Mitsubishi A2NCPUR21-S1 PLC CPU Module

The Mitsubishi A2NCPUR21-S1 is a CPU module for the Melsec A series programmable logic controllers (PLC). This unit serves as the central processor for medium to large scale automation systems, offering high-speed execution of ladder logic and support for up to 2048 I/O points. It is widely used in legacy industrial machinery and process control systems.

Processing Performance

The A2NCPUR21-S1 features a 16-bit microprocessor with a basic instruction execution time of 0.2 µs per contact instruction. The program memory capacity is 30k steps (RAM, battery-backed). The CPU supports 4k points of internal relay, 1k points of timer/counter, and 8k registers for data storage. It can handle up to 12 interrupt programs and 16 subroutine calls deep.

Built-in Communication

The CPU includes one RS-232C port (25-pin D-sub) for programming and monitoring via a PC or HMI. It supports the Melsec A protocol (dedicated) and non-protocol communication. Baud rates up to 19.2 kbps. An external communication module can be added for CC-Link or Ethernet.

Memory and Battery

The program and data memory are retained by a lithium battery (type BR-2/3A) with a typical life of 5 years. The battery status is monitored by the CPU, and a front-panel LED indicates low battery. The memory includes a built-in RAM for program storage and a separate flash EEPROM for parameter backup. The CPU supports memory cassettes for expansion.

Diagnostic and Status Indicators

Front-panel LEDs: RUN (green), ERROR (red), USER (yellow), BATTERY (red). The ERROR LED flashes for minor faults (e.g., programming error) and stays on for major faults (watchdog timeout). The CPU has an internal watchdog timer with a settable timeout (10-200 ms). A self-diagnosis function checks the CPU, memory, and bus at power-up.

I/O and Expansion

The A2NCPUR21-S1 can control up to 2048 I/O points using a combination of main base and extension racks (up to 4 extension racks). It supports I/O modules including digital, analog, positioning, and high-speed counter modules. The CPU’s base rack (A32B, A35B, etc.) provides the backplane bus.

Environmental and Power

Power consumption: 0.5A from the 5V backplane supply. Operating temperature: 0°C to 55°C; storage: -20°C to 75°C. Humidity: 10% to 90% non-condensing. Dimensions: 130 x 90 x 87 mm (H x W x D). Weight 0.6 kg. The module is designed for installation in a sealed control cabinet.

Applications

Commonly found in packaging machines, injection molding machines, textile equipment, and conveyor systems originally built with Melsec A series. The A2NCPUR21-S1 is also used as a replacement CPU for legacy systems.

Conclusion

The Mitsubishi A2NCPUR21-S1 CPU module provides fast logic execution, robust memory, and integrated RS-232 communication for Melsec A series PLCs. It remains a capable controller for established automation installations.