Line Monitoring

D

Thread Starter

DT99

Hi,

I am after a little more information on line monitoring. I have searched n the internet but there is no information available.

Really after a little more detail (tutorial) on the scenarios that merit line monitoring DI/DO AI/AO. interposing relay/volt free output configurations. Just after an overview really as I have very little experience in this area.

Thanks
DT
 
One way to do line monitoring is to use resisters and put them in series and parallel to increase and decrease the line resistance and therefore the voltage in a loop. For instance use a resistor in series with a switch and another one in parallel with the switch will change the resistance or the measured voltage in the loop as the switch open and close. Use a resister in series with the loop itself will tell you when the loop goes open circuit due to a broken cable since the system will in that case not see the value of the loop resister anymore. In the supervisory system or PLC you then set the line monitoring up that is there is a break in the cable the voltage or resistance will be that and if all is well the resistance and voltage will be that and if the switch is active and the line is not broken the resistance or voltage measured in the PLC will be this or that. It is basically limitless on what you can do by installing these resisters in your loop and use the variance in voltages or resistances to initiate an output or a flag in your main supervisory system. It is also a lot of fun since you can play around with all kinds of configurations. With analogue signals line monitoring you can do the same but be careful that you don’t drop the loop resistance below 250 Ohm since that is the minimum requirement for HART communications in a 4-20mA loop.
 
T

Timothy Niemczyk

DT,

Although a previous post has listed one way to line monitor, I believe you should look up Pretech Solutions Inc. at pretechsolutionsinc.com as they have a way to detect all conditions of line monitoring (Normal, Active, Open, Short,and Ground conditions) as well as cold coil monitoring that will detect (Normal "De-energized", Active "Energized", Open, Short and Main Power conditions). These solutions give the operators a complete status of the line conditions.

V/R,
Tim
 
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