Pinout for current loop interface

[Warner Losh]

On Fri, Aug 16, 2019, 10:56 PM Ethan Dicks via cctalk <cctalk at classiccmp.org>
wrote:

> On Sat, Aug 17, 2019 at 12:12 AM Charles via cctalk
> <cctalk at classiccmp.org> 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.
>

The DEC rainbow has B pins for talking to a modem dialer via a special
cable... but it is basically useless for anything other than a few
characters.


Warner

>