I came across a machine the other day with a check station. This station is checking for continuity on a previously installed wiring harness. I was wondering if there is another way of doing it. They seem to be just feeding +24vdc to one pin and waiting for it to come back to the PLC card on the other pin. This works, but the operators have figured out how to short the pin to the frame of the machine, which results in blowing the fuse or shorting the card.

If I were to isolate a separate voltage supply, say five volts, and not ground it to the common ground, then it should not short basically I think.

 

Use a series resistor to limit the current. Look up the specs of the PLC input card to get the input impedance and use that to calculate a suitable resistor size (large enough to limit the current to a reasonable value, but small enough to keep the voltage within input card specs).

 

Hi,

You could try sending out serial data (a single character, etc.) or some other simple pulse train, then read it back on the input. This would make it difficult to fake.

Kevin

 

Try adding a current limiting resistor in series with the +24V supply to the harness.

 

Depends on how clever/sophisticated you want to get.

Option 1:
To ensure that a system was earthed at the correct point, we used to use a circuit where there was an AC supply, a remotely mounted diode and DC powered relay. Unless the connection was made at the correct point, the diode wasn't in the circuit and the supply that was still AC wouldn't turn the relay on.

Option 2:
You could do something similar using an off the shelf switched operated relay by placing End Of Line resistors in your circuit. This is what you can do for a Process Switch circuit to detect cable faults (open or closed circuit), you are basically making a volt free contact look like a prox switch and the relay monitors the current in the loop to decide what state the switch/loop wiring is in and sets the output contacts accordingly.

Steve
www.mtl-inst.com