20mA cabling questions

From: Ethan Dicks <ethan_dicks_at_yahoo.com>
Date: Wed Dec 15 01:03:01 1999

The PDP-8/L restoration continues. I have two devices to attach to my -8/L
for testing - a VT220 (all have 20mA current-loop connectors) and an ASR-33.
One problem: the connector on the VT220 is female and the cable coming from
the ASR-33 is male. While this might not seem like a problem, it becomes one
when I have to use either device from the same W076 card. Many years ago, I
rigged up a working *male* cable from a W076 card for use with this VT220. It
still works. I have a quick-n-dirty Switch Register->TTY test program and
can emit all sorts of ASCII characters for testing. When I stick a different
W076 card in the -8/L, one with a female connector, within the limitations
of this particular ASR-33, all is well (the type cylinder doesn't always
retract all the way, nor does the hammer always return to the proper position,
but I currently suspect that the replacement rubber pad I have attached to the
lever arm/hammer does not have the correct resiliency).

So... my first goal is to come up with some cables that will solve my W076
problem. Is there a good 20mA primer out there? I have a basic understanding
that it's all based on 12VDC loops, send and receive take four wires, but
there are six coming out of the W076 card... the extra two are reader-run
relay signals, obviously not needed for a VT220, but very important for the

A) Is there a color standard for the 20mA connector cabling? I have three
W076 cards with three different arrangements.

B) When I make a pin-one-to-pin-one male-to-male 20mA cable, it does *not*
work with the female-equipped W076 and the VT220. A few moments with a DVM
show that the arrangement of a W076 and my straight-through cable do not
match the pinout of my long-ago-homemade W076-to-male connector that does work
with the VT220. Is there a special wiring trick for male-to-male 20mA cables?

C) Somewhere, I have a small box of 20mA cables that have, IIRC, four
conductors. Presumably, these are for VT52s. I have no reason to expect
that they won't work with the VT220, once I find them. I take it, then, that
in the "old days", one never used an ASR-33 as a tape input device except on
the console interface? I'm thinking of some of the ancient distribution panels
for other DEC serial cards that had EIA (RS-232) _or_ current-loop connectors.
Am I missing something?

Right now, because my TTY print mechanism is acting up (must press down on the
type cylinder between chars, and the line-feed mechanism sticks 20% of the
I have been testing by inserting one W076 in the backplane for paper tape
reads and the other W076 in the backplane for interactive testing. Obviously,
this can't go on forever. I suppose I could rig up a *really* bizarre cable
that sends the output to the VT220 and takes input from the TTY. Maybe I'll
just work that angle. It would save on paper. I don't have much cause to
punch stuff right now, and until I get the primary box working I won't even
attempt to attach my outboard 8Kw to this CPU (the eventual goal of all of this
is to set up for 8K-papertape-BASIC and play Star Trek. I have all the tapes
to do this, but I do have to replace the floating-point tape that tore the last
time I loaded it (a long, sliver of a tear, not a perpendicular, parting tear).

I still know next to nothing about the pr/s01 except that it is *not* 110
baud. Another one of my ancient projects is to replace the M452 variable
clock module with a home-made digital baudrate generator. I suppose I'd
have to wire a switch on the backplane as well to select the M706/M707 from
two stop bits to one when the baud rate was 150 or higher, but I have a
prototype module and I have the COM 8116 clock chip and I have the proper
crystal to drive it. What remains (besides sitting down and finally *making*
it) is to decide if I want to build in the ability to drive transmit and
receive at different rates as was common with VT52s. I could receive at 150
or 300 baud and transmit as fast as possible, perhaps 9600 (I'd have to look
into the M706/M707 designs to even hazard a guess, but even 150/2400 is an
improvement). The goal would be to be able to use the pr/s01 with a TTL -8.


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Received on Wed Dec 15 1999 - 01:03:01 GMT

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