The QX28 is Mitsubishi Electric’s 16-point DC digital input module for the MELSEC-Q programmable controller series, accepting 24 VDC sink or source input signals from field devices including proximity switches, photoelectric sensors, pushbuttons, and limit switches, with both sink (NPN) and source (PNP) wiring configurations supported on the same module without hardware modification. As a versatile mixed-wiring input module, the QX28 eliminates the need to specify separate NPN and PNP input modules for installations where both sensor types are present in the same zone, reducing spare parts inventory and simplifying panel wiring design. Atlantech Drives holds stock of the QX28. Contact us for fast worldwide delivery and competitive pricing.
What Is the QX28?
The QX28 is a MELSEC-Q series digital input module providing 16 input points organised in a single common group, each capable of accepting 24 VDC signals in either sink (NPN, common positive) or source (PNP, common negative) wiring topology. The module mounts in any peripheral slot of a MELSEC-Q main or extension base unit and communicates with the CPU via the MELSEC-Q high-speed internal bus, with input status refresh occurring every scan cycle. The QX28 uses optical isolation between the field-side input circuits and the internal bus, providing 2,500 VAC isolation to protect the CPU and other modules from field-side voltage transients. Field wiring connects to a 40-pin connector (FCN-361 series or equivalent) on the module front panel — Mitsubishi offers pre-wired connector assemblies and terminal block adapters for the QX28 to simplify field wiring termination. The QX28 is programmed as standard digital input data in GX Works2, with input points mapped to the X device area of the CPU’s I/O address space.
Key Technical Specifications
- Model: QX28
- Input Points: 16
- Input Voltage: 24 VDC (20.4–28.8 VDC operating range)
- Input Current: Approx. 4 mA per point at 24 VDC
- Wiring Type: Sink (NPN) and Source (PNP) — selectable per common group
- ON Voltage: 14.4 VDC minimum
- OFF Voltage: 6.0 VDC maximum
- Input Response Time: 1 ms (standard), 0.1 ms to 70 ms (adjustable)
- Isolation: Optical, 2,500 VAC between field and internal bus
- Common Configuration: 16 points per common (1 common terminal)
- Connector Type: 40-pin FCN connector
- Operating Temperature: 0°C to 55°C
- Current Consumption (5 VDC internal bus): 0.10 A
- Weight: Approx. 0.16 kg
Compatibility & System Integration
The QX28 is compatible with all MELSEC-Q base units (main and extension) and all MELSEC-Q CPU modules. In GX Works2 and GX Works3, the QX28’s input points are automatically mapped to the X device area based on the module’s slot position — no special configuration is required for standard input mapping. For applications requiring high-speed input response, the QX28’s input filter time can be reduced from the default 10 ms to 0.1 ms in GX Works2’s I/O assignment settings, enabling reliable detection of pulse signals from incremental encoders or high-speed proximity switches without missing edges. The QX28 is compatible with Mitsubishi’s A6TBXY36 and A6TBX70 terminal block adapters, which convert the 40-pin FCN connector to screw-terminal or spring-clamp termination blocks for easier field wiring — these adapters are particularly recommended for installations requiring frequent maintenance access to individual input terminals. For multi-CPU MELSEC-Q systems, the QX28 can be assigned to any CPU in the base using GX Works2’s I/O assignment editor, enabling the same physical module to report input status to a specific CPU in a multi-domain control architecture.
Expert Tips
When wiring the QX28 in a mixed NPN/PNP installation, verify the common terminal polarity before connecting field devices — in sink (NPN) wiring, the common terminal is connected to 24 VDC positive, and field device outputs pull the input terminal low to signal ON. In source (PNP) wiring, the common terminal is connected to 0 VDC, and field device outputs push the input terminal high to signal ON. Mixing NPN and PNP devices on the same common group without changing the common wiring will result in incorrect input states — if both sensor types must coexist in the same input group, use separate input modules (one per sensor type) or specify a module with individually configurable commons. For installations with long field wiring runs (over 50 m), add a 0.1 µF ceramic capacitor between each input terminal and the common terminal at the module end of the wiring to suppress high-frequency noise induced by cable capacitance and adjacent power wiring — this is particularly important in environments with variable frequency drives or servo amplifiers running adjacent cable trays, where switching noise is prevalent. Always verify that the field device’s output voltage under load remains above the QX28’s 14.4 VDC minimum ON threshold, as some NPN open-collector sensors with high saturation voltage may not reliably turn ON the QX28’s input at the end of a long cable run.
Frequently Asked Questions
Q: Can the QX28 be used with 2-wire (NAMUR) proximity sensors?
A: No. The QX28 is designed for 3-wire NPN or PNP transistor output sensors and is not compatible with 2-wire NAMUR sensors, which require a different input circuit with a defined current sink in the OFF state. For NAMUR sensor integration, Mitsubishi’s QX40-S1 module (which includes NAMUR-compatible input circuits) or an external NAMUR signal conditioner between the sensor and QX28 input should be used.
Q: What is the difference between the QX28 and the QX40 digital input module?
A: The QX28 supports both sink and source wiring on the same module (selectable by common terminal wiring), while the QX40 is a sink-only (NPN) 16-point input module. The QX40 is typically specified in pure NPN installations for its lower cost, while the QX28 is the preferred choice when PNP sensors or mixed NPN/PNP wiring is required. Both modules provide 16 input points with identical response time adjustment options and FCN-40 connector termination.
Q: How do I adjust the input filter time on the QX28 in GX Works2?
A: In GX Works2, navigate to PLC Parameters → I/O Assignment, select the slot containing the QX28, and click the Switch Setting button. The input response time (filter time) can be set individually for each 4-point group from 0.1 ms to 70 ms. The default setting is 10 ms for all groups. Reducing the filter time to 0.1 ms enables detection of high-speed pulse inputs but also increases sensitivity to noise — use the minimum filter time only for inputs that genuinely require it.
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