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Troubleshooting a PLC system down to the faulty component or software issue should take no longer than 20 minutes max.  Much less, if there is an HMI and some Messaging on it.  This is because the PLC it’s self gives you so many trouble shooting tools.  The easiest ones, that come with every PLC, are the Status Lights.

The Status lights are used to check the functionality and fitness of the field devices and the wiring to and from the devices.  In the case of the CPU Status, the health of the CPU.  The Communications Status lights will tell us the health of our network.

Using the Status lights will, in just a few minutes, determine if the problem is in the field wiring/device, the I/O module section, or the program.

EXAMPLE: The Operator comes to you and says: “The Machine is not working!”.

Your first question should be; “The whole machine, or just part of it?” Operator, “The coolant pump doesn’t come on when I hit the Start button.”

From this statement alone, we know that the problem is isolated to a small section of the equipment, and not a faulty PLC.  If the PLC was at fault (or Faulted) nothing would work.  Checking the CPU Status lights will confirm this.  Expect to see a GREEN CPU Status light. If just the pump is not working.  If you have anything other than a steady Green Status light on the CPU lights, your problem is bigger than just an I/O fault.

This is a typical problem.  Let’s look at how the status lights can help troubleshoot this problem.  Here are two typical Input circuit configurations.  Where the Status light is in the circuit can make a difference.  Finding out where it is wired in the circuit might be difficult to find out, so our procedure will eliminate the need to know.  But just so that you understand the difference we will cover it.

 

In this circuit the Status light is on the full Input voltage side of the circuit.  This will test the actual wiring/device condition.  It does not check the circuitry inside the input module.

 

In this circuit the Status light is on the CPU side of the circuit.  This will test the actual wiring/device condition and most of the circuitry in the Input module.

In general, if we activate the field device the associated Status Light should turn ON or OFF accordingly.  If the field device is N.C. (Normally Closed) the status light should be ON in its normal state, and go OFF, when the device is actuated.

If the field device is N.O. (Normally Open) the status light should be OFF in its normal state, and go ON, when the device is actuated.

In this case we could check the Start Button and the associated field wiring by having the operator push the Start button.  The Status light should go on.  If it does, we immediately know that the Start button and the wiring are just fine.

If the Status light does not come on, we would check the voltage at the Input terminal for the Start button.  If there is no change in voltage as the Start button is pushed we have confirmation that it is a field device/wiring issue.

If the light and the voltage match the operation of the Status light, then you can eliminate an Input hardware problem.  Now we can move to the Outputs.  Again, we have a Status Light that we can check.  As with the Inputs, the position of the Status Light in the Circuit makes a difference.

In this circuit the Status light is on the CPU side of the circuit.  It will simply tell us that the CPU has told the Output to turn “ON”. It does not tell us that the Output circuitry did actually turn ON.

In this circuit the Status light is on the Real World side of the circuit.  It will tell us that the actual Output device (Relay, Transistor, Triac, etc) has actually turned “ON”. It does not tell us that the actual Output device has turned ON.

In our example we would look on the I/O diagram/schematic for the address of the Status light for the Coolant Pump contactor.   When the Operator pushes the Start button, does this Output Status light come ON?  If the light comes ON, the program has seen the Start signal, and the program has turned ON the pump output.

If the pump did not come ON, then the problem must be on the Output section.  At this point we would use our meter and check the voltage at the Output terminal.  It should be equal to the Supply Voltage when the Status light is ON and 0V when the Status light is OFF.

If the Status light is ON, and the voltage at the terminal is 0V, there are a couple of things you can check.  Place your meter on the SUPPLY terminal and make sure the module is getting voltage.  If not check why.

If the Output module supply is at the right voltage, replace the Output module.

If the Voltage is correct, and goes ON and OFF in conjunction with the Status Light, the problem is in the field wiring/device.

If the Output Status light is OFF and there is zero volts on the Terminal, and the field device is off, all is well.  If the Status light is ON and there is Supply voltage at the terminal and the device is on, again things are good.

If the Status Light is ON, and there is supply voltage at the terminal, but the field device is OFF, there is a problem with the Field device or the wiring going to it.