PLC Mainframe Issue

"... the PLC does not recognize or accepts the signal." How do you know this? Are you examining the inputs directly (using e.g. RSLogix), or otherwise? Are the monitoring indicator lamps lighting for these inputs?

Along with JohnG's questions, was the unit in service and quit working or are you just trying to program. If the later did you set-up the I/O card in the processor?

Also I assume that the main frame that you refer to is the processor!

We are using RSLogix5. The original install is about 11 years old. We have inputs on the input table from limit switches that are registeering a 0 when the input wire at the input card from the limit switch is checked with a Solenoid Voltage Tester, there is 120 vac. This is giving us closed faults on our open valves that we have to force on to show they are open in order to continue running. We tried new input cards are a new mainframe. We still have the problem. What is weird is that the input problems are all bit 6 or bit 16 on the input cards.

To add, the mainframe did include the processor and power supply. Which were both changed out. We were having power supplies drop out and decided to be proactive in this case.

One quick question and then possibly some follow ups: Do the LED indicators on the front of the input modules correctly show the state of the input?

... a question I've already asked, but it was not answered. CriscoKid?

"..PLC does not recognize or accept the signal..."

Some things I would check:

1. Is your common wire at the input card loose? Is it a true neutral common?

2. Are the LED's in the input card lighting up when you receive power?

3. If you are monitoring through RSLogix, is the ladder for the section of logic you are using currently being scanned or is it being inhibitted by another condition?

4. If it is a remote rack, have you lost communication to your main rack?

Sorry about the lamp question.

1.No, the lights do not indicate they are receiving power.

2.Yes, the nuetral is tight and a true nuetral.

3.We are using RSLogix5 and there are no inhibiters that are seen.

4.We also tried an auto config in RsLogix to make sure it was still there.

5.Not all bits are acting up. Some other inputs are just fine.

I work straight night shift so excuse the lateness on my replies.

If the input indicator is not lighting, you can forget the PLC itself^[1] and all software issues.

It's a hardware/wiring fault of some kind.

[1] Unless the input circuit of the PLC is faulty, which seems unlikely as it happens on several cards, including new ones. The remaining possibility here is that the PLC input circuits have all been damaged by overvoltage. Is this a possibility in your circuitry? What is the AB type number of the input card?