MITSUBISHI ELECTRIC MELSEC A-series PLC Hardware Maintenance and Troubleshooting

MITSUBISHI ELECTRIC MELSEC A Series PLC Hardware Maintenance and On site Troubleshooting Complete Guide

In the field of industrial automation, Mitsubishi Electric's MELSEC A-series PLC has been the core controller for many factory production lines for decades due to its modular design, high reliability, and rich I/O expansion capabilities. Despite the subsequent launch of new generation products such as the Q series and iQ-R series, a large number of A series (especially compact AnS series such as A1SCPU, A2SHCPU, etc.) are still in service. For maintenance engineers, it is an essential skill to systematically troubleshoot faults in the power supply, substrate, I/O modules, and even the CPU itself when facing an old PLC that suddenly shuts down, has abnormal indicator lights, or experiences communication interruptions.

This article is based on the core content of the A-series hardware manual, extracting a complete practical guide from installation specifications, power and I/O wiring, daily maintenance to fault code interpretation, aiming to help engineers quickly locate problems on site and take correct measures.

Installation and Environment: The First Line of Defense

The A-series PLC belongs to the Open Type equipment and must be installed in a control cabinet with a protection level of at least IP54 to prevent electric shock and foreign objects from entering. Special attention should be paid to the following points during installation, as many early failures stem from this.

1.1 Substrate installation and heat dissipation

Use M4 screws to fix the substrate on a flat mounting surface with a tightening torque of 78-118 N · cm. Uneven installation surfaces can cause deformation of printed circuit boards, leading to intermittent faults.

A ventilation gap of at least 30mm (main substrate) or 80mm (extended substrate) should be reserved above and below the substrate. It is strictly prohibited to install horizontally or upside down, and must be installed vertically (with ventilation ducts facing up/down).

Avoid installing the substrate near strong vibration sources such as large capacity contactors and circuit breakers without fuses. It is recommended to install these devices on another panel or maintain a distance of at least 100mm.

1.2 Module installation sequence

Insert the fixed hook at the bottom of the module correctly into the hook hole of the substrate, then press the module towards the substrate direction, and finally tighten the module fixing screw (M4, torque 78-118 N · cm).

Never leave an empty slot on the left side when installing I/O modules in slots with dangerous voltages of 100/200VAC. If there is an empty slot, a blank cover plate (A1SG60) must be installed, otherwise the exposed circuit board may cause electric shock or short circuit.

1.3 EMC Compliance Precautions (CE Mark)

For applications that require compliance with EMC directives, conductive control cabinets must be used, and the cabinet and doors must be grounded with low impedance through braided grounding wires. The LG (wire grounding) and FG (frame grounding) terminals of the power module must be connected to the same grounding point using the shortest wire (<30cm). Signal cables must use shielded cables, and the shielding layer must be grounded through a large area of metal clamps (rather than through a thin wire) to effectively suppress high-frequency noise.

Power module: selection, wiring and protection

The power module is the energy source of the entire PLC system. The AnS series offers a variety of power modules, and incorrect selection or wiring can directly cause the system to fail to start or even be damaged.

2.1 Power module selection table (key parameters)

Model Input Voltage Output (5VDC) Output (24VDC Auxiliary) Applicable Scenarios

A1S61P 100-120VAC 5A without North American/Japanese standard voltage

A1S62P 100-120VAC 3A 0.6A requires 24VDC sensor power supply

A1S63P 24VDC 5A without DC power supply system (note: requires SELV power supply)

A1S61PEU 200-240VAC 5A without European/Asian standard voltage, CE certification

A1S61PN 100-240VAC 5A No Wide Voltage Input, CE Certified

A1S62PN 100-240VAC 3A 0.6A wide voltage input with 24VDC output

Key selection points:

Calculate the total 5VDC current consumption of all modules on the substrate, which should not exceed 80% of the rated value of the power module.

If 24VDC is required for external sensors such as proximity switches and photoelectric switches, models with 24VDC output (such as A1S62P/A1S62PN) should be selected, but the 24VDC output capacity is limited (usually 0.6A) and cannot be used to drive high-power loads.

It is absolutely forbidden to connect the 24VDC outputs of two power modules in parallel to supply power to the same I/O module, as this can cause overheating or even fire in the power modules.

2.2 Wiring and Grounding

On the input terminals of the power module, the LG and FG terminals must be grounded. For A1S61P/A1S62P/A1S63P, it is necessary to use a short jumper to connect LG and FG before grounding; For new models such as A1S61PEU, there is no need for short circuiting.

