MITSUBISHI PLC Programmable Logic Controller FX3U-2HC FX3U2HC

Mitsubishi FX3U-2HC | FX3U High-Speed Counter Module — 2×200kHz / A/B Quadrature / Pulse-Direction / Up-Down / 4 DO Outputs / 5VDC Bus-Powered

Why a Dedicated Hardware Counter Module

The FX3U base unit includes built-in high-speed counters, and for many applications they are adequate. The FX3U-2HC applies when an application requires counting reliability independent of the PLC scan cycle.

At 200kHz, pulses arrive every 5 microseconds. The FX3U-2HC's counter hardware increments on every incoming pulse in real time, completely independently of whatever the CPU's scan cycle is doing. The current count is always correct in the module's buffer memory. The PLC programme reads the count via FROM instructions each scan — the read is always current, never stale by a scan cycle.

For a 2000ppr encoder on a servo at 3000 rpm (counting at 100kHz), the FX3U-2HC never misses a pulse. This hardware independence is the fundamental reason to specify the dedicated module over relying on software-based counting.

Key Specifications

Three Counting Modes

A/B Phase (Quadrature): Two-phase encoder input. The counter increments on A-phase edges; B-phase determines direction. Detects both position and direction simultaneously — the standard for rotary encoders on positioning axes.

Pulse / Direction: Single pulse train with a separate direction signal. Each pulse increments or decrements depending on the direction input state. Used with stepper motor controllers and servo systems with pulse-train position outputs.

Up / Down: Two separate inputs — one for count-up, one for count-down. For dual-sensor systems, bidirectional flow measurement, or separate forward/reverse sensors.

Both channels can run different modes simultaneously. CH1 in quadrature for an encoder, CH2 in pulse-direction for a stepper — both counting at 200kHz with complete independence.

Application Scenarios

Encoder-based positioning: Rotary encoder in A/B quadrature mode. The module counts in real time; the PLC reads position from buffer memory and controls the servo based on current position.

High-speed production counting: 200kHz photoelectric sensor counting parts at high speed. Hardware output activates a diverter gate at exact batch count.

Dual-axis length measurement: Two measuring wheel encoders on CH1 and CH2. Both count simultaneously, giving the PLC two real-time length values each scan.

FAQ

Q1: Can the FX3U-2HC be used with FX3G or FX3UC base units?

No — it is specified for the FX3U series only. The FX3UC and FX3G have different special function module interfaces. Confirm compatibility with the installed base unit from the Mitsubishi MELSEC-F hardware documentation.

Q2: How many FX3U-2HC modules can be installed per FX3U system?

The FX3U supports up to eight special function modules total in the expansion chain. Multiple FX3U-2HC units can be installed within this limit — each occupies one special function module position.

Q3: How does the programme read the counter value?

Using the FROM instruction to read buffer memory (BFM) registers. The current count for each channel is at defined BFM addresses. Preset comparison values are written using TO instructions. The FX3U-2HC instruction manual provides the complete BFM address map.

Q4: Does the FX3U-2HC support ring counter (cyclic reset) operation?

Yes. The counter can be configured to reset to zero at a preset value, supporting cyclic position applications — rotary table indexing, cam follower tracking, and similar repeating-cycle positioning tasks.

Q5: Does the FX3U-2HC accept differential RS-422 encoder signals?

Yes — it accepts both differential (RS-422 / 5V line driver) and open-collector (NPN / 5–24V) encoder signals. Wiring differs between the two types. Confirm the connection method from the FX3U-2HC hardware manual for the specific encoder output type.