Mitsubishi A2NCPU Melsec A Series CPU Unit

The Mitsubishi A2NCPU is a CPU unit from the Melsec A series programmable logic controllers. Designed for legacy automation systems, this CPU module provides a compact processing core for controlling I/O modules, special function units, and communication adapters. The A2NCPU supports up to 512 I/O points and 64 special function modules, making it suitable for medium-scale industrial applications such as assembly lines, conveyor systems, and packaging machinery. Its robust design and straightforward programming environment (MELSEC MEDOC or GX Developer) make it a reliable choice for maintaining existing installations.

Technical Specifications

• Model: A2NCPU (also known as 132659)

• Series: Melsec A series

• Control Method: Stored program, cyclic operation

• I/O Points: 512 max. (including remote I/O)

• Program Capacity: 14K steps (RAM/EEPROM)

• Processing Speed: 1.0 µs per basic instruction (LD)

• Constant Scan: 10–190 ms (adjustable in 10 ms increments)

• Programming Languages: Ladder diagram, instruction list

• Memory Backup: Lithium battery (3V) for program and data retention

• Built-in Communication: RS‑422 (programming port)

• Expansion Capability: Up to 64 special function modules (analog I/O, positioning, etc.)

• Operating Temperature: 0°C to 55°C

• Protection Rating: IP20

• Dimensions: Standard A series single-slot module

Functional Description

The A2NCPU executes the user program stored in its internal RAM. At power‑up, the CPU reads I/O status, executes the ladder logic, and writes outputs in a continuous scan cycle. The processing speed of 1.0 µs per basic instruction enables fast response for most discrete control tasks. The CPU supports both refresh and direct I/O access modes, allowing optimization between performance and response time. The constant scan function ensures deterministic cycle times, which is beneficial for applications requiring predictable timing, such as synchronous motion sequences.

Memory and Data Management

User programs are stored in RAM with lithium battery backup. Program memory capacity is 14K steps, sufficient for medium‑complexity control logic. Data memory includes internal relays, timers (256), counters (256), and data registers (1024). An optional EEPROM or EPROM memory cassette can be installed for program backup without battery dependency. The battery life is approximately 5 years under normal operating conditions.

I/O and Module Expansion

The A2NCPU resides in the main base rack of the A series system. It can control up to 512 I/O points through a combination of local I/O modules and remote I/O networks. Special function modules (positioning, analog, temperature, etc.) are installed in the same rack and accessed via dedicated FROM/TO instructions. Expansion base units (up to 4) can be connected using extension cables to increase the total I/O count. The CPU automatically recognises I/O modules and assigns addresses based on rack and slot positions.

Communication and Programming

The built‑in RS‑422 port is used for programming and monitoring. It connects to a MELSEC programming console or to a PC running GX Developer or MEDOC software. An RS‑232C conversion adapter (A1SD51‑S1) can be added for connection to HMIs or host computers. The CPU does not have an integrated Ethernet port; network connectivity requires an Ethernet interface module (e.g., AJ71E71‑S3).

Diagnostic Capabilities

The A2NCPU features self‑diagnosis functions, including program checksum verification, I/O bus error detection, and watchdog timer monitoring. An LED indicator panel shows CPU status: POWER (green), RUN (green), and ERROR (red). Error codes are stored in a system register and can be read through the programming software.

Applications

Typical applications include conveyor control, packaging machinery, injection molding machines, and assembly line automation. The A2NCPU is commonly encountered in retrofit or replacement scenarios for maintaining older production lines where the original CPU has failed.

Conclusion

The Mitsubishi A2NCPU provides a proven, reliable processing core for Melsec A series PLC systems, offering adequate performance and expansion capabilities for a wide range of industrial control applications.