Pinout for current loop interface

Charles charlesmorris800 at
Fri Aug 16 23:12:32 CDT 2019

Is there any standard pinout for 20 ma current loop using a DB-25 connector, 
analogous to the well-documents RS-232 serial interface?

My PDP-8/A drives an ASR-33, and having just restored an ADM-3A I want to be 
able to unplug the TTY and plug in the ADM.
I somewhat arbitrarily put the transmit data + on pin 2 and receive + on pin 
3, and picked two uncommitted RS-232 pins for the - legs of both loops.

The ADM-3A receives on pins 23 & 25, and transmits on pins 24 & 17. Polarity 
doesn't matter since both pairs use bridge rectifiers.
If this is some kind of de facto standard, I'll change the bulkhead 
connector on the PDP-8 and the TTY to match.
Otherwise I'll just make yet another unique cable to hook up the ADM-3A to 
the PDP-8 as it's wired.

Or would you recommend I use a different connector entirely? The reason I 
used the DB-25 to begin with is that I had a DEC rack-mount plate that 
already takes one.
Maybe a Jones 4-pin would make more sense.

thanks for any tips.

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