20mA cabling questions

From: Truthan,Larry <truthanl_at_oclc.org>
Date: Wed Dec 15 08:32:19 1999


The 20 ma current loop is not always 12 Volt loops - its 20 ma Loops.

I have the IBM tech ref for the IBM PC Asynch Port which shows one Cuurent
Loop interface implemented on
25 pin D-shell pins 9,11, 18 & 25. (Only when an option jumper block selects
current loop.)

For Xmit they have 8250 (ttl) S-OUT buffered through an LS125. The LS125
output "fans out" feeding both the RS-232 driver as well as three 7406
inverters in parallel for "ample" 20ma drive. In series beyond the three ||
7406 inverters is a 100 ohm resistor routed to pin 11 labelled " - xmit CL
out". Pin 9 labelled "+ xmit CL ret" is pulled high to 5V through a 49.9
Ohm resistor.

For RX they show an opto coupler, a 20 ma diode anode tied to pin 18
labelled "+ RCV CL data. Its cathode is tied to pin 25, labelled "-RCV CL
ret". The switching transistor of the opto coupler drives the input (Low)on
an LS04 inverter which is routed through jumper selection to "serial in"
(SIN)on the 8250.

So you need to determine if your third CL pair drives, or reads, the
"reader" relays. Each pair is either a source or a sink for current.

Another good source for reference on computer controls interfacing is a
Burr- Brown publication called "The Handbook of Personal Computer
Instrumentation" I have a spare Fourth edition Published in 1989. Stop by
OCLC and pick it up.


Larry Truthan

-----Original Message-----
From: Ethan Dicks [mailto:ethan_dicks_at_yahoo.com]
Sent: Wednesday, December 15, 1999 2:03 AM
To: Discussion re-collecting of classic computers
Subject: 20mA cabling questions

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
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.
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
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
but I currently suspect that the replacement rubber pad I have attached to
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
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
with the VT220. Is there a special wiring trick for male-to-male 20mA

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,
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
for other DEC serial cards that had EIA (RS-232) _or_ current-loop
Am I missing something?

Right now, because my TTY print mechanism is acting up (must press down on
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.
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
is to set up for 8K-papertape-BASIC and play Star Trek. I have all the
to do this, but I do have to replace the floating-point tape that tore the
time I loaded it (a long, sliver of a tear, not a perpendicular, parting

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
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 - 08:32:19 GMT

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