Mitsubishi A3ACPUP21 / A3ACPU-P21 CPU Module (MELSEC A Series)
The Mitsubishi A3ACPUP21 (model also written A3ACPU-P21) is a high-end CPU module for the MELSEC A series programmable logic controllers. It is a direct upgrade from the A2C series, providing 32K steps of program memory, faster processing (0.2 µs per step), and a built-in RS-232C port for programming or host communication. The CPU occupies the main slot in an A series base unit (e.g., A38B, A68B).
Processor and Memory
Processor: 16-bit proprietary Mitsubishi MCU at 20 MHz. Program capacity: 32K steps (max). Device memory: 8K D registers (data), 8K M relays (internal), 2K S registers (accumulator), 1K T timers, 1K C counters. Built-in 256 KB of flash ROM for user program and parameters (no battery needed for program retention). RAM for device memory is backed by a lithium battery (CR2032, typical life 5 years).
Communication and I/O
Built-in RS-232C port (9-pin D-sub) supports programming with Mitsubishi MELSEC-PC software or SCADA via dedicated protocol. Baud rate up to 115.2 kbps. The CPU can control up to 2048 I/O points (with extension racks). The backplane communication bus handles up to 64 special modules (analog, positioning, high-speed counters). A special function (PID control) is built into firmware.
Diagnostics and LEDs
Front panel LEDs: POWER (green), RUN (green), ERROR (red), BATTERY (red flashing when low), RS-232 (yellow for tx/rx). A 7-segment display shows error codes (e.g., “E.0” for no error, “E.0L” for low battery). The CPU has a RUN/STOP switch and a RESET button. The error history buffer stores the last 20 errors with time stamps (based on a real-time clock that requires battery backup).
Environmental and Mechanical
Dimensions: 130 mm x 34.5 mm x 90 mm (H x W x D). Weight: 0.3 kg. Operating temperature: 0°C to 55°C. Storage: -20°C to 70°C. Humidity: 10% to 90% non-condensing. Protection: IP20. The module draws 0.4 A from the 5 V backplane. It meets CE, UL, and CSA standards.
Programming and Compatibility
The A3ACPUP21 is programmed in MITSUBISHI MEDOC or GX Developer (using A series mode). Languages: ladder diagram and instruction list. Up to 32K steps can be organized into multiple program files. The CPU supports “online editing” (small changes without stopping). It is compatible with all A series I/O modules (A1SX, A1SY, A1SJ, etc.) and special function modules (A1SD62, A1SD75). For replacement, the A3ACPUP21 can directly replace A3ACPU (older) but not A3ACPUP21R (different pinout).
Applications
Used in complex machine control applications such as injection molding machines, printing presses, packaging lines, and automated assembly systems requiring large program memory and fast scan times (typical scan time for 10K steps: 10 ms). The built-in RS-232 port is ideal for connection to an HMI or barcode scanner. Battery low alarm can be read via special relay M9006.
