Mitsubishi A2NCPU-P21 CPU Module for Melsec A Series PLC

The Mitsubishi A2NCPU-P21, also known as A2NCPUP21, is a central processing unit (CPU) module for the Melsec A series programmable logic controller (PLC) systems. This CPU module provides program execution, I/O control, and network communication capabilities for medium to large-scale automation applications. With high-speed processing and built-in MELSECNET data link functionality, the A2NCPU-P21 is suitable for complex machine control, process automation, and line control systems.

Program Memory and Execution Speed

The A2NCPU-P21 features a program capacity of 14K steps, providing ample memory for complex ladder logic, sequential function charts, and structured text programs. Basic command processing speed reaches 1.0µs for LD instructions in refresh mode, with direct mode ranging from 1.0 to 2.3µs. The USB-compatible programming and monitoring method, along with constant scan selectable from 10 to 190ms in 10ms increments, enables deterministic control timing required by precise machinery. Expanded memory is supported via memory cassettes up to maximum capacity.

I/O Capacity and Expansion

The CPU supports up to 512 I/O points (X/Y 0 to 1FF), making it suitable for applications with moderate I/O density. Input/output control method allows selection of refresh or direct access modes, providing flexibility between deterministic update times and immediate I/O response. The system can accommodate up to 4,032 additional points configurable in 64-point increments through expansion racks. The I/O system supports diverse module types including digital input/output, analog I/O, temperature control, positioning, and high-speed counter modules.

Device Memory Configuration

The A2NCPU-P21 provides extensive internal device memory: internal relays (M): 1000 points (M0 to M999, shared with S and L for 2048 total), latch relays (L): 1048 points, step relays, communication relays (B): 1024 points, timers (T): 2048 points with configurable time bases, counters (C): 256 points, data registers (D): 1024 points (D0 to D1023), communication registers (W): 1024 points, file registers (R): 4096 points, annunciators (F): 256 points for alarm handling, accumulators (A): 2 points, index registers (V and Z): 2 points, pointer (P): 256 points for subroutine and jump targeting, interrupt pointers (I): 32 points for event-driven tasks, special relay (M): 256 points (M9000 to M9255) for system status, and special register (D): 256 points (D9000 to D9255) for diagnostic data.

MELSECNET Data Link Function

The CPU includes built-in MELSECNET coaxial cable data link communication, enabling high-speed data exchange between multiple PLC stations in a distributed control network. The A-series MELSECNET architecture supports fiber optic or coaxial cable media with automatic loopback for fault tolerance and station-to-station communication at rates of up to several hundred meters. This networking capability allows coordinated control across production lines or multi-equipment systems. Combined with MELSECNET II functionality, the CPU supports flexible network configurations, enabling remote I/O control and station-to-station data sharing without additional programming overhead.

Programming Languages and Software

The CPU supports multiple programming languages as defined by IEC 61131-3 standards, including ladder logic (IL), logic symbolic language, and MELSAP-II for sequential function chart (SFC) programming. Programming is performed using Mitsubishi's GX Developer or GX Works software packages, which provide offline simulation, online monitoring, and debugging capabilities. Programs can be developed in languages appropriate to the application: ladder logic for discrete control, SFC for sequential processes, and structured text for complex calculations and data manipulation. Program development is supported by offline simulation, online monitoring, and debugging tools within the programming software.

Control and Operational Features

Control method employs stored program repeated operation, typical of PLC architectures. Self-diagnostic functions monitor for CPU, memory, I/O, battery, and bus errors with automatic detection of abnormal conditions. An instant power failure allowance depends on the attached power supply module. The built-in constant scan feature ensures predictable execution times for time-critical applications.

Physical Specifications

External dimensions are 254mm height by 79.5mm width by 100mm depth, with a weight of approximately 0.92 kg. The CPU mounts on a Melsec A series base unit (such as A32B, A35B, A52B, A55B, A62B, A65B, A68B, etc.) and connects via the backplane bus. DC5V internal current consumption is 1.4A, and the AC power supply tolerance covers 100-120V / 200-240V ranges with 3A at DC24V and 0.6A at DC5V output. The CPU requires a dedicated power supply module (such as A61P, A62P, A63P, or A65P) which should be selected based on system load and input voltage requirements.

Applications

The A2NCPU-P21 is suitable for industrial automation including packaging machinery, injection molding machines, material handling systems, and conveyor control. The MELSECNET data link enables coordinated control across machine cells or line sections, while the extensive device memory supports complex control logic and data management.

Conclusion

The Mitsubishi A2NCPU-P21 CPU module provides reliable processing, extensive I/O capacity, and integrated network capability for Melsec A series PLC installations. Its combination of fast instruction execution, comprehensive device memory, and MELSECNET data link support makes it suitable for medium-scale automation projects requiring robust control performance.