MITSUBISHI ELECTRIC MELSEC A-series PLC Hardware Maintenance and Troubleshooting

Common reasons and countermeasures for error codes and incorrect names

The result of 50 Operation ERROR operation exceeds the range (such as BCD conversion result>9999), or the device number exceeds the specified range. Check the use of BCD conversion instructions and index registers in the program.

70 BATTERY ERROR The battery voltage is too low or not connected. Replace the battery (when powered on).

Daily and Regular Maintenance: Extend the Life of PLC

The maintenance plan should include daily inspections and semi annual/annual inspections.

6.1 Daily Inspection (Visual)

Power LED: Is the POWER on.

RUN LED: Is it constantly on (should not flash or turn off).

ERROR LED: Whether it is off (whether it is constantly on or flashing needs to be investigated).

I/O LED: Is the input/output indicator light consistent with the actual physical state. For example, after pressing the emergency stop button, the corresponding input LED should turn off; It should light up after being released.

6.2 Regular inspection (every 6-12 months)

Environment: The temperature inside the control cabinet is between 0-55 ℃, the humidity is between 10-90% RH, and there is no condensation or corrosive gas.

Wiring: Check if the terminal screws are loose (torque M3.5=59-88N · cm), and if there is dust or wire debris in the cable tray.

Battery: Use programming software to monitor the status of M9006/M9007. Once these two signs turn ON, the battery should be replaced within a few weeks. If the battery has exceeded its nominal lifespan (usually 3-5 years), it should be replaced proactively even if there is no alarm.

Substrate and module: Check if the fixing screws of the module are loose, and if there are any burnt or deformed marks on the substrate.

6.3 Battery replacement steps (standard procedure)

Keep the CPU powered on (or complete within 5 minutes after power failure).

Open the battery cover on the front of the CPU.

Remove the old battery and be careful not to short-circuit.

Insert the new battery (A6BAT) and ensure that the connector is securely inserted.

Close the lid.

(Optional) Reset battery error: Clear the error in programming software or switch from STOP to RUN.

Troubleshooting Example: A Typical WDT Error

Phenomenon: An injection molding machine using A2SHCPU suddenly shuts down after running for about 2 hours, with the CPU's RUN LED flashing and ERROR LED constantly on.

Troubleshooting steps:

Connect the CPU using programming software (such as GX Developer) and read the error code (D9008)=22 (WDT ERROR).

Viewing scan time (D9012): The normal value is 40ms, but a peak of 220ms was recorded before the error occurred.

Analysis program: It was found that a program used a large number of floating-point operations (due to product formula calculation), and the program was executed every scan cycle, with the annotation indicating "formula calculation".

Solution: Put the computational program into a subroutine and use a "jump instruction" (CJ) in conjunction with a timer contact (e.g. executed every 1000 scans) instead of executing every cycle. The maximum scanning time has been reduced to 45ms after modification, and errors have been eliminated.

Deep root cause: The computation speed of the old CPU is slow (A2SHCPU is 0.25 μ s/step, but complex operations still require multiple cycles), and when the fluctuation of process data leads to a sudden increase in computation, the scanning time exceeds the watchdog threshold.