Pinout for current loop interface

Ethan Dicks ethan.dicks at
Fri Aug 16 23:56:01 CDT 2019

On Sat, Aug 17, 2019 at 12:12 AM Charles via cctalk
<cctalk at> wrote:
> Is there any standard pinout for 20 ma current loop using a DB-25 connector,
> analogous to the well-documents RS-232 serial interface?

>From checking a couple of devices, I don't think so.  Here's how the
original IBM Async adapter for the 5150  did it...

Pin 18 +receive current loop data
Pin 25 -receive current loop return
Pin 11 -transmit current loop data
Pin 9 +transmit current loop return

One BlackBox adapter I found used different pins.

> The ADM-3A receives on pins 23 & 25, and transmits on pins 24 & 17.

I'm used to using 15 and 17 for sync serial clock.  They are typically
NC for async.  23-25 have alternate uses that are, again, typically
NC. (I've never seen a device that used pin 25 for 'test').  There are
Sun workstations that have two serial ports over one DB25 which might
be the only use of the "B" pins I've encountered.


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