Schneider Quantum Modicon PLC

Hughes838,

I looked it up on Schneider's website. That is a 32-point, 24VDC input with inputs grouped into 4 groups of 8 each. Each group has common terminal wired to DC+ and 8 inputs wired to DC-. Therefore they are "ON" when they have -15 to -24VDC on the terminal, and "OFF" when they have 0 to -5VDC on the terminal. I'm not sure what you meant when you said you were grounding the input wire to the PLC. You would need to read the voltage between that group's common terminal (10, 20, 30, or 40) and the corresponding input. If the voltage is in one of the two ranges given above, then the input should be ON or OFF depending on the voltage you read. If it is between -5 and -15VDC, then its status is in doubt.

If the power supply for this module has its negative grounded (a fairly common practice), then "grounding" the input would have made it be ON (which is what you observed).

Now, if you have no wire on the terminal, I agree that the input should be OFF. If the status by a voltage check disagrees with the status on the LED's for that input, then you have a defective input module. If you have a computer with programming software, you could enable forces and then force this input OFF. Otherwise you are going to have to find someone who has the software and the expertise to solve your problem

--JMM

Hello JMM, thanks very much for your comments. You're right the common terminal is the negative for the 24 vdc power supply (ground). The logic says open contact (high - 24 vdc) and closed (low - 0 vdc) to ground. The output lead (to the terminal block) from the PLC was reading positive to ground. Yet on the PLC terminal block of the DDI 353 10 it was 0 vdc (the other end of the wire??). I thought it was miss wired but this morning it went high, yet the alarm still won't clear. I agree, it must be the module itself that's bad. Thanks again, I thought that maybe if the module could be deenergized, it might clear the problem. I'm afraid we don't have the software you mentioned so it looks like we'll have to buy a new module.

ERH

Hughes838,

Its been quite a few since I last worked with a Modicon Quantum PLC. My memory is that it was fairly unusual among PLC's because I/O cards could be removed or inserted while it was powered on. In most PLC's this would result in a major error and fault the processor. When that happens, the only recourse is to hook up a progamming terminal and clear the fault, then set it back to RUN mode.

It is entirely possible that the input in question comes via some intervening equipment (did I mention this in my previous post?). If so, there is an additional location to check for what is happening with that signal. I have, of course, seen fried inputs and fried outputs, so changing out the module is reasonable. Another source of problems sometimes has been in the removable terminal block, with a whisker of a wire being where it shouldn't be.

Work safely and I hope you succeed in this problem.

--JMM

Faced this problem before in a Premium PLC. I had changed the module.

cheers.