Dersch <derschjo at gmail.com>, Date: Tue, 17 Sep 2013 21:09:46 -0700
Sorry to reply to my own post here, I had a bit more
time to screw with
this this evening. Turns out it was a programming error (actually a
program entry error :)) that was (apparently) causing the 40mA current
instead of 20mA -- the >little "echo" program I coded up wasn't
properly
waiting for data to come in before echoing it back, so the 8/L was
constantly sending 377 back across the loop, which I'm assuming accounts
for the difference?
Do you have a picture of your setup you can share? I'm wondering what the
other end of the wiring coming of your W076 card looks like.
This style of Molex connector was rather common (in the DEC world
for sure)
http://www.vintagecomputer.net/digital/PDP11-05/M9970/wires_fr_1105_to_TTY_…
There's one of these on the back of a VT220, for example, the first
terminal I ever hooked to my PDP-8 (it was only later I got a TTY).
As has been pointed out already, there are 2 current loops - one for
Tx, one for Rx, and the other two wires are the Reader Run control,
essential for reading in RIM and BIN tapes. RR won't matter for
a simple echo test.
From your subject line, it sounds like you have a
20mA<->RS232
adapter. If so, what model and how is it wired to your W076?
-ethan