Mitsubishi MELSEC-Q Series Network Modules: QJ71LP21S-25

Model: QJ71LP21S-25

Brand Mitsubishi Electric

Series MELSEC-Q

Model QJ71LP21S-25

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Technical Dossier

Product Details And Specifications

Mitsubishi Electric MELSEC-Q Series: Comprehensive Module Range and Technical Overview

The Mitsubishi Electric MELSEC-Q Series is a modular programmable logic controller (PLC) platform deployed across global heavy industry sectors including petrochemical refineries, nuclear power generation facilities, steel manufacturing plants, and offshore oil & gas installations. Its multi-CPU architecture, high-speed backplane bus (up to 400 Mbps), and deterministic scan-cycle performance have made it the reference platform for process-critical automation in facilities requiring IEC 61511 functional safety compliance. The QJ71LP21S-25 is a MELSEC-Q optical loop network module supporting MELSECNET/H optical loop topology at 25 Mbps, used for peer-to-peer inter-PLC communication in distributed control architectures.

The Evolution of MELSEC-Q Series Architecture

The MELSEC-Q platform was introduced in the late 1990s as the successor to the MELSEC-A and MELSEC-QnA series, consolidating Mitsubishi Electric's industrial PLC lineup into a unified high-performance backplane architecture. Early Q-series systems used the QnA-compatible CPU modules (Q02, Q02H) with a 32-bit RISC processor and a dedicated system bus operating at 50 Mbps. The introduction of the Universal model CPUs (Q03UD, Q04UD, Q06UD, Q13UD, Q26UD) in the mid-2000s expanded memory capacity, added built-in Ethernet ports, and increased the internal bus speed to 400 Mbps, enabling tighter multi-CPU synchronization. Network communication evolved in parallel: MELSECNET/10 coaxial loop was superseded by MELSECNET/H optical and coaxial variants, which in turn were complemented by CC-Link IE Field and CC-Link IE Control for Gigabit-class deterministic networking. As of 2024, Mitsubishi Electric has announced the end-of-sale transition for several Q-series CPU and network modules, with the iQ-R Series (R-series) designated as the long-term replacement platform. Installed Q-series bases remain in service under extended lifecycle support agreements, making spare parts availability a critical operational requirement for asset-intensive industries.

MELSEC-Q Full Catalog & Functionalities (SKU List)

CPU Modules

Q02UCPU: Universal model CPU, 20K steps, 256KB memory, built-in USB port.
Q03UDECPU: Universal model CPU, 30K steps, built-in Ethernet, SD card slot.
Q06UDEHCPU: Universal model CPU, 60K steps, dual built-in Ethernet, high-speed processing.
Q13UDEHCPU: Universal model CPU, 130K steps, extended I/O and network capacity.
Q26UDEHCPU: Universal model CPU, 260K steps, maximum program capacity in Q-series.

Network Communication Modules

QJ71LP21S-25: MELSECNET/H optical loop master/local, 25 Mbps, ST-type fiber connector.
QJ71LP21-25: MELSECNET/H optical loop, 25 Mbps, standard fiber, predecessor to S-25 variant.
QJ71LP21G: MELSECNET/H optical loop, 10 Mbps, GI fiber, legacy coexistence support.
QJ71BR11: MELSECNET/H coaxial bus master/local module, 10 Mbps.
QJ71E71-100: Ethernet interface module, 100BASE-TX, MELSOFT/SLMP protocol support.
QJ71GF11-T2: CC-Link IE Field master/local module, 1 Gbps, twisted-pair.
QJ61BT11N: CC-Link V2 master/local module, up to 10 Mbps, 64-station support.
QJ71C24N: Serial communication module, RS-232/RS-422/RS-485, 2-channel.

Digital I/O Modules

QX40: 16-point DC input module, 24VDC, sink/source compatible.
QX81: 32-point DC input module, 24VDC, high-density terminal block.
QY10: 16-point transistor output module, 12–24VDC, 0.5A per point.
QY80: 32-point transistor output module, 24VDC, high-density, source type.

Analog I/O Modules

Q64AD: 4-channel analog input, 16-bit resolution, voltage/current selectable.
Q68DAIN: 8-channel analog input, 16-bit, high-speed sampling, isolated channels.

Power Supply Modules

Q61P: 100–240VAC input power supply, 6A output, for Q-series main base.
Q62P: 100–240VAC input power supply, 6A, redundant-capable configuration.

Sourcing Hard-to-Find & Obsolete MELSEC-Q Parts

Quality Control for the MELSEC-Q Range

MELSEC-Q network modules such as the QJ71LP21S-25 incorporate a proprietary optical transceiver and MELSECNET/H ASIC that require protocol-level validation beyond standard power-on testing. DriveKNMS applies the following test procedures to all Q-series network modules prior to dispatch: optical power output measurement on ST-type fiber ports (reference: Mitsubishi Electric specification ±3 dBm tolerance); MELSECNET/H loop initialization and token-passing verification using a dedicated Q-series test rack; backplane bus communication integrity check at 400 Mbps using a Q06UDEHCPU reference CPU; firmware version identification and comparison against Mitsubishi Electric's published version matrix; and 48-hour burn-in cycle under operational load conditions. All test results are documented and supplied with each unit shipment.