Mitsubishi Unit A1SJ71UC24-R4 or A1SJ71UC24R4 New

Mitsubishi A1SJ71UC24-R4 | MELSEC-A Computer Link Module — RS-422/RS-485 / 0.3–19.2kbps / 500m / 2-Piece Terminal Block

Serial Communication Between MELSEC-A CPU and External Devices

The A1SJ71UC24-R4 is a computer link module for Mitsubishi's MELSEC-A PLC platform — the module that adds an RS-422/RS-485 serial communication channel to the A-series rack. It enables the MELSEC-A CPU to exchange data with external devices: computers, HMI terminals, other PLCs, and intelligent field instruments that communicate via RS-422 or RS-485 serial links.

The module supports both computer link function and multi-drop link. Computer link connects the PLC directly to a host computer or SCADA system, allowing the computer to read and write PLC device data (registers, contacts, timers, counters) using Mitsubishi's dedicated protocol. Multi-drop link allows a single communication channel to serve multiple slave stations — up to 32 station addresses are supported on one RS-422/485 segment.

The U designation in A1SJ71UC24-R4 identifies the extended capability version. In addition to the standard computer link function, the UC24-R4 supports access to devices extended by AnACPU, AnUCPU, and A2US(H)CPU, and access to other MELSECNET/10 network stations via the connected CPU. This makes it compatible with a wider range of MELSEC-A CPU types than the base A1SJ71C24-R4.

Key Specifications

RS-422 and RS-485 — Terminal Resistors Required at Line Endpoints

When the A1SJ71UC24-R4 is the first or last station on an RS-422/485 bus segment, a terminal resistor must be connected across the receive and transmit differential pairs. The correct value depends on the wiring type:

• RS-422: 330Ω, 1/4W between RDA and RDB, and between SDA and SDB
• RS-485: 110Ω, 1/2W between RDA and RDB

Missing or incorrectly valued termination causes data communication errors, particularly at higher baud rates and longer cable runs. All devices on the same RS-422/485 segment must use the same communication standard — mixing RS-422 and RS-485 devices on one segment is not supported.

Station numbers are set using the station number switches on the module face, with a valid range of 00 to 31.

FAQ

Q1: What is the difference between A1SJ71UC24-R4 and A1SJ71C24-R4?

Both are MELSEC-A computer link modules with RS-422/RS-485 interface and the same physical dimensions. The UC24-R4 (U prefix) adds extended access capability: it can access devices from AnACPU, AnUCPU, and A2US(H)CPU, and supports communication to other MELSECNET/10 stations. For CPU types beyond the basic MELSEC-A range, the UC24-R4 is the compatible choice.

Q2: How many external devices can connect to one A1SJ71UC24-R4?

The module supports up to 32 station addresses (00 to 31) on a single RS-422/485 segment in multi-drop configurations. In practice, the number of devices is limited by the cable length (500m total segment), the baud rate selected, and the RS-485 receiver unit load capacity of the connected devices.

Q3: What transmission protocols does the A1SJ71UC24-R4 support?

The module supports full-duplex and half-duplex transmission, four dedicated communication protocols, no-protocol mode for generic serial communication, and bidirectional mode. Protocol selection is made via switches on the module. The no-protocol mode allows reading and writing raw serial data from the PLC ladder programme, enabling communication with devices that do not support Mitsubishi's dedicated protocol.

Q4: Can the A1SJ71UC24-R4 communicate with third-party devices that use RS-485?

Yes, using no-protocol mode or a compatible protocol variant. For Modbus RTU communication specifically, the A1SJ71UC24-R4-S2 variant is the appropriate choice — it adds dedicated Modbus RTU support. The base R4 module uses Mitsubishi's own dedicated protocol or no-protocol mode for custom serial exchanges.

Q5: What should be checked if the module shows communication errors after installation?

Verify that terminal resistors are fitted at the first and last stations on the RS-422/485 bus segment with the correct resistance value. Confirm all devices on the segment are wired consistently (all RS-422 or all RS-485). Verify station number settings match the programmed address assignments. Check cable shielding is grounded at one end only and that cable length does not exceed 500m.