We have compiled a list of 8 common error types that PLC repair personnel need to be aware of when a PLC presents an abnormal alarm.
I. CPU Abnormalities
When the CPU alarms abnormally, check all devices connected to the internal bus of the CPU unit. The specific method is to replace potentially faulty units one by one to identify the problematic unit and then take appropriate action.
II. Memory Abnormalities
If a memory error alarm occurs and the problem is with the program memory, and reprogramming from scratch does not resolve the issue, the problem may be caused by noise interference altering the program. Otherwise, the memory should be replaced.
III. Input/output unit anomalies, expansion unit anomalies
When this type of alarm occurs, you should first check the connection status of the input/output unit and expansion unit connectors, as well as the cable connection status, to determine which unit is causing the problem, and then replace the unit.
IV. Procedure not executed
Under normal circumstances, you can check the process by following the sequence of input – program execution status – output.
(1) Input checking is done using input LED indicators or an input monitor constructed with a programmer. When the input LED is not lit, it can be initially determined that the problem is with the external input system, and then checked with a multimeter. If the output voltage is abnormal, it can be determined that the problem is with the input unit. When the LED is lit but there is no display on the internal monitor, it can be considered that the problem is with the input unit, CPU unit, or expansion unit. (2) Program checking is done through the monitor on the programmer. When the contact status of the ladder diagram is inconsistent with the result, it is a program error (such as the use of two layers of internal relays), or a problem with the arithmetic section.
(3) Output can be checked using the output LED indicator. If the calculation result is correct but the output LED indicates an error, it can be considered a problem with the CPU unit or I/O interface unit. If the output LED is lit but there is no output, it can be determined that there is a problem with the output unit or the external load system.
Because different PLC models have different methods for connecting I/O and LEDs (some connect to the I/O unit interface, and some connect to the I/O unit itself), the problem scale identified based on the LEDs also varies.
V. Some procedures are not executed.
The inspection method is the same as the previous one. However, if the input time of the counter, stepper controller, etc. is too short, there will be no response problem. In this case, the input time should be checked to see if it is large enough. The check can be performed according to the relationship between the input time (maximum response time of the input unit + operation scan time multiplied by 2).
6. If the power supply is interrupted for a short period of time, the program content will also be lost.
(1) First check if there is a problem with the battery.
(2) Check by repeatedly turning the PLC power supply on and off. To ensure the microprocessor starts correctly, the PLC has an initial reset circuit and a program saving circuit when the power is off. If this circuit malfunctions, the program cannot be saved. Therefore, you can check by turning the power supply on and off. (3) If the battery abnormality alarm still occurs after replacing the battery, it can be determined that it is caused by an abnormal increase in leakage current in the memory or external circuit.
(4) The switching on and off of the power supply always occurs synchronously with the machine system. At this time, the noise generated by the machine system can be checked. Since the disconnection of the power supply is a problem that often occurs at the same time as the operation of the machine system, most of the time it is caused by the strong noise generated by the motor or winding.
7. PROM is not working
First, check if the PROM connection is good, then determine if the chip needs to be replaced.
8. The operation stops after the power is restarted or reset.
This problem can be attributed to noise interference or poor internal contact within the PLC. Noise is generally caused by reduced capacitance of small capacitors on the circuit board or malfunctioning components. Poor contact can be checked by gently tapping the PLC body, as well as by inspecting the connections of cables and connectors.
