How to test Ground Probe Circuit Board?

[JamesArthurHof]

I am an industrial electrician. I would like to start bench-testing these Ground Probe circuit boards. I tried setting something up, yet need help. The inputs on the board are wired to plunger switches that ground out on steel parts in a press—a part present indicator. The board’s outputs are wired to a Modicom True Low (TL) PLC Input card. I have supplied photos and schematics. I think it should be pretty simple for you electronics smart guys. I have supplied appropriate photos and schematics.

[ajb]

Just a functional test? 

The schematic shows a positive ground system, so the probe inputs are activated by connecting them to the positive side of the 24V supply.  The outputs are open collector aka sinking outputs, meaning they will be pulled to the negative side of the 24V supply when active.  So a test fixture could be a momentary pushbutton from each probe input to 24V+ and a 24V lamp between each output and 24V+.  Press each button, check that the corresponding lamp illuminates.  Of course you could do this with just jumper wires instead of switches and a voltmeter instead of lamps. 

The trim pots set the sensitivity of each probe input, presumably to account for resistance in the probe contact with the part.  If you need to adjust those to a specific level then you could connect the probe inputs via a resistor of suitable value, but you'd need to identify what value that is based on the probe/part characteristics.  Probably better to dial those in on the machine anyway. 

One other thing you could bench test would be to measure the voltage between pin 20 and the negative side of the supply.  The pinout says that pin 20 is no connect, but the schematic shows that it's the reference point the card compares the inputs to.  It should measure about 5.5V with a 24V supply.  If not, then there's a problem with R17/R18 or U1/U2.

It looks like there's probably a negative ground version of that board as well, for which you'd need to connect the probe inputs to the negative side of the 24V supply.  The schematic makes it look like it could be the same PCB with different components fitted and some jumpers moved, but there are no jumpers apparent on the board in the photo so it's probably a different PCB but hard to be sure.  Just something to watch out for if you have a bunch of boards of mixed or unknown provenance.  I'm also a little suspicious of the schematic because D1 is clearly connected wrong--as drawn its pins are shorted making it entirely useless.  I'm pretty sure that's just an error in the schematic drawing and the wire parallel to it isn't supposed to be there.