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: