MITSUBISHI A-series PLC Troubleshooting Replacement

MITSUBISHI MELSEC A-series PLC: Complete Guide to Replacing and Troubleshooting Discontinued Controllers

In the field of industrial automation, Mitsubishi MELSEC A-series (especially AnS series) programmable controllers have long occupied an important position in medium-sized control systems due to their compact structure, high reliability, and rich I/O expansion capabilities. However, as the product lifecycle changes, many A-series models (such as A1SHCPU, A2SHCPU, etc.) have gradually been discontinued, making it difficult to obtain spare parts. When the old controller in the on-site equipment malfunctions or requires functional upgrades, engineers face two major challenges: how to seamlessly replace it with existing or alternative models while ensuring safety and performance; And how to quickly diagnose and eliminate various errors that occur during operation. This article combines the core content of the A-series hardware manual, providing a systematic engineering solution from installation specifications, EMC compliance, wiring points, fault safety design to common error code analysis.

Product Overview and Key Points for Replacement Selection

The MELSEC AnS series includes multiple sub series such as A1S, A2S, A1SH, A2SH, etc. Its CPU module, power module, I/O module, and expansion substrate all adopt a unified structure. When replacing, the following points should be noted:

CPU performance difference: The processing speed of A1SH CPU is 0.33 μ s/step, much faster than A1SCPU's 1 μ s; A2SHCPU is also faster than A2SCPU. If the original system is sensitive to scanning time during replacement, a CPU of the same level or higher performance should be selected.

I/O points: The maximum actual I/O points for A1SHCPU are 256 points, A2SHCPU are 512 points, while A1SCPU only has 256/512 points. When replacing, it is necessary to confirm whether the I/O capacity of the new CPU meets the existing configuration.

File register capacity: A1SHCPU supports a maximum of 8192 points (R0~8191), while A1SCPU only has 4096 points. If the original program uses a large number of file registers, a larger capacity CPU should be selected.

CC Link specific instructions: A1SHCPU and A2SHCPU support 11 CC Link specific instructions (such as RLPA, RRPA, RIRD, etc.), while early A1SCPU did not support them. If a CC Link network has been deployed on site, a CPU with the suffix "H" or A2ASCPU series must be selected.

Memory card compatibility: There are compatibility differences between traditional memory (such as A1SNMCA-2KE) and new models (such as A1SNMCA-2KE), as detailed in Appendix 1 and 2.

Environment and mechanical installation: avoid "hidden" failure

1. Environmental specification requirements

According to Chapter 1 of the manual, A-series PLCs must be installed in the following environment:

Working temperature: 0~55 ℃ (temperature inside the control cabinet)

Storage temperature: -20~75 ℃

Humidity: 10~90% RH, no condensation

Anti vibration: Complies with JIS B 3501 (10~57Hz: amplitude 0.075mm; 57~150Hz: 4.9m/s ²)

Impact resistance: 147m/s ² (15G), 3 times in each of the X/Y/Z directions

Altitude: ≤ 2000m

Pollution level: ≤ 2 (inside the control cabinet, IP54 protection level)

Installation location: inside the control cabinet (open equipment)

Key reminder: If the on-site environment is humid or there is conductive dust, the PLC must be installed in a closed control cabinet with a protection level of not less than IP54, otherwise the insulation performance may decrease and cause short circuits or misoperations.

2. Dimensions and spacing for substrate installation

Substrate mounting hole spacing: Depending on the model, the width ranges from 155mm (A1S52B) to 430mm (A1S38B), and the height is uniformly 130mm.

Heat dissipation gap: Leave at least 30mm (1.18 inches) between the top and bottom of the module and the cabinet or other components. If using A5 | B or A6 | B expansion substrate, the gap must be ≥ 80mm.

Left and right clearance: at least 50mm between the device that generates noise (such as contactors, frequency converters).

Installation direction: It must be installed horizontally (face up), and it is strictly prohibited to install vertically or upside down, otherwise it will affect heat dissipation.

3. Substrate fixation and DIN rail installation

Secure with M4 screws, tightening torque: 78~118 N · cm (8~12 kg · cm). Attention: Screws that are too long may damage the internal circuit board.

If using DIN rails (TH35-7.5Fe/TH35-7.5Al/TH35-15Fe), the spacing between the rail mounting screws should be ≤ 200mm to prevent deformation.

