The QD75MH4 is Mitsubishi Electric’s 4-axis high-speed positioning module for the MELSEC-Q programmable controller series — providing differential line driver pulse train output at up to 4 Mpps per axis for positioning control of up to 4 independent servo axes via MR-J4-A, MR-J3-A, MR-J2S-A, or any compatible servo amplifier with differential pulse train input, delivering precise multi-axis coordinated motion control from a MELSEC-Q base unit without requiring a dedicated motion CPU. The QD75MH4 is available through Atlantech Drives — quote within 24-48 hours, worldwide express shipping, 12-month warranty.
What Is the QD75MH4?
The QD75MH4 is a MELSEC-Q series intelligent positioning module providing 4 independent servo axis channels, each with differential line driver (RS-422A) pulse output at up to 4 Mpps and encoder feedback input for closed-loop position verification. The «MH» designation indicates the high-speed differential pulse output variant (as opposed to the «D» open-collector variant in the QD75D series) and the «4» indicates 4-axis capacity. Each axis channel independently manages position command pulse generation, encoder pulse counting for position feedback, home return operation, speed control, and position data management — the MELSEC-Q CPU program reads positioning results and writes positioning commands to the QD75MH4 via the module’s buffer memory using FROM/TO instructions or auto-refresh, while the QD75MH4’s internal positioning processor executes the motion profiles autonomously between CPU scan cycles. The QD75MH4 supports linear interpolation between any two axes in the module (X-Y, X-Z, Y-Z, or any other pair), enabling coordinated 2-axis linear path generation directly from the module without requiring motion controller software — the motion profile (start position, end position, feed rate) is configured in the QD75MH4’s positioning data table, and the CPU triggers execution via a single positioning start command. The module also supports speed-position switching control, enabling a single motion sequence to transition between speed control and position control at a defined trigger position — useful for conveyor-to-stop positioning and press approach sequences.
Key Technical Specifications
- Model: QD75MH4
- Axes: 4 independent positioning axes
- Pulse Output Type: Differential line driver (RS-422A), up to 4 Mpps per axis
- Encoder Input: Differential line driver (RS-422A), up to 4 Mpps per axis
- Position Data: Up to 600 positioning points per axis (stored in module non-volatile memory)
- Interpolation: 2-axis linear interpolation (any axis pair)
- Home Return Methods: Near-point dog, count, data set, stopper, and others
- Control Modes: Position control, speed control, speed-position switching, position-speed switching
- Compatible CPUs: MELSEC-Q Q02UCPU, Q03UDECPU, Q04UDEHCPU, and other Q-series CPUs
- Installation: Q-series base unit intelligent function module slot
- Power Consumption: 5 VDC internal bus, approx. 0.75 A
- Operating Temperature: 0°C to 55°C
- Dimensions (W × H × D): 27.4 mm × 98 mm × 90 mm
- Weight: Approx. 0.15 kg
Architecture & Design Overview
The QD75MH4 implements a distributed motion control architecture where the module’s internal positioning processor handles the real-time aspects of motion control — pulse generation, encoder counting, position error calculation, and positioning data management — independently of the MELSEC-Q CPU’s scan cycle. This distributed architecture decouples servo axis response time from the CPU scan time: the QD75MH4 generates positioning pulses at up to 4 Mpps continuously regardless of the CPU scan time, while the CPU program accesses positioning status and commands positioning operations at its normal scan rate (typically 1–10 ms) without affecting servo response. Each of the QD75MH4’s 4 axis channels maintains its own positioning data table (up to 600 position points per axis) in non-volatile flash memory — the positioning data (target position, command speed, dwell time, M-code output) is pre-loaded into the module’s positioning table using GX Works2’s QD75 positioning data editor, enabling complex multi-step positioning sequences to execute from a single CPU start command. The QD75MH4’s 2-axis linear interpolation function generates coordinated pulse output on two selected axes simultaneously, computing the required pulse rate ratio to maintain a straight-line motion path between the start and end positions — the motion path accuracy is determined by the servo amplifier’s encoder resolution and the QD75MH4’s interpolation algorithm, providing straight-line path accuracy suitable for most machine tool auxiliary axes and Cartesian robot applications. The module’s hardware differential encoder input (up to 4 Mpps) enables the QD75MH4 to count encoder pulses from the servo motor’s encoder (via a signal splitter or the servo amplifier’s encoder output) for position verification without a dedicated position feedback card.
Frequently Asked Questions
Q: What is the difference between the QD75MH4 and the QD75D4 positioning module?
A: The QD75MH4 uses differential line driver (RS-422A) pulse output capable of up to 4 Mpps per axis, providing superior noise immunity and higher pulse frequency than the QD75D4 which uses open-collector pulse output limited to 200 kpps per axis. The QD75MH4 is the recommended specification for all new designs using MR-J4-A or MR-J3-A servo amplifiers with differential pulse input — the 4 Mpps capability enables higher axis speeds at the same encoder resolution, and the RS-422A differential output is far less susceptible to EMI from adjacent motor cables than open-collector outputs. The QD75D4 is appropriate only when the servo amplifier’s command input is open-collector only, or when the installation’s lower maximum axis speed makes the 200 kpps open-collector limit adequate.
Q: Can the QD75MH4 control all 4 axes simultaneously in coordinated motion?
A: The QD75MH4 supports 2-axis linear interpolation between any pair of its 4 axes — simultaneous coordinated motion of 2 axes in a straight-line path. True 3-axis or 4-axis coordinated motion (for circular interpolation, 3D path generation) requires a MELSEC-Q dedicated motion CPU (Q172DSCPU or Q173DSCPU) with SSCNET III/H servo amplifiers — the QD75MH4 does not support circular interpolation or more than 2-axis simultaneous interpolation. For 4-axis independent simultaneous positioning (where all 4 axes move simultaneously to different independent targets without coordinated path control), all 4 axes can be started simultaneously by the CPU program and will execute their respective positioning profiles independently.
Q: Does the QD75MH4 support absolute position detection with MR-J4-A servo amplifiers equipped with 22-bit absolute encoders?
A: Yes. When the QD75MH4 is configured for absolute position detection (parameter setting in GX Works2’s QD75 system parameter), the module reads the absolute position from the MR-J4-A servo amplifier at power-up via the encoder feedback input — the servo amplifier transmits the absolute encoder position as a serial data signal on the encoder output (ENC) terminals, which the QD75MH4 reads to establish the current axis position without requiring a homing cycle. This absolute position detection eliminates the homing requirement at each power-up, enabling immediate machine operation after power restoration.
Do you need more references from this brand? Explore our full catalog of Mitsubishi here: https://atlantechdrives.com/manufacturers/mitsubishi/
