The FX3U-128MR/ES-A is the 128-point relay output CPU unit of the MELSEC FX3U series from Mitsubishi Electric, combining 64 DC inputs and 64 relay outputs in a single base unit powered by 100–240 V AC supply. It is the highest I/O density model in the FX3U standalone CPU range, designed for complex machine control applications requiring large numbers of relay-switched outputs without the addition of extension modules. Available now at Atlantech Drives with worldwide express delivery and 12-month warranty — complete our quote form for pricing and availability.
What Is the Mitsubishi FX3U-128MR/ES-A?
The FX3U-128MR/ES-A is the maximum built-in I/O configuration within the FX3U CPU family. The suffix breakdown is: 128 = total I/O points, M = main unit (CPU), R = relay output, ES = CE-marked international version, A = AC power supply (100–240 V AC). With 64 inputs and 64 relay outputs built in, this unit covers a wide range of machine control applications within the CPU housing alone. The FX3U platform supports up to 256 additional I/O points via extension modules (FX3U-series extension blocks or FX2N-series blocks via adapter), giving a maximum system I/O of 384 points. The built-in relay outputs switch AC and DC loads up to 240 V AC / 30 V DC, making the FX3U-128MR/ES-A particularly suitable for retrofit and upgrade projects in legacy machine environments where AC output switching is standard.
Key Technical Specifications
- Total built-in I/O: 128 (64 inputs + 64 outputs)
- Input type: DC sink/source, 24 V DC
- Output type: Relay contact (volt-free)
- Output voltage range: 5–240 V AC / 5–30 V DC
- Output current per point: 2 A (resistive), 80 VA (AC inductive)
- Program memory: 64,000 steps (with memory cassette)
- Basic instruction execution: 65 ns
- Built-in communication: RS-422 (programming port), USB (mini-B)
- Optional communication: FX3U-232-BD, FX3U-485-BD, FX3U-ENET-ADP
- High-speed pulse output: 100 kHz, 3 axes (transistor output only — not applicable on relay output model)
- Power supply: 100–240 V AC, 50/60 Hz
- Certifications: CE, UL
- Operating temperature: 0 to 55°C
- Dimensions (W × H × D): 480 × 90 × 87 mm
Architecture & Design Overview
The FX3U-128MR/ES-A uses a dual-bus architecture that separates the high-speed CPU processing bus from the I/O refresh bus, enabling the 65 ns instruction execution speed to be maintained independently of the I/O point count. The 64 relay output points are organised in groups of 8 with a shared common per group, allowing different load voltages (AC or DC) to be assigned per common group — a significant wiring flexibility advantage over modules with a single shared common. The built-in AC power supply accepts 100–240 V AC without manual range selection, simplifying installation in facilities with varying supply voltages. The FX3U-128MR/ES-A includes a built-in USB mini-B port for direct laptop programming connection using GX Works2 without requiring an RS-422-to-USB converter, which was a significant practical improvement over earlier FX3U models requiring the SC-09 cable. Memory is expandable to 64,000 steps using an FX3U-FLROM-64L flash ROM cassette inserted into the CPU memory slot.
Installation Advice
Due to the 480 mm width of the FX3U-128MR/ES-A, verify DIN rail length and panel depth before specifying — the unit requires a minimum panel depth of 130 mm with the terminal block cover closed. For relay output wiring, connect load supply voltage to each 8-point common (COM) terminal corresponding to the output group — consult the FX3U-128MR/ES-A hardware manual wiring diagram to identify which COM terminal serves which output group, as incorrect COM wiring is the most frequent installation error on high-density relay output CPUs. Install surge suppressors (RC snubbers for AC loads, flyback diodes for DC inductive loads) across all inductive loads to protect relay contacts and extend electrical life beyond the rated 100,000 operations. For programming via USB, install the Mitsubishi FX USB driver before connecting the unit to a PC — Windows may not auto-detect the device correctly without the manufacturer driver, particularly on Windows 10 and Windows 11.
Frequently Asked Questions
Q: Can the FX3U-128MR/ES-A perform high-speed pulse output for servo or stepper control?
A: No. High-speed pulse output (up to 100 kHz) is only available on transistor output models (FX3U-128MT/ES-A). Relay contacts have a mechanical switching time of approximately 10 ms, making them entirely unsuitable for pulse train generation. For combined relay output switching and servo pulse control, use a transistor output CPU (FX3U-128MT) and add relay output extension blocks for the AC-switched circuits.
Q: What is the maximum number of extension modules that can be added to the FX3U-128MR/ES-A?
A: Up to 8 extension blocks can be connected, adding a maximum of 256 additional I/O points. However, since the CPU already has 128 built-in points, the practical extension count is typically limited by the FX3U’s maximum system I/O of 384 points rather than the 8-module physical limit.
Q: Is the FX3U-128MR/ES-A compatible with GX Works3?
A: GX Works3 does not fully support FX3U series CPUs — GX Works2 is the correct programming software for FX3U. GX Works3 is required for FX5U series CPUs only. FX3U programs created in GX Works2 can be imported into GX Works3 for migration purposes, but ongoing FX3U development and maintenance should be performed in GX Works2.
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