Practical lesson: In a certain automotive parts production line, A2SHCPU frequently experiences "WDT ERROR" (watchdog timeout). Upon inspection, it was found that the substrate was installed next to a highly vibrating compressor and the fixing screws were loose. After re reinforcing and adding vibration damping pads, the fault disappeared.

EMC compliant installation: key measures to meet CE certification

To comply with the EU EMC Directive (89/336/EEC), A-series PLCs need to be installed according to the following requirements:

1. Control cabinet processing

Use conductive metal cabinets to maintain good electrical contact between panels (remove paint).

Flat braided wires are used to connect multiple points between the cabinet door and the main body, reducing high-frequency impedance.

The cross-sectional area of the cabinet grounding wire should be ≥ 22mm ², and the length should be as short as possible.

Cabinet opening diameter ≤ 10cm to prevent high-frequency noise radiation.

2. Connection between power supply and grounding wire

Set up a grounding point near the power module and connect the LG and FG terminals with the shortest wire (≤ 30cm).

For A1S61P/A1S62P/A1S63P, a noise filter needs to be installed at the input end (recommended SCHAFFNER FN343-3/01 or TDK ZHC2203-11).

For CE compatible models (A1S61PEU, A1S62PEU, A1S61PN, A1S62PN), no external filter is required, but LG and FG must be short circuited (unless otherwise specified).

Twist the power cord and ground wire together to enhance common mode noise discharge.

3. Signal cable shielding

All I/O signal lines and communication lines (RS-232C, RS-422, etc.) leading out of the control cabinet must use shielded cables.

The shielding layer contacts the cabinet grounding through a large area of metal clips (welding leads cannot be used due to high high-frequency impedance).

For MELSECNET/II modules (such as A1SJ71AR21), a three coaxial cable must be used and a ferrite magnetic ring (TDK ZCAT3035 is recommended) must be installed at the cable outlet.

The AUI cable of Ethernet module A1SJ71E71-B5 needs to be stripped of insulation and grounded extensively.

Attention: If shielded cables are not used or shielded grounding is poor, the immunity will decrease from 2kV (IEC801-4) to below 1kV, which may cause false signals.

Selection and replacement of power module and I/O module

1. Power module replacement guide

Prototype Number Recommended Replacement (CE Compatible) Remarks

A1S61P (100-120VAC) A1S61PN (100-240VAC) 5V/5A output, requires an external filter

A1S62P (200-240VAC) A1S62PN (100-240VAC) 5V/3A + 24V/0.6A

A1S63P (24VDC input) without CE compatibility model requires external SELV power supply

Replacement precautions:

Overcurrent protection: When the 5V output current exceeds 5.5A (A1S61P) or 3.3A (A1S62P), the power module will automatically turn off the output and turn off the LED. At this point, the load short circuit must be eliminated before power can be restored.

Overvoltage protection: If the 5V voltage rises to 5.5-6.5V, the protection will activate. Power off and then power on to reset. If it still cannot be restored, replace the power module.

Allowable instantaneous power down time: A1S63P is about 20ms under 24VDC input; A1S61P/A1S62P is over 20ms under AC input.

2. Key points for replacing input modules

The rated voltage of AC input modules (such as A1SX10, A1SX20) is 100-120VAC or 200-240VAC. It should be noted that CE compatible models (such as A1SX10EU, A1SX20EU) have a dielectric strength of 1780VAC and the same operating temperature range.

DC input modules (such as A1SX40, A1SX41, A1SX80, etc.) support 12/24VDC, input current 3-7mA, and response time is usually 10ms (high-speed models such as A1SX42 can reach 0.1ms). Attention should be paid when replacing:

Source/Drain Type: The A1SX80 series supports both sink and source connections, while the A1SX40 only supports the sink type.

Response speed: If the original system uses high-speed counting (such as encoder signals), a model with a response time of ≤ 0.2ms (such as A1SX42) must be selected, otherwise pulses will be lost.

3. Output module replacement

Relay output (A1SY10, A1SY14, A1SY18A): Contact capacity 240VAC/24VDC, 2A/point, 8A/common terminal. When replacing, pay attention to the higher dielectric strength (2830VAC) of CE compatible models (A1SY10EU, A1SY14EU, A1SY18AEU). The lifespan of the relay is 20 million mechanical cycles and 100000 electrical cycles (rated load).

