P0926AH-B P0924JH FOXBORO Remote Output/Input Module

Description

Any PLC has self diagnosis function, and when the PLC is abnormal, its self diagnosis function should be fully utilized to analyze the cause of the fault. When there is an abnormality in the PLC, first check the power supply voltage, whether the screws and connectors of the PLC and I/O terminals are loose, and whether there are any other abnormalities. Then, based on the status of various LED indicators set on the basic unit of the PLC, check whether there are any abnormalities in the PLC itself and externally.

The following is an example of using the FX series PLC to illustrate the method of diagnosing the cause of PLC faults based on the status of LED indicators.

1. Power indicator ([POWER] LED indicator)
When supplying power to the PLC basic unit, the [POWER] LED indicator light set on the surface of the basic unit will light up. If the power is turned on but the [POWER] LED indicator light is not on, please confirm the power wiring. In addition, if the same power supply has drive sensors, please confirm whether there is a load short circuit or overcurrent. If it is not for the above reasons, it may be due to the inclusion of conductive foreign objects or other abnormal situations in the PLC, causing the fuse in the basic unit to blow. At this time, the problem can be solved by replacing the fuse.

2. Error indication ([EPROR] LED flashing)
When a program syntax error occurs (such as forgetting to set a timer or counter constant, etc.), or there is abnormal noise, conductive foreign objects mixed in and other reasons that cause changes in the content of the program memory, the [EPROR] LED will flash, the PLC is in the STOP state, and all outputs will turn OFF. In this case, the program should be checked for errors, as well as for the presence of conductive foreign objects and high-intensity noise sources.

When an error occurs, one of the values 8009, 8060, and 8068 is written to the special data register D8004. Assuming that the content written to D8004 is 8064, the error code can be determined by checking the content of D8064. The actual error content corresponding to the error code can be found in the error code table in the PLC user manual.

3. Error indication ([EPROR] LED light on)
Due to the mixing of conductive foreign objects inside the PLC or the influence of external abnormal noise, the CPU loses control or the calculation cycle exceeds 200ms, resulting in a WDT error. The [EPROR] LED light is on, the PLC is in STOP, and all outputs become OFF. At this point, a power outage reset can be performed. If the PLC returns to normal, please check for any abnormal noise sources and the mixing of conductive foreign objects. Additionally, please check if the grounding of the PLC meets the requirements.

If there is a change in the [EPROR] LED light from on to flashing during the inspection process, please perform a program check. If the [EPROR] LED remains on all the time, please confirm if the program calculation cycle is too long (monitor D8012 to determine the maximum scanning time).

If after all inspections, the [EPROR] LED light cannot be released, it should be considered that there is some internal fault in the PLC. Please contact the manufacturer. Japanese American Network.

4. Input indication
Regardless of whether the LED light of the input unit is on or off, please check whether the input signal switch is indeed in the ON or OFF state. If the rated current capacity of the input switch is too large or due to oil intrusion or other reasons, it is easy to cause poor contact. When the input switch and LED light are connected in parallel with a resistor, even if the input switch is OFF but the parallel circuit is still connected, the PLC can still be input. If input devices such as light sensors are used, the sensitivity may change due to dirt adhering to the emitting/receiving parts, which may not fully enter the “ON” state. Unable to receive ON and OFF inputs in a shorter period of time than the PLC operation cycle. If different voltages are applied to the input terminals, it will damage the input circuit.

5. Output indication
Regardless of whether the LED light of the output unit is on or off, if the load cannot be turned on or off, it is mainly due to overload, load short circuit, or impact current of capacitive load, which causes the output contact of the relay to bond or poor contact due to poor contact surface.