Mitsubishi AJ71C21-S1 Serial Communication Module

The Mitsubishi AJ71C21-S1 is a dedicated serial communication module for the MELSEC A Series PLC platform. It provides a flexible interface, allowing the PLC to exchange data with external devices such as barcode readers, weigh scales, printers, third-party controllers, and operator terminals. By relieving the main CPU from the overhead of complex byte-by-byte serial data handling, the AJ71C21-S1 ensures reliable communication without affecting machine cycle times.

Multi-Protocol Support and Dual Interfaces

The AJ71C21-S1 supports both the RS-232C and RS-422/485 physical interfaces, making it compatible with a wide range of industrial devices. The module is capable of communicating using the standard MELSEC communication protocol (bidirectional protocol) as well as a non-procedural protocol, where the user program can directly control data transmission and reception frames. This design allows connection to a variety of peripheral devices, including printers for data logging and operation panels for human-machine interaction.

Data Communication with Dedicated Instructions

Integration is accomplished via a set of special-purpose communication instructions. The module's engineering approach is based on simple buffer memory exchanges. The user program writes the data to be transmitted into the module's buffer memory and then executes the appropriate instruction (such as "XMIT" for transmission or "RECV" for reception). This concept isolates the program logic from the low-level details of the start bit, stop bit, and parity checks, simplifying the design of complex data exchange tasks.

Transmission Speeds and Configuration

The AJ71C21-S1 supports standard transmission speeds (baud rates) configurable from 300 bps up to 19.2 kbps. The data format (start bits, data bits, parity, stop bits) can be matched exactly to the requirements of the connected partner device. All communication parameters are set using the GX Developer software via a simple parameter dialog, avoiding the need for complex DIP-switch settings. The module draws its logic power via the MELSEC A series backplane bus.

Diagnostics and Troubleshooting

Reliable troubleshooting of communication failures is possible thanks to the module's integrated diagnostic LEDs. LEDs on the front panel provide immediate visual feedback on the transmission status (SD) and reception status (RD), allowing maintenance personnel to quickly ascertain whether data is flowing on the line. Additionally, the module's detailed status information is mapped into the PLC's buffer memory. A user program can monitor this status and trigger an alarm on the machine's HMI if a communication timeout or a partner device error is detected.

Application in Legacy Systems

The AJ71C21-S1 is frequently used in existing industrial plants to connect older MELSEC A series controllers to modern data acquisition systems (SCADA). By acting as a data gateway, this module allows plant managers to extract production counters, machine states, and fault logs from their legacy machinery and integrate them into a central production control center (MES). This helps extend the service life of existing equipment, as the automation platform can still be integrated into new company networks via this module.