The FX3U-128MT/ESS is Mitsubishi Electric’s maximum-capacity transistor sink output base unit for the MELSEC FX3U compact PLC series, providing 64 digital inputs and 64 NPN open-collector transistor outputs in a self-contained AC-powered controller — the NPN sink counterpart to the FX3U-128MT/ES-A source model, and the largest I/O density available in any single FX3U base unit for North American and Japanese machine builders where NPN sink output wiring convention is standard. With 128 total I/O points, built-in 100 kHz pulse output for 2-axis positioning, and unlimited transistor output switching life, the FX3U-128MT/ESS delivers the full FX3U capability set at maximum I/O density for complex standalone machine automation without expansion modules. In stock at Atlantech Drives. Request your quote within 24-48 hours with worldwide shipping and 12-month warranty.
What Is the FX3U-128MT/ESS?
The FX3U-128MT/ESS is a MELSEC FX3U series compact PLC base unit with transistor sink (NPN open-collector) outputs, providing 64 input points (X0–X77, octal numbering) and 64 transistor output points (Y0–Y77). The «ESS» suffix designates sink (NPN) transistor outputs with AC power supply — the output common terminal connects to the external +24 VDC supply positive, and the NPN transistors pull individual output terminals to 0 VDC when activated, sourcing current through the load from the common terminal. This wiring polarity is compatible with PNP-input field devices — sensors and actuators where the input activates when the input terminal is pulled to 0 VDC — standard in Japanese automation practice and widely used in North American installations. The FX3U-128MT/ESS is electrically identical to the FX3U-128MT/ES-A in all respects except output transistor polarity, sharing the same CPU architecture (0.065 µs per basic instruction), 64,000-step program memory, 6 built-in high-speed counter inputs, and 100 kHz pulse generation on Y0 and Y1 for direct stepper and servo positioning without additional hardware.
Key Technical Specifications
- Model: FX3U-128MT/ESS
- Total I/O Points: 128 (64 inputs + 64 outputs)
- Input Type: 24 VDC sink (X0–X77)
- Output Type: Transistor sink/NPN open-collector (Y0–Y77), 0.5 A per point, 5–30 VDC
- Output Polarity: Sink (NPN) — common terminal to +24 VDC positive
- High-Speed Pulse Output: Y0 and Y1 up to 100 kHz; Y2 up to 10 kHz
- Power Supply: 100–240 VAC, 50/60 Hz
- Program Memory: 64,000 steps
- Execution Speed: 0.065 µs per LD instruction
- Built-in High-Speed Counters: 6 inputs (100 kHz)
- Communication: RS-422 (built-in)
- Real-Time Clock: Built-in
- Battery Backup: Yes
- Expansion: Up to 8 extension units (max 256 total I/O), up to 4 special adapters
- Operating Temperature: 0°C to 55°C
- Dimensions (W × H × D): 285 mm × 90 mm × 87 mm
- Weight: Approx. 1.55 kg
Compatibility & System Integration
The FX3U-128MT/ESS integrates with the complete FX3U expansion ecosystem — FX3U extension I/O units (FX3U-16EX, FX3U-16EYT, FX3U-16EYR) connect to the right side for additional I/O beyond 128 points, up to a maximum of 256 total points. FX3U special adapter modules (FX3U-4AD-ADP, FX3U-4DA-ADP, FX3U-4LC temperature control, FX3U-1PG and FX3U-2PG pulse positioning adapters) connect to the left side for analogue, temperature, and multi-axis positioning without consuming extension bus capacity. The built-in RS-422 port supports programming via GX Works2 and GX Works3, and communication with GOT HMI terminals, Modbus RTU masters, and inverter drive networks using Mitsubishi’s built-in communication protocols. For Ethernet connectivity, the FX3U-ENET-ADP Ethernet adapter installs on the left-side special adapter port, providing 100BASE-TX for MC protocol communication with SCADA systems, MES platforms, and remote GX Works2 programming access over the plant LAN. CC-Link master functionality is available via the FX3U-16CCL-M option board installed directly on the base unit’s option board connector — this configuration enables the FX3U-128MT/ESS to manage up to 64 CC-Link remote I/O stations and intelligent device stations simultaneously with the main CPU program, providing a distributed I/O architecture when the 256-point direct expansion limit is insufficient.
Troubleshooting Guide
For an FX3U-128MT/ESS with specific output points that fail to activate, the first diagnostic step is to use GX Works2’s device monitor to force the affected Y output ON while the machine is in a safe state — if the output activates correctly under force, the fault is in the ladder program logic rather than the output hardware. If the output does not activate under force, check the external 24 VDC load supply voltage at the output common terminal — a supply voltage below 21.6 VDC causes unreliable output transistor saturation and can prevent full output activation. For multiple outputs failing simultaneously, check the output common terminal connection — a loose common terminal causes all outputs in the connected group to fail simultaneously, which can be misidentified as a CPU fault. For outputs that activate intermittently with no consistent pattern, use GX Works2’s buffer monitor to check special relay M8028 (output error detection flag) — if M8028 turns ON coinciding with the intermittent failures, the output circuit is detecting overcurrent events caused by a marginal short circuit or inductive load without adequate suppression. For the high-speed pulse output points Y0 and Y1, if positioning moves start but the motor does not reach the commanded position, verify that the stepper driver’s step resolution setting matches the FX3U-128MT/ESS positioning instruction’s step count — a mismatch between the FX3U’s commanded pulse count and the stepper driver’s configured microstep division causes position errors that accumulate with each move cycle.
Frequently Asked Questions
Q: What is the maximum total I/O achievable with the FX3U-128MT/ESS including all extension units?
A: The FX3U-128MT/ESS supports up to 8 FX3U extension I/O units, each providing up to 16 additional I/O points. With 8 extension units of FX3U-16EX (16 inputs each) and FX3U-16EYT (16 outputs each) in combination, the total I/O can reach 256 points maximum — the absolute hardware limit of the FX3U CPU’s I/O address space. In practice, the mix of input and output extension units determines the achievable total, as the 256-point limit applies to the combined input and output address range.
Q: Can the FX3U-128MT/ESS connect to a Mitsubishi GOT2000 HMI directly without additional communication hardware?
A: Yes. The FX3U-128MT/ESS’s built-in RS-422 port supports direct connection to a GOT2000 HMI using a standard FX3U-GT cable (GT11H-C cable assembly). The GOT2000 communicates with the FX3U-128MT/ESS using the FX3U CPU communication protocol, providing full read/write access to all PLC device data — D registers, M relays, Y outputs, X inputs, T timers, and C counters — from the HMI screen without requiring additional communication modules or Ethernet connectivity.
Q: Is the FX3U-128MT/ESS suitable for safety-rated machine control applications?
A: The FX3U-128MT/ESS is a standard (non-safety-rated) PLC and is not certified for SIL or PLd safety applications. For machine control applications requiring IEC 62061 SIL2 or EN ISO 13849 PLd safety functions, safety-related control functions (emergency stop monitoring, safety gate monitoring, two-hand control) must be implemented using a dedicated safety controller or safety relay module — the FX3U-128MT/ESS handles the standard machine control logic while the safety functions are managed in a separate, certified safety system.
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