To suppress lightning surges, it is recommended to parallel surge absorbers (varistors) at the power input end, with a maximum allowable voltage higher than the power input voltage. The grounding point (E1) of the surge absorber should be set separately from the grounding point (E2) of the PLC.

2.3 Overcurrent and Overvoltage Protection

Overcurrent protection: When the output current of 5VDC or 24VDC exceeds the specified value, the protection circuit will cut off the output, and the LED indicator light of the power module will turn off or dim. At this point, it is necessary to eliminate the cause of overload (such as output module short circuit, substrate pin bending, etc.), and then power on again.

Overvoltage protection: If the 5VDC voltage rises to 5.5-6.5V, the protection circuit will activate and turn off the output, and the LED will turn off. At this point, it is necessary to turn off the power and then turn it back on; If it still cannot be restored, the power module is damaged and needs to be replaced.

I/O module wiring and diagnosis

The I/O module is a bridge that connects PLC with on-site sensors and actuators. The manual provides detailed specifications for various input and output modules, among which wiring errors and external faults are the most common non PLC faults on site.

3.1 Common wiring issues with input modules

The A series offers two main categories: AC input (100/240VAC) and DC input (12/24VDC). Key parameters:

ON/OFF voltage threshold: For 24VDC input modules (such as A1SX40), the ON voltage should be ≥ 14VDC and the OFF voltage should be ≤ 6.5VDC. If the on-site signal is located in the critical zone due to line voltage drop or power fluctuation, it will cause unstable input state.

Response time: The OFF → ON and ON → OFF response times of the DC input module are usually 10ms (there are also high-speed models such as A1SX41-S2 with 0.1ms). When high-speed counting is required, high-speed type must be selected.

Maximum number of simultaneous input points: For a 100VAC input module (such as A1SX10), it can be turned on 100% at 110VAC, but only 60% at 132VAC. Exceeding this limit can cause the module to overheat.

Wiring example: For a sink type DC input module, the common terminal is connected to 0V and the input signal is connected to 24V+. For a source type module, the opposite is true. Be sure to confirm the polarity of the module, reverse connection will not damage it but the signal cannot be read.

3.2 Selection and Protection of Output Modules

The A-series output modules are divided into relay output, thyristor output, and transistor output.

Output type characteristics, typical models, and precautions

Relay is universal for both AC and DC, with high load current (2A/point) and slow response (10-12ms). A1SY10 has a limited lifespan and needs to be downgraded for frequent switching

Thyristors only communicate AC, have no contacts, respond quickly (1ms), have leakage current A1SY22, suitable for resistive loads such as heaters, and inductive loads require RC absorption

The transistor is only DC and has a high-speed response (0.1-2ms), which can meet the positioning requirements of A1SY40. Pay attention to the load current (0.1A or 0.5A), overload will burn out

Important protective measures:

When the relay output module drives inductive loads (such as contactor coils), it is necessary to parallel freewheeling diodes (DC) or varistors (AC) at both ends of the load, otherwise the high-voltage back electromotive force generated during shutdown will burn out the relay contacts.

The transistor output module itself has short-circuit protection, but long-term overload can cause damage to internal components. If the "ERR." LED of the module lights up, it indicates that the external power supply is missing or the output circuit is faulty.

For output modules with fuses (such as A1SY10), if the fuse is blown, the fuse blow indicator light on the module panel will light up, and the CPU will detect the "FUSE BREAK OFF" error (error code 32). At this point, it is necessary to replace the fuse with the same specification and investigate the cause of the output short circuit.

3.3 Isolation and Safe Voltage of I/O Modules

For I/O modules with rated voltage ≥ 50VAC or ≥ 75VDC (such as 100VAC input modules), reinforced insulation is used between them and the internal circuits of the PLC to meet the requirements of the Low Voltage Directive (LVD). The left and right sides of these modules must be covered with blank covers or adjacent modules to prevent contact with dangerous voltages.

All 24VDC I/O modules are Safety Extra Low Voltage (SELV), but must be powered by a power supply that meets SELV requirements and must not be mixed with hazardous voltages in the same slot.

CPU module: indicator lights, switches, and self diagnosis

The CPU module is the core of the system. By understanding the panel indicator lights and button functions, one can quickly determine the type of fault without using programming software.

4.1 Interpretation of indicator light status

Meaning and Countermeasures of LED Status

The POWER power module is not outputting or the CPU module is not installed properly. Check the power module indicator light and substrate connection.

RUN is constantly on and executing the program normally.

Flashing causes a program execution error (such as watchdog timeout) or is performing a latch clear operation. Error code needs to be read.

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

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.