MITSUBISHI ELECTRIC MELSEC A-series PLC Hardware Maintenance and Troubleshooting

Turn off STOP mode, remote STOP status, or the bottom switch is in the STOP position.

ERROR is constantly on, and self diagnosis has detected a serious error (such as instruction code error, parameter error, WDT error). Need to read the error code of special registers such as D9008.

The F (alarm) coil in the flashing program is set to ON, indicating that a user-defined warning condition has been triggered, which does not affect program execution but should be noted.

4.2 RUN/STOP/LCLR switch and latch clearing

RUN/STOP: Used for manually starting/stopping program execution. When debugging on site, one should develop the habit of "stopping the CPU first, and then modifying the program or wiring".

L. CLR (Latched Range Clearing): Used to clear data within the latched range. Operation method: When in the STOP position, quickly turn the switch towards L.CLR several times until the RUN LED starts flashing, then turn it towards L.CLR again, and the RUN LED will turn off to complete the clearing. This operation will not clear the program itself, but will erase the data held by the power outage.

4.3 Battery and Memory Maintenance

The CPU module uses lithium batteries (model A6BAT) to hold data (programs, registers, latch data) in RAM. When the battery voltage is low, the special auxiliary relays M9006/M9007 will turn on, or the "BATTERY ERROR" error code 70 will appear.

Battery replacement must be carried out while the PLC is powered on (or quickly replaced within 5 minutes after disconnecting the power), otherwise RAM data will be lost. After replacement, the "BATT" alarm needs to be reset (usually by clearing the error or restarting).

4.4 Remote STOP/PAUSE and RS-232C Communication (A1SCPUC24-R2)

For A1SCPUC24-R2 with computer link function, its front panel has a 9-pin RS-232C interface (D-sub male). The standard wiring is as follows:

2 (RD) ← External device sends data (TXD)

3 (SD) → External device receives data (RXD)

5 (SG) - Signal Ground

4 (DTR) → Data Terminal Ready

6 (DSR) ← Dataset ready

7 (RTS) → Request to send

8 (CTS) ← Clear Send

1 (CD) ← Carrier detection (If external devices do not support it, CD detection can be disabled in the software)

The baud rate is set through the SW05-07 switch on the panel and supports 300 to 19200bps. When encountering communication failure, you can first perform a Self loopback test: short-circuit the sending (SD) and receiving (RD) of RS-232C, use programming software to send data and verify whether the same data is received to confirm whether the CPU's communication hardware is normal.

Error code interpretation: from self diagnosis to on-site countermeasures

When the ERROR LED stays on or the RUN LED flashes, the CPU stores the error code in a special register D9008 (some models also have detailed codes in D9091/D9092). The following table lists the most common error codes on site and their handling steps.

5.1 Program related errors

Common reasons and countermeasures for error codes and incorrect names

The 10 INSTRUCT CODE ERR program contains instruction codes that the CPU cannot recognize. The possible reason is that the memory card is damaged or the instructions are corrupted by noise. Read out the error steps using programming software, correct the program or replace the ROM.

11 Parameter ERROR parameter setting error or poor memory card contact causing parameter reading failure. Rewrite the parameters and check if the memory card is securely installed.

There is no END instruction at the end of the program. Add 'END' at the end of the program.

The destination pointer (P) of the 13 CAN 'T EXECUTE (P) jump instruction (CJ/CALL) does not exist or the FOR/NEXT nesting does not match. Check the pointer numbers to ensure that FOR/NEXT appear in pairs.

22 WDT ERROR scan time exceeded the watchdog timer set value (default 200ms). Optimize the program (such as using CJ instructions to skip unnecessary segments), or increase the watchdog setting value; Check for momentary power outages.

5.2 Hardware and configuration errors

Common reasons and countermeasures for error codes and incorrect names

20 RAM ERROR CPU internal RAM read/write test failed. Hardware malfunction, CPU module needs to be replaced.

21 OPE CIRCUIT ERR CPU operation circuit abnormality. Hardware malfunction, replace CPU.

After powering on, it was detected that the configuration of the I/O module was inconsistent with the actual installation (the module was unplugged or replaced). Check and reinstall the correct I/O module, then reset the CPU.

The fuse of the 32 FUSE BREAK OFF output module is blown. Replace the fuse and check if the external output circuit is short circuited.

Control bus communication error between 40 Control-BUS ERR CPU and special function module. It may be a hardware failure of the substrate, CPU, or special module, which needs to be replaced and investigated one by one.

The configuration of the 44 SP. UNIT LAY. ERR special module exceeds the CPU limit (such as installing 3 or more computer link modules, or 2 or more data link modules). Reduce the number of special modules that exceed the limit.

5.3 Battery and operational errors