Mitsubishi A2ACPU21-S1 CPU Module

The Mitsubishi A2ACPU21-S1 is a CPU module for the MELSEC A series programmable logic controller family. Designed for small to medium-scale control systems, this compact CPU provides reliable sequence processing and communication capabilities. It is often used in legacy machinery upgrades or maintenance replacements where a compact A series CPU is required.

Technical Specifications

The A2ACPU21-S1 is built around a 16-bit microprocessor with control method: stored program cyclic scan. Program language: ladder diagram and instruction list. Program memory: 2.8k steps (RAM, battery-backed). Execution speed: 0.6 µs per basic instruction (LD). I/O points: maximum 64 points (integrated I/O or via expansion). The CPU includes 256 internal relays, 256 timers (0.1-999s), 256 counters (0-9999), and 256 data registers (16-bit). The built-in communication port is RS422 (programming/MELSEC protocol). Power supply: 5V DC from backplane, consumption approx. 0.3A. The unit has dimensions 90mm (H) x 20mm (W) x 85mm (D). Operating temperature 0-55°C.

Key Features

• Compact CPU for MELSEC A series (A1S, A2S)

• Program capacity 2.8k steps (RAM)

• Maximum 64 I/O points

• Built-in RS422 programming port (115.2 kbps)

• Real-time clock (requires battery)

• Battery backup for RAM and clock (CR2032, approx. 3 years)

• Status LEDs: RUN, ERROR, USER

• Mounts on A series base rack (5V backplane)

Memory and Data

The user program is stored in RAM protected by a lithium battery (CR2032, field-replaceable). When battery voltage drops, the "BAT" LED lights; user program may be lost if not replaced soon. The CPU does not include built-in flash memory; an optional EEPROM or EPROM memory cassette can be installed on the side for program backup. Data registers are also battery-backed. The CPU supports up to 128 file registers (optional cassette).

Programming and Software

Programming is done using Mitsubishi GX Developer or MEDOC software, connecting via RS422 to the CPU's programming port. The programming cable is typically the SC-09 (RS232 to RS422 converter). The ladder logic supports basic instructions (LD, AND, OR, OUT, TIM, CNT, etc.) and advanced functions (arithmetic, move, compare, PID). An optional numeric keypad and display unit (A0J2-U1) can be attached for on-site monitoring.

I/O Expansion

The A2ACPU21-S1 can be used stand-alone with its integrated I/O (if present, but this module itself has no built-in I/O; it requires separate I/O modules on the same rack). The CPU can control up to 64 I/O points from modules placed on the same base unit. For expansion, additional base units (e.g., A1S32B) and extension cables can be added, extending the total I/O count up to 256 points (with additional power supplies).

Communication

The RS422 port supports programming and monitoring, but can also communicate with Mitsubishi HMIs (e.g., GOT series) or other devices using the MELSEC protocol (formats 1-4). For networking, the CPU can be equipped with a communication module (e.g., A1SJ71C24-R2 for RS232/RS485, or Ethernet module A1SJ71E71).

Applications

Typical applications include small assembly machines, conveyor control, packaging equipment, and specialized machinery. Many legacy systems still use A series CPUs; the A2ACPU21-S1 serves as a direct replacement for older A2ACPU21 or A2ACPU21-S0 models. Its small footprint allows integration into tight control panels.

Diagnostics and Maintenance

Front LEDs: RUN (green) indicates normal operation, ERROR (red) indicates a program error or watchdog timeout, USER (yellow) is user-defined. The CPU supports self-diagnostics including checksum, battery check, and watchdog timer. If the ERROR LED flashes, use GX Developer to read the error code. Replace the battery annually or when the BAT LED lights. Always upload the program before replacing the battery.

Environmental and Certifications

The A2ACPU21-S1 is CE compliant and designed for pollution degree 2 environments. It meets EN 61131-2 for PLCs. Storage temperature -20°C to 70°C, humidity 10-95% non-condensing. Vibration resistance 2g (10-55 Hz).

Conclusion

The Mitsubishi A2ACPU21-S1 CPU module offers a compact and reliable solution for MELSEC A series control systems. Its proven architecture and compatibility with legacy installations make it a practical choice for maintenance or small-scale projects.