All brands of PLC  have self-diagnostic functions. The skill of PLC maintenance lies in that when the PLC is abnormal, its self-diagnosis function should be fully utilized to analyze the cause of the fault. For example, if an abnormality occurs in the PLC, first check whether the power supply voltage, the screws of the PLC and I/O terminals and the connectors are loose, and whether there are other faults. Then, according to the indicator status of various LEDs set on the basic unit of the PLC, check whether the PLC itself and the outside are abnormal.
PLC is essentially a computer dedicated to industrial control. Its hardware structure is basically the same as that of a microcomputer.

Steps/methods

1

Tips for PLC maintenance – Power indication ([POWER] LED indication) When power is supplied to the PLC base unit, the [POWER] LED on the surface of the base unit is lit. If the power is turned on but the [POWER] LED does not light, confirm the power wiring. Also, if there is a drive sensor or the like on the same power supply, check for short circuit or overcurrent in the load. If it is not for the above reason, it may be that the PLC is mixed with conductive foreign matter or other abnormal conditions, so that the fuse in the basic unit is blown, and this can be solved by replacing the fuse.

2

Tips for PLC maintenance – error indication ([EPROR] LED flashes) When the program syntax is wrong, for example if you forget to set timer or counter constants, or if there is abnormal noise, conductive foreign matter, etc. The [EPROR] LED flashes, the PLC is in the STOP state, and the outputs all go OFF. In this case, check the program for errors and check for conductive foreign matter and high-intensity noise sources.

3

Tips for PLC maintenance – error indication ([EPROR] LED is on) Check procedure If the [EPROR] LED changes from light to flash, check the program. If the [EPROR] LED is still on, check whether the program operation cycle is too long. Monitor the D8012 to see the maximum scan time.

4

Tips for PLC maintenance – Input instructions Regardless of whether the LED of the input unit is on or off, check if the input signal switch is actually ON or OFF. When the input switch is connected in parallel with the LED light resistance, the PLC can still be input even if the input switch is OFF but the parallel circuit is still conducting. If you use an input device such as an optical sensor, the sensitivity may change due to dirt or other stains on the light-emission/light-receiving area, and the ON state may not be completely entered. The ON and OFF inputs cannot be received within a time shorter than the PLC operation cycle. When different voltages are applied to the input terminals, the input circuit may be damaged.

5

PLC maintenance skills – output indicating unit output LED lights whether bright or off, if the load can not be turned on or off, mainly due to overload, load short circuit or capacitive load inrush current, causing the relay output contact bonding, or Poor contact surfaces cause poor contact.

Precautions

If the power supply is burned during the PLC repair process, after the rectifier bridge filter electrolytic capacitor has been blown open, the fuse is burnt black, need to use a multimeter to check whether the blasting filter capacitor is short-circuited, and check whether other components are burned one by one. If not, you can replace the fuse and filter electrolytic capacitor and power it on. After detecting that each group of power supply is normal, install the whole unit and then use it for power.

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