Semi OT: 20ma loop troubleshooting

From: Paul Thompson <>
Date: Thu Oct 30 19:58:41 2003

On Thu, 30 Oct 2003, Mike Cukr wrote:

> > 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:

One current loop terminal is hooked up so that it can transmit and
receive, the other can receive only. They must both be the same ID
because they both see the same data: the receive only terminal can see
what the send/receive types and what the fire system sends. I will look
in more detail to see how this was wired. Below is a little more
information which might clarify what I am trying to do.

> 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).

Will do. That has been the tough part so far since there are xmit and
recv lights on the black box which don't light.

> 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.

I am sure the main fire alarm system provides the current. I will check
the voltage.

> 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.

The terminals themselves are current loop native devices and are working
fine. Right now the receive only terminal has a linux box hanging off its
serial port which analyses the messages and pages people. My hope is to
get rid of the read-only terminal with just the linux box and black box
hooked directly on the 20ma loop. The read-write terminal will remain.

Thanks for your thoughts.


Received on Thu Oct 30 2003 - 19:58:41 GMT

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