The QJ71MT91 is Mitsubishi Electric’s MODBUS/TCP communication module for the MELSEC-Q programmable controller series — providing a dedicated 100BASE-TX Ethernet port implementing the MODBUS/TCP protocol as both master and slave for integrating MELSEC-Q systems with third-party devices, SCADA systems, and industrial Ethernet networks that use Modbus TCP as the standard communication protocol, without requiring the MC protocol driver or socket programming that the general-purpose QJ71E71-100 Ethernet module demands. In stock at Atlantech Drives. Request your quote within 24-48 hours with worldwide shipping and 12-month warranty.
What Is the QJ71MT91?
The QJ71MT91 is a MELSEC-Q series intelligent function module providing a dedicated MODBUS/TCP interface via 100BASE-TX Ethernet (RJ45, auto MDI/MDI-X, 10/100 Mbps auto-negotiation) for reading and writing MELSEC-Q device data (D registers, M relays, X inputs, Y outputs) to and from Modbus TCP client devices (SCADA systems, HMI panels, other PLCs) or for reading data from Modbus TCP server devices (drives, meters, sensors with Modbus TCP server interface). The module’s dedicated MODBUS/TCP protocol implementation eliminates the socket communication programming required when using the general-purpose QJ71E71-100 for Modbus TCP — the QJ71MT91’s communication is configured via GX Works2 parameter settings without PLC ladder programming for the Modbus TCP data exchange, substantially reducing engineering time for Modbus TCP integration projects. As a Modbus TCP master, the QJ71MT91 reads holding registers, input registers, coils, and discrete inputs from connected Modbus TCP slave devices and writes holding registers and coils to slave devices — auto-refreshing the exchanged data to/from specified MELSEC-Q device addresses each communication cycle without FROM/TO buffer memory instructions in the PLC program. As a Modbus TCP slave, the QJ71MT91 exposes specified MELSEC-Q device addresses to Modbus TCP master clients, enabling SCADA systems and HMI panels with Modbus TCP masters to read and write MELSEC-Q data using standard Modbus function codes (FC01–FC06, FC15, FC16) without the Mitsubishi MC protocol driver requirement of the QJ71E71-100.
Key Technical Specifications
- Model: QJ71MT91
- Network Standard: 100BASE-TX (IEEE 802.3u), auto-negotiation 10/100 Mbps
- Connector: RJ45 (auto MDI/MDI-X)
- Protocol: MODBUS/TCP (master and slave simultaneously)
- Master Function: Read/write Modbus TCP server devices (up to 32 slave connections)
- Slave Function: Expose MELSEC-Q devices to Modbus TCP master clients (up to 16 master connections)
- Supported Modbus Function Codes: FC01, FC02, FC03, FC04, FC05, FC06, FC15, FC16
- Auto-Refresh: Configurable data exchange between module buffer and MELSEC-Q devices without PLC instructions
- Compatible CPUs: MELSEC-Q Q02UCPU and compatible Universal/High Performance model CPUs
- Installation: Q-series base unit intelligent function module slot
- Power Consumption: 5 VDC internal bus, approx. 0.52 A
- Operating Temperature: 0°C to 55°C
- Dimensions (W × H × D): 27.4 mm × 98 mm × 90 mm
- Weight: Approx. 0.14 kg
Typical Industrial Applications
The QJ71MT91 is deployed in MELSEC-Q systems requiring Modbus TCP communication with third-party devices that do not support Mitsubishi’s MC protocol — a common requirement in facilities with mixed automation vendor equipment where Modbus TCP is the agreed interoperability standard. In energy management systems where power meters, energy analysers, and substation automation equipment (all with Modbus TCP server interfaces) must report energy consumption data to a MELSEC-Q system for production-correlated energy monitoring, the QJ71MT91 acts as a Modbus TCP master, polling each energy meter’s holding registers for kWh, kW, voltage, and current data and auto-refreshing the values to MELSEC-Q D registers for PLC-based energy reporting logic. Water and wastewater treatment plants with mixed PLC and SCADA infrastructure use the QJ71MT91 to integrate MELSEC-Q pump station controllers into Modbus TCP SCADA systems (Wonderware, iFIX, AVEVA) without requiring the SCADA vendor to implement the Mitsubishi MC protocol driver — the SCADA Modbus TCP master reads pump status, flow totals, and alarm states from the QJ71MT91’s slave interface using standard Modbus FC03 read holding registers. In building automation systems where a MELSEC-Q sequence PLC controls mechanical and electrical systems, the QJ71MT91’s slave interface enables the building management system (BMS) Modbus TCP master to read setpoints and override output states without dedicated BMS-to-PLC integration programming. Pharmaceutical production facilities with strict batch record requirements use the QJ71MT91 to provide Modbus TCP data access from the MELSEC-Q PLC to MES (Manufacturing Execution System) platforms that use Modbus TCP as their PLC interface standard, enabling automated batch record population with production parameter data from the MELSEC-Q control program.