Controllable silicon output (A1SY22, A1SY28A): suitable for AC loads, with a response time of approximately 1ms+0.5 cycles. When replacing, pay attention to the load current (0.6A/point, 2.4A/common terminal).

Transistor outputs (A1SY40, A1SY41, A1SY42, etc.): used for DC loads, with a response time of 2ms, and some high-speed types (such as A1SY71) can reach 2ms. Special attention should be paid to:

Output current: A1SY40 is 0.1A/point, A1SY42 is 0.5A/point, A1SY50 is 2A/point. When replacing, be sure to check the original load current to avoid overloading and burning out.

Surge suppression: The transistor output is internally integrated with a Zener diode, but for inductive loads, it is still recommended to use an external parallel freewheeling diode.

Fuse: Most transistor output modules do not come with fuses and require external installation (fast melting is recommended).

Code of practice for electrical wiring

1. Power wiring

Connect the live wire (L) and neutral wire (N) of the AC power supply to the corresponding terminals of the power module. LG is functional grounding, FG is protective grounding. FG must be connected to the grounding bar of the control cabinet.

For A1S63P (24VDC input), an external power supply that complies with SELV (Safety Extra Low Voltage) must be used, with the 24V positive pole connected to "24V" and the negative pole connected to "COM".

It is strictly prohibited to parallel the outputs of multiple power modules to the same I/O module for power supply, otherwise it may cause module heating or even fire.

2. Input module wiring

For DC input modules, the common terminal can be connected to the positive or negative pole to achieve source or drain type. For example, when the COM of A1SX40-S1 is connected to 0V, the input terminal is connected to 24V effectively (source type).

To improve anti-interference, the input signal line should be laid separately from the power line, with a spacing of ≥ 100mm. If necessary, shielded cables should be used, and the shielding layer should be grounded at one end.

For the communication input module (A1SX10EU), attention should be paid to the power supply frequency of 50/60Hz, input current of 6mA, and minimum ON voltage of 80VAC.

3. Wiring of output module

The common terminal (COM) of the relay output module can be connected to AC or DC power supply, but it must be consistent with the power supply of the load. Inductive loads must be connected in parallel with RC absorption circuits (AC) or freewheeling diodes (DC).

The "external power" terminal of the transistor output module must be connected to 24VDC, otherwise the output cannot conduct. The manual clearly requires' SELV power required '.

When driving incandescent lamps or capacitive loads, the impulse current may reach 10 times the rated value and needs to be reduced for use.

4. Communication wiring (A1SCPUC24-R2)

The RS-232C interface adopts a 9-pin D-sub male connector, with pin definitions of 2-RD, 3-SD, 5-SG, 7-RS, and 8-CS. Cable length ≤ 15m, it is recommended to use 7/0.127mm shielded twisted pair.

If the external device cannot provide the CD signal (pin 1), the RS232C CD terminal with buffer address 10BH needs to be checked as "not executed" in the program.

Self loop test: Short circuit 2-3 and 7-8 to verify if the interface hardware is functioning properly.

Fault safety circuit design

According to Chapter 5 of the manual, a safety circuit must be constructed outside the PLC to prevent danger caused by power failure or PLC hardware failure.

1. Essential external circuits

Emergency stop circuit (hard wired, not through PLC).

Positive and reverse interlock (contactor electrical interlock).

Hardware travel switch for upper/lower limit positioning control.

Output module monitoring circuit: For a single output point that may cause serious accidents (such as a motor contactor), an external relay should be connected in parallel, and its contacts should be connected to the PLC input for program monitoring of its status.

2. Power sequence control

To avoid accidental output during power on, the following sequence is recommended:

First, power the PLC.

Provide external load power supply (such as the load power supply of 24V output module).

After a delay of 0.5 seconds through the PLC program, connect the main contactor (MC) and allow the output action.

The specific example circuit is shown in Figure 5.2 (pages 5-2 of the manual). For the DC output module, voltage relay RA2 can be used to detect the establishment of the load power supply, and then a timer can be used to delay the connection of MC.

3. Safety status in case of PLC failure

When the PLC detects the following faults, all outputs will be turned off:

Power module overcurrent/overvoltage protection action.

CPU watchdog timer timeout (WDT ERROR).

Program execution error (such as instruction code error, no END instruction).

