PLC troubleshooting


To proceed the basics of PLC troubleshooting. it is assumed that the analyzed PLC system was working correctly at some time in the recent past, so the problems of debugging the program and wiring errors that are more typical of a start situation will not be solved.

It is also assumed that the PLC is programmed using some form of ladder logic and not a higher level language, and the discussion is limited to the most common types of I / O modules namely, those that support digital inputs and outputs and analogue.

Troubleshooting inputs and outputs

The most common problem in the solution of input and output problems, the main objective of solving I/O problems is to find out why the internal state of the PLC (what the PLC thinks is happening) does not agree with the external situation (what is really happening).

The first thing to do is to determine the relationship between the physical I / O modules and the I / O instructions in the PLC program. This is done by using the addressing scheme for the particular PLC, this scheme differs from one manufacturer to another.

Once this scheme is understood, each problem can be isolated in a single I / O module and a program monitoring device (usually a portable unit, terminal or personal computer) can be used to verify the internal status of the input or exit in question.

Troubleshooting digital input modules:

The function of a digital input module is to determine the ON / OFF status of a signal or signals and communicate that information to the PLC processor. The digital input modules detect changes in voltage levels and are available with various AC, DC or universal ratings, and universal modules generally accept a fairly wide range of AC or DC signals.

There are two types of entries: isolated and not isolated. Problem-solving varies depending on the type you are dealing with. The isolated input module is electrically separated from the others and may have a different power source. On the other hand, one side of each input channel in an uninsulated module is connected to a common reference.

  • If power is present for the input in question, since faults in the wiring and field devices can burn a fuse, trip a switch or cause another power interruption.

  • If there is no input power, determine and rectify the cause of the failure before continuing. If there is input power, connect a voltmeter through the input.

  • Activate the input device in the field and measure the voltage at the input of the PLC to determine if it changes properly when the field device changes state.

  • If it does not, the field device or wiring are most likely at fault.

  • If an adequate voltage change is observed, the power and / or logic indicators in the module should change when the voltage is applied, and the location addressed in the PLC, when monitored with the programming device, should also change state.

  • If the indicators do not correctly reflect the status of the input, replace the input module. If the input module works correctly but the PLC does not register the input internally, the problem lies in the system used to communicate the input information of the module to the processor.

Troubleshooting analog input modules

Instead of monitoring the on / off status of an input, the analog inputs measure the actual value of a voltage or current and communicate it to the processor.

First determine if the input is isolated or not isolated, and determine the source of energy and verify that it is present. Next, change the voltage or current level generated by the field device, verify that the change is reflected at the input module terminals and verify that the address content associated with the input reflects the change in voltage or current.

There are two additional complications introduced by analog modules, however. First, there usually is no indication on the module to reflect the level of the input, so an external meter must be relied upon.

The second is the problem of scale: you must determine what voltage or current range is designed to measure the module and what numerical scale is associated with that range in the PLC.

  • An input with a 1-5 VDC range may be expected to generate a change from 0 to 1000 in a PLC register.

  • The signal transmitter can be connected directly to the input module, and if the module does not respond correctly, it must be replaced.

  • If it responds correctly, it is most likely that the problem is in the field device or the wiring. Field wiring can be tested by temporarily replacing the field device with a signal transmitter and observing the reaction in the PLC to signal changes.

Troubleshooting digital output modules:

The output modules are designed to cause some change in the external world in response to an instruction in the PLC processor. The digital outputs will often be used to perform tasks such as starting engines, turning on indicator lights and activating solenoid valves.

The energy to control PLC outputs, such as inputs, is not usually supplied by the module, so it is important to find out where that power comes from.

Faults in field wiring and devices can burn that fuse, so you should check its condition before continuing. Many modules are equipped with a “blown fuse” indicator that shows which channel or module has a blown fuse.These fuses may be accessible from the front of the module, or the module may have to be removed or even disassembled in order to gain access to them.

  • The programming device must be connected to the PLC, and the address associated with the output in question must be determined. The output can be “forced” ON or OFF internally in the PLC, and the module can be observed for a reaction.

  • If the indicators in the module do not reflect the forced condition, change the output module.

  • If the indicators are observed to be reacting to the forced state, measure the voltage across the output device to see that it’s changing as the state of the output changes.

  • If the voltage is changing but the device is not reacting, the problem is in the output device. If the voltage is not changing, the problem can most likely be found in the field wiring.

If field wiring is in doubt, it can be temporarily disconnected and a test load can be connected to the module. If the test load is working correctly, the problem lies in the field wiring or the field device. It is important that a test load be used instead of simply disconnecting the field wiring.

Troubleshooting analog output modules:

Analog outputs are used to generate a variable voltage or current that is normally used to perform tasks such as speeding up a variable speed drive, adjusting the position of a control valve or driving a panel indicator.

  • You must determine what range of voltage or current the module is designed to produce and what numerical scale is associated with that range in the PLC. An output with a 4-20 mA DC range may be expected to react to a change from 0 to 1000 in a PLC

  • A good approach to testing analog outputs is to “force” the number in the PLC register associated with the output in question to minimum, half scale, and maximum values, and to measure the voltage or current generated at the output.