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1
CPU Abnormality:
When the CPU malfunctions, check all components connected to the internal main line of the CPU. The specific method is to sequentially replace components that may cause the fault, identify the faulty component, and take appropriate action.
2
Storage Device Abnormality:
When an alarm occurs for the storage device, if it is a problem with the program storage device, the fault may reoccur after recompiling. This situation may be caused by noise interference affecting the program, otherwise, the storage device should be replaced.
3
Input/Output Module Abnormality, Expansion Module Abnormality:
When such an alarm occurs, first check the insertion status of the input/output module and the expansion module connectors, the connection status of the cables, and determine which module has the fault before replacing it.
4
Program Not Executing:
Generally, you can check the input-output steps by using the input LED indicators or using a programmer to check the input monitor. When the input LED is off, it may indicate a fault in the external input system, and a multimeter should be used for further checks. If the output voltage is abnormal, it can be determined that there is a fault in the input module. When the LED is on but the internal monitor shows nothing, it can be concluded that there is a fault in the input module, CPU module, or expansion module.
(1) The input check is done using the input LED indicator or by using a monitoring device made up of the input components. When the input LED is not lit, it can be preliminarily determined that there is a fault in the external input system, and then a multimeter should be used for checks. If the output voltage is not normal, it can be determined that there is a fault in the input module. When the LED is lit but the internal monitor shows nothing, it can be recognized as a fault in the input module, CPU module, or expansion module.
(2) The program execution check is done through the monitoring device on the programmer. When the state of the ladder diagram contacts does not match the results, it indicates a programming error (such as internal relay dual use, etc.), or a fault in the calculation unit.
(3) The output check can use the output LED indicator. When the calculation result is correct but the output LED indicates an error, it can be recognized as a fault in the CPU module or the 1/0 interface module. When the output LED is on but there is no output, it can be determined that there is a fault in the output module or the external load system.
Additionally, due to different PLC models, the connection methods for 1/0 and LED may vary (some connect to the 1/0 module interface, while others connect to the 1/0 module). Therefore, the fault range determined based on the LED also differs.
5
Partial Program Not Executing:
The checking method is the same as the previous item. However, if the time for the counter, step controller, etc., is too short during input, a non-response fault may occur. At this time, it should be verified whether the input time is sufficient, and the verification can be done by input time < maximum response time of the input module + calculation scan time multiplied by 2.
6
Power Short Interruption, Program Content May Also Be Lost:
(1) At this time, in addition to checking the battery, the following checks should also be performed.
(2) Check the PLC’s power supply by repeatedly interrupting it. To ensure that the microprocessor starts correctly, the PLC has initial reset point circuits and program saving circuits when the power is disconnected. If this circuit fails, the program cannot be saved. Therefore, it is necessary to check the availability of power supply.
(3) If the battery abnormal alarm still occurs after replacing the battery, it can be determined that it is due to the leakage current of the storage device or the external circuit.
(4) Power interruptions usually occur simultaneously with mechanical systems. At this time, check the noise generated by the mechanical system. The power disconnection is often caused by strong noise generated by motors or windings.
7
PROFIBUS Not Operating:
First, check whether the PROFIBUS connector is good, and then determine whether the chip needs to be replaced.
8
Power Re-injection or Reset, Operation Stops:
This fault can be recognized as interference from noise or poor internal contact of the PLC. The cause of noise is usually due to the reduction of capacitance in the circuit board or poor performance of components. Poor contact can be checked by lightly tapping the PLC body. It is also necessary to check the insertion status of the cables and connectors.
A typical PLC system includes a field PLC station, and a host computer connected via high-speed data lines, as well as a simulated screen PLC station. The host computer is used to display various graphs and data, while the simulated screen PLC station drives the light-emitting diodes on the simulated screen. The entire PLC system connects to external devices, forming an automatic control system.
Commonly, the PLC is treated as a black box, and we can simply determine the fault location based on the I/O signals. There are two situations for determining faults, namely the flashing fault signal on the simulated screen and the equipment running without display on the simulated screen.
1Flashing Fault Signal on the Simulated Screen
According to the PLC control station diagram, first check the status of the equipment displayed in the simulated screen PLC cabinet. If it matches, check the status of the on-site PLC cabinet. If it also matches, continue checking the I/O terminals of the PLC cabinet and the I/O terminals of the external devices, and deduce whether the fault is in the device or the PLC.
Determining whether the PLC cabinet I/O terminals and the external devices I/O terminals match the status signals is very simple: just measure the voltage value with a multimeter. If it shows 24V, it indicates disconnection, with no signal; if it shows 0V, it indicates connection, with a signal.
2The Equipment Running Has No Display on the Simulated Screen
At this time, it should be determined whether the PLC did not provide a running signal, or if it provided a running signal but the equipment cannot run due to a fault.
We can observe whether there is a signal displayed in the output module address of the on-site PLC cabinet, then check whether the output relay of the PLC station has a pull-in, and then check whether the external device’s electrical cabinet has a drive signal. If there is a signal but the device is not running, then it indicates that the device is faulty. If the device is running normally, then we should check the input from the external device back.
If the device is running, then check according to the following framework.
For testing simulated signals, since the instrument uses 4~20mA input, a multimeter should be connected in series at the simulated signal input terminal to measure the current value and compare it with the output value of the PLC to determine whether the value is correct.
There is a simple method to quickly determine whether the fault is with the PLC or with the electrical equipment, which is to use the short circuit method: disconnect the external device status input line, connect a wire between the input terminal and the common line, which means giving the PLC a connected signal. If the PLC shows a signal, then the PLC is normal; otherwise, it indicates a PLC fault.
After finding the fault point, appropriate measures should be taken. Generally speaking, the possibility of a PLC fault is relatively small. Most of the causes of faults are due to loose connections or incorrect wiring, or faults in relays. There may also be cases where the PLC board is burned out, in which case the PLC board must be replaced. Remember to always disconnect power during operations, otherwise, it may easily burn out a good board or may also involve the PLC processor.
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