QJ71C24N is a serial communication module for Mitsubishi Electric’s MELSEC-Q series programmable logic controller platform, providing two independent serial channels — one RS-232 and one RS-422/RS-485 — for data exchange with external devices such as barcode readers, weighing terminals, robot controllers, and third-party instrumentation. The module supports MC Protocol (SLMP), non-procedure communication, and predefined protocol communication, making it one of the most versatile serial interface options in the MELSEC-Q ecosystem. Atlantech Drives holds stock of the QJ71C24N. Contact us for fast worldwide delivery and competitive pricing.
What Is the QJ71C24N?
The QJ71C24N is a Q-series intelligent function module that occupies one slot on a MELSEC-Q base unit and communicates with the CPU module via the Q-bus backplane. It supersedes the earlier QJ71C24 and QJ71C24-R2 variants by combining both RS-232 and RS-422/RS-485 interfaces in a single module body, eliminating the need for two separate communication modules in mixed-interface installations. Channel 1 (CH1) operates as RS-232 at speeds up to 115,200 bps, while Channel 2 (CH2) operates as RS-422 or RS-485 at speeds up to 115,200 bps with multi-drop capability for up to 32 nodes. Both channels operate independently and simultaneously, each with its own buffer memory and interrupt-driven data handling. The module is compatible with all Q-series CPU types including the Basic, High Performance, Process, Redundant, and Universal model CPUs, as well as the QnA-compatible mode.
Key Technical Specifications
- Number of Channels: 2 (CH1: RS-232, CH2: RS-422/RS-485)
- CH1 Interface: RS-232, D-sub 9-pin male connector
- CH2 Interface: RS-422/RS-485, D-sub 9-pin female connector
- Transmission Speed: 300 to 115,200 bps (both channels)
- RS-485 Multi-drop: Up to 32 nodes per CH2
- Communication Modes: MC Protocol (Format 1/4), Non-procedure, Predefined Protocol, Bidirectional
- Send/Receive Buffer: 4,096 bytes per channel
- Isolation: Photocoupler isolation between channels and Q-bus
- Current Consumption: 5 V DC, 0.30 A (from Q-bus)
- Operating Temperature: 0 °C to 55 °C
- Compatible CPUs: All MELSEC-Q series CPU modules
- Dimensions (W × H × D): 27.4 × 98 × 90 mm
- Weight: Approx. 150 g
Typical Industrial Applications
The QJ71C24N is a standard integration module in MELSEC-Q systems where legacy serial devices must be incorporated into a modern PLC architecture. In automotive assembly lines, CH2 in RS-485 multi-drop mode is commonly used to poll up to 32 torque wrenches or fastening controllers sequentially, with the PLC logging torque results to a data historian via the Q-series Ethernet module. In pharmaceutical manufacturing, the module connects via RS-232 on CH1 to checkweighers and metal detectors from manufacturers such as Mettler-Toledo and Fortress, using non-procedure communication to capture rejection signals and weight data directly into the PLC data registers. In water treatment facilities, the QJ71C24N drives Modbus RTU communication on CH2 to RS-485 networks of flow meters, pressure transmitters, and valve positioners — all from a single module slot. The predefined protocol function (supported in firmware version B or later) allows engineers to configure communication sequences graphically in GX Works2 without writing custom ladder communication routines, significantly reducing commissioning time for complex ASCII-protocol instruments.
Maintenance Tips
The QJ71C24N requires no scheduled preventive maintenance beyond standard Q-series module practices, but several field conditions commonly cause degraded communication performance. RS-232 cable runs should not exceed 15 m — beyond this distance, signal integrity degrades and framing errors increase, particularly at speeds above 19,200 bps; use RS-422 on CH2 for longer runs or electrically noisy environments. Inspect the D-sub connector solder joints on cables annually in high-vibration environments, as intermittent pin contact is the most frequent cause of sporadic communication timeouts that are difficult to diagnose remotely. If CH2 is used in RS-485 half-duplex mode, verify that termination resistors (typically 110–120 Ω) are fitted only at the two physical ends of the cable run — missing or incorrectly placed termination is the leading cause of reflections and data corruption in multi-drop RS-485 networks. Monitor the ERR LED on the module front panel: a steady ERR indication typically signals a hardware fault requiring module replacement, while a flashing ERR indicates a communication timeout that can often be resolved by checking cable continuity and baud rate matching on the connected device.
Frequently Asked Questions
Q: Can the QJ71C24N run Modbus RTU master communication on CH2?
A: Yes, using the Predefined Protocol function available in GX Works2, the QJ71C24N can act as a Modbus RTU master on CH2 in RS-485 mode. Mitsubishi provides a Modbus RTU predefined protocol library that can be imported directly into GX Works2, allowing function code 03 (Read Holding Registers) and 06/16 (Write Registers) transactions to be configured without writing raw ASCII ladder communication sequences.
Q: Is the QJ71C24N compatible with the Q00UJ or Q00U CPU modules?
A: Yes. The QJ71C24N is compatible with all Universal model Q-series CPUs including the Q00UJ, Q00U, Q01U, Q02U, Q03UD, Q04UDH, and Q06UDH, among others. It is mounted on any I/O slot of the main base unit or an extension base, and is configured via the intelligent function module parameters in GX Works2.
Q: What is the difference between QJ71C24N and QJ71C24N-R4?
A: The QJ71C24N-R4 provides two RS-422/RS-485 channels instead of the QJ71C24N’s RS-232 + RS-422/RS-485 combination. Choose the QJ71C24N when one RS-232 device and one RS-422/RS-485 network must be connected simultaneously; choose the QJ71C24N-R4 when both channels require RS-422/RS-485 multi-drop capability.
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