The QD70P4 is Mitsubishi Electric’s 4-axis high-speed counter and simple positioning module for the MELSEC-Q programmable controller series, combining pulse train input counting and pulse train output positioning in a single intelligent function module — enabling simultaneous position feedback measurement from four encoder channels and pulse train position commands to four axes of stepper or servo drives without requiring separate counter and positioning modules. Designed for applications requiring compact, cost-effective multi-axis position control where the full functionality of the QD75 dedicated positioning modules is not required, the QD70P4 provides a streamlined pulse I/O solution for conveyor position tracking, rotary indexer control, and simple pick-and-place automation. In stock at Atlantech Drives. Request your quote within 24-48 hours with worldwide shipping and 12-month warranty.
What Is the QD70P4?
The QD70P4 is a MELSEC-Q series intelligent function module providing four channels of high-speed pulse counter input and four channels of pulse train output simultaneously. Each channel independently operates as either a high-speed counter (accepting AB quadrature, single-phase pulse, or CW/CCW pulse inputs from encoders, proximity sensors, or pulse generators) or a simple positioning output (generating step/direction or CW/CCW pulse trains to stepper drivers or servo drives with pulse input interfaces). The module’s dual counter/positioning architecture differs from the dedicated QD75 positioning modules — the QD70P4 provides simpler positioning functionality (preset position, home return, JOG operation) without the QD75’s advanced features such as S-curve acceleration, complex multi-step positioning patterns, or linear interpolation between axes. This streamlined feature set makes the QD70P4 the appropriate choice for applications requiring basic position control across four axes at lower system cost than four separate QD62/QD75 modules. The module communicates with the MELSEC-Q CPU via the high-speed internal bus using FROM/TO instructions or GX Works2 auto-refresh.
Key Technical Specifications
- Model: QD70P4
- Channels: 4 (each independently configurable as counter input or pulse output)
- Counter Input Types: AB quadrature (×1, ×2, ×4 multiplication), CW/CCW, single-phase
- Maximum Counter Input Frequency: 200 kpps per channel
- Counter Range: -2,147,483,648 to +2,147,483,647 (32-bit signed)
- Pulse Output Type: Open-collector, step/direction or CW/CCW
- Maximum Pulse Output Frequency: 200 kpps per channel
- Positioning Functions: Preset position, home return, JOG, speed control
- External Signals: Near-point DOG, limit switches (per channel)
- Current Consumption (5 VDC internal bus): 0.60 A
- External Power Supply: 24 VDC ±10%, 0.15 A (required)
- Operating Temperature: 0°C to 55°C
- Compatible CPUs: All MELSEC-Q series CPU modules
- Weight: Approx. 0.28 kg
Architecture & Design Overview
The QD70P4 implements a unified pulse I/O architecture where each of the four channels shares a common hardware interface capable of both input counting and output generation, with the channel direction (input or output) configured via buffer memory parameters in GX Works2. This unified architecture allows the module’s channel allocation to be adapted to different application requirements without hardware changes — a system initially requiring three counter inputs and one positioning output can be reconfigured to two inputs and two outputs by changing software parameters, without replacing the module. Each channel’s counter and position data is stored in dedicated buffer memory registers updated every 0.5 ms by the module’s internal processor, independent of the MELSEC-Q CPU scan cycle — this ensures that high-frequency encoder pulses are counted accurately even during CPU scan time peaks caused by communication processing or complex ladder execution. The module’s position preset function allows the counter value to be set to any value within the 32-bit range via buffer memory write, enabling absolute position systems where a reference position is established by a home sensor rather than by counting from power-up. For speed measurement applications, the QD70P4 calculates the rotational speed of the connected encoder by measuring the pulse interval and converting it to a configurable engineering unit (RPM, m/min, Hz) stored in the buffer memory for the CPU to read.
Troubleshooting Guide
If a QD70P4 counter channel reports incorrect or drifting counts, the first diagnostic step is to verify the encoder signal quality using an oscilloscope at the module input terminals — check that the A, B, and Z signal amplitudes are within the module’s specified input voltage range (typically 5V differential or 12/24V single-ended depending on the configured input type), that the signal rise and fall times are below 1 µs at the maximum counting frequency, and that there is no noise-induced spurious pulsing during signal transitions. Common causes of miscounting in industrial environments include capacitive coupling from adjacent VFD cables causing false pulses on encoder lines — verify that encoder cables are routed separately from motor power cables with at least 200 mm physical separation, and that the encoder cable shield is connected to earth at the module end only. If the pulse output channel is not generating pulses despite the CPU ladder program initiating a positioning operation, check the buffer memory’s axis operation status register — if the status shows «error» rather than «positioning in progress,» read the error code register to identify the specific fault. Common pulse output errors include missing external power supply (24 VDC not connected to the module’s external power terminals), limit switch input active (upper or lower limit switch in the activated state at the start of the move direction), and positioning data out of range (commanded position exceeds the configured software limit values).
Frequently Asked Questions
Q: Can the QD70P4 perform electronic gearing, synchronising a pulse output axis to a counter input axis?
A: The QD70P4 does not have a built-in electronic gearing function. Speed synchronisation between a counter input and a pulse output must be implemented in the MELSEC-Q CPU ladder program — the CPU reads the measured speed from the counter input channel’s buffer memory and writes a corresponding speed command to the pulse output channel’s buffer memory each scan cycle, implementing the gear ratio calculation in the ladder program. For applications requiring deterministic electronic gearing without CPU scan time dependency, the QD75P4N’s built-in speed synchronisation function or the QD77MS SSCNET III/H servo module provides a dedicated hardware gearing function.
Q: What is the maximum encoder frequency the QD70P4 can count reliably?
A: The QD70P4 accepts counter input frequencies up to 200 kpps in single-phase or CW/CCW mode. In AB quadrature mode with ×4 multiplication, the physical encoder frequency is up to 50 kHz (200 kpps ÷ 4 edges per cycle). For higher encoder frequencies, the QD62E (high-speed counter module, up to 500 kpps) should be specified for the counting function, with a separate pulse output module for the positioning function.
Q: Can the QD70P4 be used to measure the position of a linear scale encoder with 5 µm resolution?
A: Yes, provided the linear scale output frequency does not exceed 200 kpps at maximum traverse speed. For a 5 µm resolution linear scale, the output frequency at 1 m/s traverse speed is 200,000 pulses/second — exactly at the QD70P4’s maximum input frequency, leaving no margin for overspeed conditions. For linear scales with higher speed requirements or finer resolution (1 µm), the QD62E high-speed counter module supporting 500 kpps is a more appropriate specification.
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