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Digitally monitoring end of line resistors

Pascal666

Aug 20, 2012
18
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Aug 20, 2012
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I have a few old Omega D1711 digital I/O to RS232 modules I was hoping to repurpose to monitor door and window switches from my old burglar alarm. I had planned to simply wire one end of each switch to ground, the other to one of the lines on a D1711, and hook a pull-up resistor to each line. So as long as the switch is closed the line is grounded, otherwise it is pulled up to the 20V that fed the old alarm panel (and will now power the D1711).

However, when I went to hook everything up I figured out that the alarm system used 1K Ohm end of line resistors (in series with the switches) to monitor the wires for shorts. I can compensate for this by using larger pull-up resistors to create a voltage divider, so instead of the lines being grounded they would at least be below 1V.

I'm wondering though, if there is some way to monitor the EoLRs instead of just compensating for them. Perhaps some way to wire the switches across two D1711 lines to monitor for the three possible states: open, shorted, and 1k Ohm. Thoughts?

Omega D1711 specs:
15 digital input/output lines
Input voltage levels: 0-30V without damage.
Input switching levels: High, 3.5V min., Low, 1.0V max.
Outputs: Open collector to 30V, 100mA max. load.
More info: http://www.omega.com/pptst/D1000.html
 

dorke

Jun 20, 2015
2,342
Joined
Jun 20, 2015
Messages
2,342
Well Pascal,
Here is a possible solution for you:
Using 2 lines of the D1711 A and B instead of the current single line A.
note:
line A can act as a short indication(active low).
line B can act as the closed/open indication.
but you are free to use them as you like.
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