Semi OT: 20ma loop troubleshooting

From: Mike Cukr <cukr_at_massnet1.net>
Date: Thu Oct 30 14:35:54 2003

----- Original Message -----
From: "Paul Thompson" <thompson_at_new.rr.com>
To: <cctalk_at_classiccmp.org>
Sent: Thursday, October 30, 2003 2:26 PM
Subject: Semi OT: 20ma loop troubleshooting


>
> I am trying to set up a Black Box '20ma loop to serial' converter box on
> an existing, known working 20ma loop which currently drives a couple of
> 20ma terminals.
>
> There is a note in the black box manual that the device won't work on
> analog current loop. Am I correct in assuming that an analog current loop
> is one where an analog instrument is sending its 'telemetry' over the
> loop? A 20ma loop terminal obviously would not fit into that category I
> am assuming.

That is correct
>
> The folks I am working with seem to have tried the obvious. They only
> want the black box device to receive data so they have hooked on the
> appropriate wires so far no output. Any obivous gotcha's with this
> technology?

There are always gotcha's... I'm unclear about what you mean by 'recieve
only'
Are the terminals only to display data sent to them? or are they data
entry only (keyboard)? Three things come to mind that could be problems:

1. Assumng the 'black box' does no data buffering, the baud rate has to
be the same on both sides of the 'black box', ie if the terminals are 300
baud
then the serial (RS-232) has to be 300 baud too. Confirm that serial
data is reaching the 'black box' from the host (if that's what you mean
by recieve).

2. Which device is expecte to provide the loop current? Most terminals
don't provide loop current and expect the host to provide it. Take a meter
reading across the loop lines, you should see some like 20-60 vdc, if not
the terminals are configured to have the host provide the loop current.
Check the docs for the terminals to see what's expected. The 'black box'
may or may not be able to provide the loop current.

3. Assuming that data is being sent to the terminals for display, you may
have to 'address' the terminal, that is unless the data is proceeded by a
a unique set of characters, the terminal may just ignore the data, again,
check the terminal docs and configuration settings.

Good Luck
Mike
>
> Paul
>
>
>
> --
>
>
Received on Thu Oct 30 2003 - 14:35:54 GMT