Mitsubishi A2ACPU-R21 PLC CPU Module

The Mitsubishi A2ACPU-R21 (also designated A2ACPUR21) is a CPU module for the Melsec A series programmable logic controllers. It is a compact, high‑speed processor designed for small to medium automation tasks. The module features 14K steps of program memory, a built‑in RS‑232 port, and operates on 24V DC power. It is typically used in packaging machines, conveyor systems, and small assembly lines.

Processor Performance

The CPU uses a 16‑bit microprocessor with a basic instruction execution time of 1.25 µs (LD). Application instructions (MOV, ADD) execute in 2–5 µs. The A2ACPU-R21 supports both cyclic and interrupt processing. Scan time for a typical 5K‑step program is approximately 8 ms.

Memory

• Program memory: 14K steps (RAM, battery backed).

• Data memory (D): 4K words (16 bits).

• Internal relays (M): 2K points.

• Timers (T): 256 (100 ms, 10 ms, 1 ms).

• Counters (C): 128 (16‑bit).

An optional memory cassette (A6NV‑E2) can expand program memory to 28K steps.

Built‑in Communication

One RS‑232C port (9‑pin D‑sub) supports MC protocol, non‑procedural, and bidirectional communication. Baud rate: 300–19200 bps. A peripheral port (6‑pin mini DIN) is also available for connecting a programming console (e.g., A6GPP).

I/O and Expansion

The CPU mounts on a Melsec A series base (A3B, A5B, A8B) and can control up to 512 I/O points using local I/O modules (AX, AY series). Up to 32 special function modules (analog, positioning, temperature) can be connected. High‑speed counters: 2 channels (10 kHz).

Battery and Memory Retention

A lithium battery (type A6BAT) retains the program and data registers for approximately 5 years (25°C). Battery low is indicated by a yellow LED (BAT). In case of power failure, the CPU automatically saves retentive areas. A supercapacitor provides 1 minute of backup during battery replacement.

Diagnostics and LEDs

Front panel LEDs: RUN (green), ERROR (red), USER (yellow programmable), BAT (yellow). The CPU runs power‑on self‑tests (ROM, RAM, bus) and runtime diagnostics (watchdog timer, instruction error). The watchdog timer (WDT) is adjustable from 10 to 500 ms.

Power Supply

The A2ACPU-R21 requires 24V DC (20.4–28.8V) from the backplane or external supply. Power consumption: 0.3A at 5V DC (internal). The CPU also provides a 24V DC sensor supply (200 mA) for inputs.

Environmental and Physical

Dimensions: 130 mm height x 40 mm width x 100 mm depth. Weight: 0.35 kg. Operating temperature: 0 to 55°C; storage: -20 to 70°C. Humidity: 10% to 90% non-condensing. Vibration: 2 g (10–55 Hz). The module meets CE and UL 508 standards.

Programming Software

Compatible with GX Developer (version 8 or higher) and GX IEC Developer. Languages: ladder diagram and instruction list. Online editing (insert/overwrite) is supported. Password protection (4 digits) prevents unauthorized access.

Ordering and Documentation

Mitsubishi order number: A2ACPU-R21 (A2ACPUR21). Includes a terminal cover and a battery (A6BAT). Manual: A Series CPU Hardware Manual (IB-66724).