Mitsubishi A1SJ71QC24-R2 Retrofit-Compatible Serial Comms Module

Mitsubishi A1SJ71QC24-R2 Retrofit-Compatible Serial Communications Module for Legacy MELSEC-A Systems

The Mitsubishi A1SJ71QC24-R2 is a serial communications module designed for the MELSEC-A series AnS-format compact rack systems. Originally engineered to provide RS-232C and RS-422 interface capability within the A1S backplane, this module remains a critical component for facilities operating legacy Mitsubishi programmable logic controller platforms that have not yet completed full migration to the MELSEC-Q or MELSEC-iQ-R architecture. As production lines age and OEM support windows close, the A1SJ71QC24-R2 continues to serve as a reliable retrofit solution for engineers tasked with extending system life, replacing failed communications hardware, or bridging legacy serial protocols to modern supervisory networks.

At NINERMAS, every A1SJ71QC24-R2 unit is pre-shipment tested under simulated rack conditions to verify RS-232C channel integrity, RS-422 differential signal levels, module address recognition, and backplane communication handshake with the A1SCPU or A1SHCPU host controller. Units are supplied with a 12-month warranty and documented test records, supporting your incoming inspection and quality assurance requirements.

Upgrade Compatibility Table

Retrofit Planning for Existing Automation Systems

Replacing the A1SJ71QC24-R2 within an operating MELSEC-A control system requires a structured approach that accounts for the interdependencies between the communications module and the broader control architecture. The module typically interfaces with an A1SCPU or A1SHCPU central processing unit mounted in the same A1S52B, A1S55B, or A1S58B compact rack. Before initiating any replacement, engineers should audit the rack’s power supply — commonly an A1S61PN (5 VDC, 3 A) or A1S62PN (5 VDC, 6 A) — to confirm that the existing power budget accommodates the QC24-R2’s 5 VDC draw alongside all installed I/O modules, including digital input modules such as the A1SX40 or A1SX80, digital output modules such as the A1SY10 or A1SY40P, and any analog I/O modules like the A1S68AD or A1S62DA that may be present in the same rack.

Terminal wiring is a critical checkpoint. The RS-232C channel (CH1) uses a DB-9 or DB-25 connector depending on the connected peripheral — typically an GOT1000-series HMI (GT1055-QSBD or GT1150-QLBD), a barcode reader, or a third-party SCADA serial gateway. The RS-422 channel (CH2) uses screw-terminal connections and is commonly wired to inverters, servo drives, or remote I/O adapters operating on Mitsubishi’s computer link protocol. Document all existing wiring, including signal ground references and shield termination points, before disconnecting the failed module.

Module address configuration is performed via the rotary switch on the QC24-R2 front panel. The address must match the I/O assignment defined in the host CPU’s parameter settings, which are stored in the A1SCPU’s built-in RAM or in an A1SMCA memory cassette. If the memory cassette is present, verify that the parameter data is intact before powering down the rack, as some older cassettes may lose data during extended power interruptions. For systems using GX Developer for program management, back up the current project — including the network parameter file and the QC24 buffer memory initialization sequence in the ladder program — before proceeding with the swap.

In control cabinets where space is constrained, confirm the physical installation clearance for the replacement module. AnS-format modules are single-slot width, but cable routing for the RS-232C DB connector and the RS-422 screw terminals may require additional horizontal clearance. If the cabinet also houses an A1SJ71AP23Q MELSECNET/MINI-S3 module or an A1SJ71E71-B2 Ethernet interface module in adjacent slots, ensure that the replacement operation does not disturb their wiring or connector seating.

For sites planning a longer-term migration from the MELSEC-A platform to MELSEC-Q, the A1SJ71QC24-R2 can serve as a bridging component while the migration is staged across multiple maintenance windows. The QJ71C24N-R2 is the functional equivalent for Q-series racks and supports the same computer link and non-procedure protocols, but requires a QA1S5□B base unit adapter or a full Q-series rack replacement, along with program conversion using GX Works2 or GX Works3. Planning this migration in parallel with the immediate QC24-R2 replacement allows engineering teams to maintain production continuity while building toward a fully supported control platform.

Downtime Control During System Migration

Minimizing unplanned downtime during a communications module replacement on a MELSEC-A system depends on preparation quality and the availability of a pre-tested spare. Because the A1SJ71QC24-R2 is a hot-swap-incompatible module — the rack must be powered down before removal — the replacement window is effectively the planned outage window. Reducing this window requires that all pre-replacement tasks be completed before the line is stopped.

The recommended sequence is: (1) back up the CPU program and parameters using GX Developer via the RS-232C programming port on the A1SCPU front panel or via an A1SJ71UC24-R4 programming cable; (2) photograph or document all terminal wiring on the QC24-R2 before disconnection; (3) set the rotary address switch on the replacement unit to match the recorded address; (4) power down the rack, remove the failed module, install the replacement, and reconnect all wiring; (5) power up the rack and verify that the CPU transitions to RUN mode without communication errors; (6) confirm that the connected HMI displays are receiving data and that any SCADA polling sequences resume correctly.

If the system includes a redundant communication path — for example, a parallel Ethernet link via an A1SJ71E71-B2 module — that path can be used to maintain supervisory visibility during the serial module replacement, reducing the operational impact of the outage. For systems without redundancy, coordinating the replacement during a scheduled maintenance window and pre-positioning a tested A1SJ71QC24-R2 spare at the site eliminates the lead-time risk that is the most common cause of extended unplanned downtime on legacy MELSEC-A installations.

NINERMAS maintains stock of the A1SJ71QC24-R2 and related MELSEC-A components to support emergency replacement requirements. All units are dispatched with pre-shipment test documentation, reducing the risk of installing a non-functional spare and extending the outage further.

Retrofit Support FAQ

Q1: Is the A1SJ71QC24-R2 a direct drop-in replacement for a failed unit in my existing MELSEC-A rack?
Yes, provided the replacement unit is installed in the same rack slot and the rotary address switch is set to the same position as the original module. No program changes are required for a like-for-like replacement. Verify that the RS-232C and RS-422 wiring is reconnected correctly and that the CPU parameter settings reference the correct I/O address for the QC24 module.

Q2: Can the A1SJ71QC24-R2 communicate with modern HMI panels and SCADA systems?
Yes. The module supports Mitsubishi computer link protocol (Format 1 and Format 4) and non-procedure protocol over both RS-232C and RS-422 channels. Modern GOT2000-series HMI panels (GT27□□-□) support direct serial connection to the QC24-R2 using the computer link driver. For SCADA systems using OPC servers, a serial-to-Ethernet converter can be used to bridge the RS-232C or RS-422 output to a TCP/IP network without modifying the PLC program.

Q3: What pre-shipment testing is performed on each A1SJ71QC24-R2 unit?
Each unit supplied by NINERMAS is tested for RS-232C channel signal integrity, RS-422 differential pair levels, module address recognition via rotary switch, and backplane communication handshake with a host A1SCPU. Test results are documented and included with the shipment. Units that do not pass all test criteria are not dispatched.

Q4: What warranty and supply terms apply to this module?
All A1SJ71QC24-R2 units are supplied with a 12-month warranty from the shipment date, covering manufacturing defects and functional failure under normal operating conditions. NINERMAS maintains ongoing stock of MELSEC-A series components to support long-term spare parts programs. Contact sale@ninermas.com or +0086 187 5021 5667 to discuss volume commitments, reserved stock agreements, or expedited delivery requirements.