Mitsubishi A2ACPU21 CPU Module: Central Processing Unit for MELSEC A Series Programmable Logic Controllers

Product Overview

The Mitsubishi A2ACPU21 is a CPU module belonging to the MELSEC A series of programmable logic controllers, providing centralized processing and I O management for industrial automation systems. This CPU unit executes the user control program stored in its internal memory, processing inputs from connected field devices, executing control logic, and updating outputs to actuators and indicators. The A2ACPU21 supports a program capacity of 14 kilosteps, providing adequate memory for medium-scale control applications ranging from machine control to process monitoring. The CPU operates with a basic instruction processing speed of 0.20 microseconds per step for LD instructions, ensuring responsive control performance suitable for real-time automation tasks.

Key Features of Mitsubishi A2ACPU21

The A2ACPU21 supports both refresh and direct I O control modes, allowing engineers to select the appropriate I O update method for their application requirements. The module includes a watch dog timer that can be configured between 10 and 2000 milliseconds, providing system monitoring that triggers a safety output state if the CPU scan cycle exceeds the configured interval. Self-diagnostic functions continuously monitor for watchdog errors, memory errors, CPU errors, I O errors, and battery errors, providing diagnostic information that assists maintenance personnel in identifying and resolving system faults. The CPU module includes a built-in real-time clock that maintains year, month, day, hour, minute, second, and day of the week information, with automatic leap year recognition and typical accuracy of -1.1 seconds per day at 0 °C.

The A2ACPU21 supports a latch range that can be configured for L, B, T, C, D, and W relays, ensuring that critical data is retained during power interruptions. The CPU can be started automatically when the RUN switch is moved to the ON position at power-on, or after power restoration following a power failure, simplifying system recovery after power interruptions. The module is compatible with memory cassettes that expand the available program and data storage capacity beyond the base 14 kilosteps, accommodating larger control programs when needed.

Technical Specifications

• Manufacturer: Mitsubishi Electric Corporation

• Part Number: A2ACPU21

• Series: MELSEC A Series

• Product Type: CPU Module / Programmable Controller

• Program Capacity: 14 kilosteps

• Basic Instruction Speed: 0.20 µs per step (LD command)

• I O Control Mode: Refresh or direct selectable

• Watch Dog Timer: 10 to 2000 ms selectable

• Self Diagnostics: WDT error, memory error, CPU error, I O error, battery error

• Real-Time Clock Accuracy: -3.9 to +0.8 seconds per day at 0 °C

• Latch Range: Configurable for L, B, T, C, D, W relays

• Memory Cassette Support: Yes

• Startup Method: Automatic on RUN switch at power-on

• Operating Temperature: 0 to 55 °C

• Storage Temperature: -20 to 75 °C

Functional Description

The A2ACPU21 executes the stored sequence program using the relay-symbol-type programming language combined with logic symbolic language and MELSAP-II SFC language. During each scan cycle, the CPU reads the status of input modules, executes the user program instructions to determine the required output states based on input conditions and internal logic, and writes the calculated output values to output modules. The watch dog timer monitors the duration of each scan cycle, and if the cycle time exceeds the configured watchdog interval, the CPU triggers a fault response, typically placing outputs into a safe state to prevent uncontrolled operation. The self-diagnostic functions run continuously in the background, providing real-time detection of system faults that could compromise control integrity.

Applications

The Mitsubishi A2ACPU21 CPU module is deployed in industrial automation systems where the MELSEC A series platform has been installed. Typical applications include machine tool control, packaging machinery, material handling systems, conveyor control, and process monitoring stations. The 14 kilostep program capacity and 0.20 microsecond instruction speed make this CPU suitable for applications with moderate program complexity and real-time response requirements. For facilities that operate legacy MELSEC A series equipment, the A2ACPU21 provides a direct replacement CPU option that maintains compatibility with existing I O modules and field wiring, extending the service life of installed automation systems without requiring complete controller replacement.