But for faults that the CPU cannot detect (such as I/O control bus damage), the output may randomly remain ON or OFF. At this point, it is necessary to rely on external safety circuits (such as the monitoring circuit or safety relay mentioned above) to cut off the hazard source.

Common error codes and troubleshooting

Based on the error code table in Chapter 7 of the manual, the most common errors and their handling methods on site are extracted.

1. Instruction code error (INSTRUCT CODE ERR., error code 10)

Phenomenon: CPU stops, ERROR LED stays on.

Possible reasons:

The memory contains instruction codes that the CPU cannot decode (such as damaged ROM or downloaded incompatible programs).

Noise causes changes in memory content.

Exclusion steps:

Read the wrong step with a programmer and check if the instruction for that step is valid.

If using an EPROM memory card, reprogram the correct program.

If it occurs frequently, check the grounding and power filtering.

2. Parameter Error (Error Code 11)

Phenomenon: CPU stops.

Possible reasons: Parameter settings exceed the CPU range (such as incorrect program capacity or file register capacity settings) or the total exceeds the memory card capacity.

Solution: Use peripheral devices to read parameters, reset and write them.

3. Missing END INS instruction (Error code 12)

Phenomenon: CPU stops.

Reason: There is no END (or FEND) instruction at the end of the program.

Solution: Write the END instruction at the end of the program.

4. Watchdog timer error (WDT ERROR, error code 22)

Phenomenon: CPU stops or ERROR flashes during RUN.

Reason:

The scanning time of the user program exceeds the set watchdog time (default 200ms).

Transient power failure occurred during scanning, resulting in a false increase in scanning time.

solve:

Check if there is an infinite loop or excessively long sequence in the program, and use the CJ instruction to jump to unrelated segments.

Monitor special register D9005. If it is not 0, it indicates power fluctuation and the power supply line needs to be checked.

The watchdog setting value can be appropriately extended (through parameter settings).

5. Fuse Break OFF (error code 32)

Phenomenon: The CPU continues to run (RUN state), but the ERR LED of the corresponding output module lights up.

Reason: The internal fuse of the output module is blown (usually due to a short circuit of the output load).

solve:

Observe the fuse indicator LED on the output module panel and replace it with a fuse of the same specification.

Read the special registers D9100~D9107 through peripheral devices to identify which module (bit 1) is specific.

6. Unit verification error (UNIT VERITY ERR., error code 31)

Phenomenon: CPU stops.

Reason: The configuration of the I/O module after power on is inconsistent with the actual module currently inserted (for example, someone unplugged and plugged in the module).

Solution: Check the special registers D9116~D9123, and the module corresponding to bit 1 has changed. Re plug or replace it.

7. Battery Error (BATTERY ERROR, error code 70)

Phenomenon: The ERROR LED flashes while the CPU is running, or an alarm is triggered when powered on.

Reason: Low battery voltage or disconnected battery.

Solution: Immediately replace the battery (model A6BAT). Before replacement, ensure that the PLC is powered on (or connected to an external power source through the programming port) to prevent program loss. Reset the "low battery" flag in the parameters after replacement.

8. Special module layout error (SP. UNIT LAY. ERR., error code 44)

Phenomenon: CPU stops.

Possible reasons:

Installed 3 or more computer link modules (up to a maximum of 2).

Installed 2 or more MELSECNET modules.

The I/O allocation in the parameters does not match the actual module type (such as setting a special module as a regular I/O).

Solution: Reduce the number of modules or correct parameter allocation.

Maintenance and battery replacement

1. Daily inspection items (recommended for each shift)

Power LED, RUN LED, ERROR LED status.

The input/output LED is consistent with the actual device status.

No abnormal noise or odor.

2. Regular inspections (every 6-12 months)

Environmental temperature and humidity (0-55 ℃, 10-90% RH).

Check if the wiring terminals are loose (torque check).

Internal dust accumulation (cleaned with a vacuum cleaner).

Battery voltage status: Monitor special relays M9006 (low battery voltage warning) and M9007 (low battery voltage error). If M9006 is ON, replacement should be arranged.

3. Battery replacement steps

Ensure that the PLC power is turned on (at least 1 minute).

Open the front panel of the CPU and unplug the old battery connector.

Insert a new battery (A6BAT), pay attention to polarity.

Close the panel. After replacement, M9006/M9007 should be reset.

If it needs to be replaced in a power-off state, it must be completed within 5 minutes, otherwise the program will be lost (backup is required in advance).