Mitsubishi AJ71C21-B1-S2 PLC Controller 424749
The Mitsubishi AJ71C21-B1-S2 is a communication module belonging to the MELSEC PLC family. It is designed to provide RS-232C serial communication capabilities for A series CPUs, enabling connection to programming devices, operator interfaces, and other serial equipment. The module carries the additional identification code 424749 and is a variant of the AJ71C21 series, specifically the S2 version with enhanced firmware.
Communication Specifications
The module features one RS-232C port (9-pin D-sub) with configurable baud rates from 300 bps to 19.2 kbps. Supported protocols include MELSEC proprietary (MC protocol), non-procedural (message exchange), and bidirectional (dedicated). Data format: 7 or 8 data bits, 1 or 2 stop bits, parity none/even/odd. The transmission distance is up to 15 meters (50 feet) at maximum baud rate. The module supports both full-duplex and half-duplex operation.
Integration with MELSEC PLC
The AJ71C21-B1-S2 occupies one slot in a MELSEC A series base unit (A1S, A2S, A3S, or AnS). It communicates with the CPU via the dedicated bus, mapping I/O addresses to buffer memory (128 words). The module can be used for:
Connection to GX Developer or other programming software.
Data exchange with Mitsubishi HMI panels (GOT series).
Peer-to-peer communication between two MELSEC CPUs (1:1 or 1:N).
Host computer link (Modbus RTU slave emulation).
Hardware Features
The module is housed in a compact plastic case with dimensions 130 mm height x 34 mm width x 100 mm depth. Weight: 0.25 kg. Front panel includes: RS-232C connector, status LEDs (PWR, RUN, SD, RD, ERR), and a rotary address switch (setting station number 0–31 for network operation). The module draws 150 mA from the 5V backplane; no external power required.
Configuration and Settings
Setup is performed via the CPU's parameter area using GX Developer. The user defines communication parameters (baud rate, data format, protocol type) and assigns buffer memory addresses. The module also features a hardware handshake (CTS/RTS) and a built-in termination resistor for RS-422 conversion (via optional adaptor). A DIP switch selects between RS-232C and RS-422 modes (the AJ71C21-B1-S2 defaults to RS-232C).
Data Buffer and Access
The module contains 128 words of buffer memory (16 bits per word), accessible via FROM/TO instructions from the CPU. For non-procedural protocol, the user can read/write data directly to the buffer. For MC protocol, external devices can access CPU registers (D, M, X, Y, etc.) using standardized frames. Maximum message length is 256 bytes.
Diagnostics and Status
LED indicators provide real-time status: PWR (green, power OK), RUN (green, module operational), SD (yellow flashing, transmitting data), RD (yellow flashing, receiving data), ERR (red, communication error or hardware fault). The module stores error codes in buffer memory (address 0) which can be read by the CPU. Common errors include parity error, overrun, and time-out.
Environmental Specifications
Operating temperature: 0 to 55°C; storage: -20 to 70°C. Relative humidity: 10% to 90% non-condensing. Vibration resistance: 2 g (10–55 Hz). The module meets CE and UL 508 standards. It is RoHS compliant (lead-free).
Compatibility
Compatible with MELSEC A series CPUs: A1SCPU, A2SCPU, A3SCPU, A1SJCPU, A2USHCPU, and A3UCPU. Can also be used with AnN series with base adaptor. Not compatible with Q series (requires QJ71C24). For newer systems, consider AJ71QC24 or QJ71C24N.
Typical Applications
This module is commonly used in legacy industrial machinery such as injection molding machines, printing presses, and packaging lines where an RS-232 interface is needed for data logging or HMI connection. It is also suitable for retrofitting old systems that require a spare communication module.
Ordering and Documentation
Part number: AJ71C21-B1-S2 (also referenced as 424749). Manual: Mitsubishi AJ71C21 User's Manual (IB-66622). The module is supplied with a 9-pin D-sub connector hood and a set of labels.
