1
CPU Abnormality:
When a CPU abnormality alarm occurs, check all devices connected to the internal bus of the CPU unit. The specific method is to replace the potentially faulty units one by one to identify the faulty unit and take appropriate action.
2
Memory Abnormality:
When a memory abnormality alarm occurs, if it is a problem with the program memory, the fault will reappear after reprogramming. This situation may be caused by noise interference affecting the program, otherwise, the memory should be replaced.
3
Input/Output Unit Abnormality, Expansion Unit Abnormality:
When this type of alarm occurs, first check the insertion status of the input/output unit and expansion unit connectors, as well as the cable connections. After determining which unit has failed, replace the unit.
4
Program Not Executing:
Generally, you can check according to the steps of input — program execution — output.
(1) Input check is performed using the input LED indicator or an input monitor constructed with a programmer. When the input LED is not lit, it can be initially determined that there is a fault in the external input system, followed by a multimeter check. If the output voltage is abnormal, it can be confirmed that the input unit is faulty. When the LED is lit but there is no display on the internal monitor, it can be considered a fault in the input unit, CPU unit, or expansion unit.
(2) Program execution check is performed through the monitor on the programmer. When the status of the ladder diagram contacts does not match the results, it indicates a program error (e.g., double usage of internal relays) or a fault in the computation part.
(3) Output check can be identified using the output LED indicator. If the computation result is correct but the output LED indicates an error, it can be considered a fault in the CPU unit or I/O interface unit. If the output LED is lit but there is no output, it can be judged as a fault in the output unit or a failure in the external load system.
Additionally, due to different PLC models, the connection method of I/O and LED varies (some connect to the I/O unit interface, while others connect to the I/O unit). Therefore, the fault range judged based on the LED also differs.
5
Partial Program Not Executing:
The checking method is the same as the previous item.
However, if the input time for devices such as counters and step controllers is too short, a non-response fault may occur; at this point, the input time should be verified to ensure it is sufficiently large. The verification can be conducted according to the relationship of input time < input unit’s maximum response time + computation scan time multiplied by 2.
6
Short Power Outage Causes Program Content to Disappear:
(1) At this time, in addition to checking the battery, the following checks should be performed.
(2) Check by repeatedly turning the PLC power on and off. To ensure the microprocessor starts correctly, the PLC has a reset point circuit and a program saving circuit when the power is turned off. If these circuits fail, the program cannot be saved. Therefore, the power on and off can be used for checks.
(3) If a battery abnormality alarm still appears after replacing the battery, it can be determined that the fault is due to abnormal leakage current in the memory or external circuit.
(4) The power on and off is always synchronized with the machine system, at this point, check for noise interference from the machine system. Because the power disconnection often occurs simultaneously with machine system operation, most of the time it is caused by strong noise generated by motors or windings.
7
PROM Cannot Operate:
First check whether the PROM is properly inserted, then determine whether the chip needs to be replaced.
8
Action Stops After Power Reset or Restart:
This fault can be considered caused by noise interference or poor internal contact of the PLC. The noise is generally caused by reduced capacitance of small capacitors in the circuit board or poor performance of components. Poor contact can be checked by gently tapping the PLC body. Also, check the insertion status of cables and connectors.
A typical PLC system includes a field PLC station and a host computer connected via a high-speed data line, as well as a simulation screen PLC station. The host computer is used to display various graphics and data, and the simulation screen PLC station is used to drive the light-emitting diodes on the simulation screen. The entire PLC system connects with external devices to form an automatic control system.
Typically, PLCs are treated as a black box, and we can simply determine the fault location based on I/O signals. There are two situations for determining faults: the flashing fault signal on the simulation screen and the running device having no display on the simulation screen.
1Flashing Fault Signal on Simulation Screen
According to the PLC control station diagram, first check the display status of the device in the simulation screen PLC cabinet. If it matches, then check the display status in the field PLC cabinet. If it matches again, continue checking the I/O terminals of the PLC cabinet and the I/O terminals of the external devices, thus inferring whether it is a device fault or a PLC fault. The above process can be represented by the following flowchart.
The method to check whether the PLC cabinet I/O terminals and the external device I/O terminals match the status signal is very simple; just measure the voltage value of the terminal number and the common terminal with a multimeter’s DC voltage range. 24V indicates disconnection, no signal; 0V indicates connection, there is a signal.
2The Running Device Has No Display on the Simulation Screen
At this point, it should be determined whether the PLC has not provided a running signal or has provided a running signal but the device cannot run due to a fault.
We can observe whether there is a signal display from the output module address of the field PLC cabinet, then check whether the output relay of the PLC station is actuating, and finally check whether there is a drive signal in the electrical cabinet of the external device. If there is, but the device does not run, then the device has a fault. If the device runs normally, then we should check back from the input end of the external device, the process is exactly the opposite of the first fault checking process. The above process can be represented by the following flowchart.
If the device is running, check according to the following flowchart.
For detecting analog signals, since the instrument uses a 4~20mA input, a multimeter should be connected in series at the analog signal input end to detect the current value of the analog signal and compare it with the PLC output value to see if the value is correct.
A simple method to quickly determine whether it is a PLC fault or an electrical equipment fault is to use the short circuit method: disconnect the input line of the external device status, connect an input port to the common line with a wire, which means giving the PLC a connection signal. If the PLC shows a display, it is normal; otherwise, it is a PLC fault.
After locating the fault point, appropriate measures should be taken. Generally speaking, the likelihood of a PLC fault is low; most of the fault causes are loose wiring, incorrect connections, or relay faults, and there are cases of burnt PLC templates, in which case the PLC template can only be replaced. Remember to always work with the power off; otherwise, it is easy to burn good templates and may also affect the PLC processor.