Expert Tips
When configuring the QJ71MT91 as a Modbus TCP master reading data from multiple slave devices (energy meters, drives, sensors), stagger the polling cycle start times for each slave in the QJ71MT91’s master parameter settings to avoid simultaneous polling requests to multiple slaves — simultaneous requests cause TCP connection establishment overhead to stack, slowing the effective data refresh rate for all slaves below the intended cycle time. Configure each slave’s polling interval in the QJ71MT91 parameters based on the data’s update frequency requirement — energy totals may only need 10-second polling while fault status bits may need 1-second polling — assigning appropriate intervals prevents unnecessary network load from over-polling stable data. For SCADA integration using the QJ71MT91’s slave interface, document the Modbus register map (which MELSEC-Q D register addresses correspond to which Modbus holding register numbers) in a formal register map document shared with the SCADA configuration team — the QJ71MT91’s register mapping is configured in GX Works2 parameter settings and may not be readily apparent to SCADA engineers unfamiliar with the module. Register map documentation prevents SCADA misconfiguration that reads the wrong data or writes to unintended PLC addresses. For QJ71MT91 master connections to Modbus TCP slaves behind NAT routers or firewalls (remote panel connections via WAN), verify that the network infrastructure allows TCP port 502 (the standard Modbus TCP port) traffic in both directions between the QJ71MT91 and the remote slave device IP addresses — Modbus TCP on non-standard ports is not supported by the QJ71MT91’s standard firmware.
Frequently Asked Questions
Q: Can the QJ71MT91 simultaneously act as a Modbus TCP master (reading from slave devices) and a Modbus TCP slave (being read by SCADA) at the same time?
A: Yes. The QJ71MT91 supports simultaneous master and slave operation — it can poll Modbus TCP slave devices (energy meters, drives) while also responding to Modbus TCP read requests from SCADA master clients. Both functions use the same 100BASE-TX Ethernet port and operate concurrently. Configure the master connections and slave address mapping independently in GX Works2’s QJ71MT91 parameter settings for simultaneous bidirectional Modbus TCP data exchange.
Q: What is the difference between using the QJ71MT91 and using the QJ71E71-100 with custom Modbus TCP socket programming for Modbus TCP integration?
A: The QJ71MT91 provides dedicated Modbus TCP protocol implementation configured entirely through GX Works2 parameters — no PLC ladder programming is required for the Modbus TCP data exchange itself. The QJ71E71-100 requires the PLC programmer to write socket communication ladder logic (open socket, construct Modbus TCP request frame, send, receive response, parse) for each slave device — significantly more engineering effort. For Modbus TCP integration projects, the QJ71MT91’s configuration-based approach reduces engineering time and eliminates the risk of socket communication programming errors in the PLC ladder. The QJ71E71-100 provides greater flexibility for non-Modbus protocols (MQTT, custom TCP applications), while the QJ71MT91 is optimised specifically for Modbus TCP.
Q: Does the QJ71MT91 support Modbus RTU (serial) in addition to Modbus TCP (Ethernet)?
A: No. The QJ71MT91 supports only Modbus TCP over Ethernet — it does not provide any serial RS-485 Modbus RTU capability. For Modbus RTU serial integration with the MELSEC-Q system, a separate QJ71C24N-R4 serial communication module (with RS-485 port and Modbus RTU master/slave protocol support) is required in addition to or instead of the QJ71MT91.
Do you need more references from this brand? Explore our full catalog of Mitsubishi here: https://atlantechdrives.com/manufacturers/mitsubishi/
