Mitsubishi A3ACPU PLC CPU Module for MELSEC A Series

The Mitsubishi A3ACPU is a central processing unit (CPU) module belonging to the MELSEC A series programmable logic controller (PLC) platform. It provides high-speed instruction execution, extensive program memory, and built-in serial communication for small to medium scale automation systems. The A3ACPU is commonly used in packaging lines, assembly machines, and material handling systems requiring robust ladder logic control.

Processing Performance

The CPU utilizes a proprietary 16-bit microprocessor with a basic instruction execution time of 1.0 microsecond per step (LD, OUT, etc.). Application instructions (MOV, ADD) execute in 2 to 10 µs. The scan time for a typical 10k step program is approximately 10 ms. The CPU supports both cyclic and interrupt processing, with up to 4 external interrupt inputs.

Program and Data Memory

User program memory capacity: 30k steps (RAM, battery backed). Data memory includes:

• Input/output relays (X/Y): 2048 points each.

• Internal relays (M): 2048 points.

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

• Counters (C): 128 (16-bit).

• Data registers (D): 4096 words (16 bits).

• Index registers (Z/V): 16 each.

Communication Ports

Built-in RS-232C port (9-pin D-sub) supports programming, data link, and connection to operator interfaces. The port configuration is adjustable via software: baud rate from 1200 to 38400 bps, parity none/even/odd, and protocol (MELSEC proprietary, Modbus RTU, or non-procedural). A second RS-422 port (6-pin modular) is available for remote programming or connection to multiple CPUs.

I/O and Expansion

The A3ACPU mounts on a base unit (A3B series) and can control up to 1024 physical I/O points (512 inputs + 512 outputs) using a combination of local and remote I/O racks. Expansion is possible via optical fiber or coaxial data link modules (A1SJ71UC24). Maximum number of special function modules (analog, positioning, temperature) is 64 per system.

Battery and Memory Protection

Lithium battery (type A6BAT) retains program and data for approximately 5 years (25°C). The battery voltage is monitored by a LED indicator (BAT LOW) and can be read from special register D8005. In case of power failure, the CPU automatically saves retentive areas. A supercapacitor provides 1 minute of backup during battery replacement.

Diagnostics and Self-Diagnosis

The CPU executes power-on diagnostics (ROM, RAM, bus) and run-time diagnostics (watchdog timer, instruction error, communication error). Watchdog timer (WDT) is configurable from 1 to 500 ms. An error code is stored in special register D8000. The CPU has an LED status panel: RUN (green), ERROR (red), USER (yellow programmable), and BATTERY (orange).

Environmental Specifications

Operating temperature: 0 to 55°C; storage: -20 to 70°C. Humidity: 10% to 90% non-condensing. Vibration: 0.5 mm amplitude (2 to 13 Hz), 2 g (13 to 55 Hz). The CPU module dimensions: 130 mm height, 40 mm width, 93 mm depth. Weight: 0.4 kg. Power consumption: 0.5 A from +5 V backplane. The unit conforms to CE and UL 508.

Programming Software

Compatible with Mitsubishi GX Developer (version 8 or higher) and GX IEC Developer. Programming languages: ladder diagram (LD), instruction list (IL), and sequential function chart (SFC). Online editing (insert/overwrite) is supported without stopping the PLC. Password protection (4-digit) prevents unauthorized reading or writing of the program.

Special Functions

The A3ACPU incorporates built-in high-speed counters: two independent 32-bit counters up to 10 kHz each. It also provides a real-time clock (year/month/day/hour/minute/second) with 2 ms time stamp resolution. Pulse train output (PTO) function for simple stepper motor control up to 5 kHz is available via Y0/Y1.

Ordering and Compliance

Mitsubishi model number: A3ACPU. The CPU is supplied with the battery A6BAT (installed) and a terminal cover. Documentation: Hardware manual IB-66724, Programming manual IB-66725. The product is EOL (end of life) but remains available for legacy system maintenance. RoHS compliant (lead-free soldering).