Mitsubishi AJ71C21-A Communication Module for MELSEC A Series PLC

Product Overview

The Mitsubishi AJ71C21-A is a communication module designed for the MELSEC A series programmable logic controllers. It provides serial communication capabilities (RS-232 and RS-422) for connecting the PLC to external devices such as operator panels, printers, barcode readers, and host computers. The module is widely used in legacy automation systems where serial data exchange is required. It mounts in a MELSEC A series base unit and communicates with the CPU via the backplane. The AJ71C21-A supports protocols such as MELSEC protocol (non-procedural) and bidirectional protocol, allowing flexible data transfer.

Key Features of AJ71C21-A

The module features one RS-232C port (25-pin D-sub) and one RS-422/485 port (terminal block), selectable via switches. It supports baud rates from 300 to 19200 bps. The module can handle up to 256 bytes of data per transmission. It includes a data buffer and parity/error checking. Configuration is done via DIP switches and sequence program. LED indicators show power, data transmission, and error status.

The AJ71C21-A can be used as a master or slave in a multi-drop network. It supports the MC protocol for direct CPU access, allowing external devices to read/write PLC devices. The module has a dedicated protocol mode for connection to Mitsubishi HMIs. It operates from 5 V DC (backplane) and consumes 250 mA. The operating temperature is 0 to 55°C. Dimensions are standard A series module size (approx. 60 mm x 250 mm x 90 mm).

Technical Specifications

• Manufacturer: Mitsubishi Electric

• Part Number: AJ71C21-A

• Series: MELSEC A

• Product Type: Serial Communication Module

• Ports: RS-232C (25-pin D-sub), RS-422/485 (terminal block)

• Baud Rate: 300, 600, 1200, 2400, 4800, 9600, 19200 bps

• Protocols: MC protocol (non-procedural), bidirectional

• Data Format: Start-stop synchronous, ASCII/Hex

• Data Buffer: 256 bytes transmit/receive

• Indicators: Power, send, receive, error LEDs

• Power Supply: 5 V DC from backplane (250 mA)

• Operating Temperature: 0 to +55 °C

• Storage Temperature: -20 to +75 °C

• Dimensions (W x H x D): 60 mm x 250 mm x 90 mm

• Certifications: CE (as part of system)

Functional Description

The AJ71C21-A exchanges data with the PLC CPU via dedicated buffer memory. The user program initializes the module (baud rate, protocol) and then triggers send/receive operations using FROM/TO instructions. The module handles the serial framing, and the PLC reads received data. In MC protocol mode, the module automatically interprets commands from an external device to read/write PLC devices, eliminating the need for custom programming. Error counters can be monitored for diagnostics.

Applications

This module is used to connect MELSEC A series PLCs to serial devices such as touch screens, weigh scales, and printers. It is also used for data logging to a PC. It serves as a replacement part for failed communication modules in legacy systems.