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Basic knowledge of PLC Module : Preliminary detection of common faults of PLC hardware module

2019-07-22

Basic knowledge of PLC Module: preliminary detection of common faults of PLC hardware module


PLC generally consists of CPU, memory, input/output module, communication interface and power supply and other hardware components, for the program part, the failure of the PLC hardware can be seen more intuitively. Replace the modules is the maintenance method. Fault detection can basically be found by the indicator lights of each module.

Let's take a look the treatment of hardware failure, Be sure to power down before replacing the module.

Power module failure

When A PLC normal power in working process, power module indicator lights such as "AC"," 24VDC ", "5VDC", "BATT" is green long bright, if a lamp constantly flashing or directly put out, then you can analyze where is the problem. "AC" lamp: the main AC power supply of PLC. When the "AC" lamp is power off, there is probably no power supply, and the whole PLC stops.At this point, we should check whether the power fuse is blown.Fuse replaced with the same type of fuse of the same specification.A fast fuse with the same current should be used when the same type of inlet fuse is not available.If the fuse repeatedly burned, the circuit board short circuit or damage, we should replace the entire power supply.

"5VDC" and "24VDC" lights: if off, there is no corresponding dc power output.The indicator light flashes when the power supply deviation exceeds 5% of the normal value.At this time, although the PLC can still work normally, but should pay attention to it , shut down for maintenance when necessary.


"BATT" lamp: this lamp is the backup power indicator, green means normal, yellow means low, red means fault.When the yellow light is on, replace the spare battery in time.The PLC manual states that lithium batteries should be replaced every two to three years.When the red light is on, it indicates that the standby power system fails and the whole module needs to be replaced.


I/O module failure

The input module is generally composed of photoelectric coupling circuit.The output module has relay output, transistor output, photoelectric output, etc., depending on the model number.Each input and output point has corresponding led indication. If there is an input signal but the point does not light up, or if there is an output signal but the output lamp does not light up, we should suspect that there is something wrong with the I/O module.

Input and output modules have 6 to 24 points, it is not economical to replace the entire module just because one point is broken.It is the common practice to find alternative points and then change the corresponding address in the program.


CPU module failure

CPU modules often include communication interfaces, EPROM slots, running switches, etc., where faults are more subtle and therefore harder to identify.CPU as the most important part of PLC, the cost of replacing CPU module is very high, so we should be very careful when analyzing and judging the fault.

Maintenance example: when the PLC is connected to the power supply, the switch cannot switch to the RUN state.The error light goes on first and then on.After the power failure is reset, the fault still exists.After changing the CPU module, it will work normally.

During the chip level maintenance, the CPU module was replaced, but the fault indicator continued to flash, and the function returned to normal after the communication interface was replaced.

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