The FX3U-128ASL-M is Mitsubishi Electric’s AS-Interface (AS-i) master station module for the MELSEC FX3U compact PLC series, enabling any FX3U base unit to act as an AS-Interface master — connecting up to 62 AS-i slave devices (standard addressing) or 124 slaves (extended addressing) on a single 2-wire AS-i cable for binary field device communication including pushbuttons, proximity sensors, limit switches, solenoid valve actuators, and pilot lights — without requiring individual signal wires from each field device to the PLC cabinet. As the most cost-effective distributed binary I/O technology for factory automation, AS-Interface via the FX3U-128ASL-M dramatically reduces field wiring costs and installation time for machines with densely distributed binary sensors and actuators. Atlantech Drives holds stock of the FX3U-128ASL-M. Contact us for fast worldwide delivery and competitive pricing.
What Is the FX3U-128ASL-M?
The FX3U-128ASL-M is a MELSEC FX3U series special function module implementing the AS-Interface (Actuator Sensor Interface) protocol at master level, compliant with EN 50295 / IEC 62026-2. AS-Interface is a two-wire network that carries both data (12 kbps, Manchester-encoded) and 24 VDC power on the same unshielded flat cable — the AS-i cable supplies up to 8 A total for powering field devices, eliminating the need for separate power supply cables to sensors and actuators. Each standard AS-i slave (digital I/O slave) provides up to 4 binary inputs and 4 binary outputs, with up to 62 slaves on a single AS-i network providing a maximum of 248 binary inputs and 248 binary outputs from a single FX3U-128ASL-M module. The module connects to the FX3U extension bus and maps AS-i slave I/O data to FX3U device memory via auto-refresh — the FX3U ladder program reads AS-i slave input states and writes output states using standard X/Y/M device instructions without special FROM/TO programming. An integrated AS-i power supply function is built into the FX3U-128ASL-M, eliminating the need for a separate AS-i power supply unit in systems with moderate field device power consumption.
Key Technical Specifications
- Model: FX3U-128ASL-M
- Function: AS-Interface master station
- Protocol: AS-Interface V2.1 (EN 50295 / IEC 62026-2)
- Maximum Standard Slaves: 62 (addresses 1–31 × 2 sub-addresses per address)
- Maximum Extended Addressing Slaves: 124 (A/B addressing)
- Maximum Binary Inputs: 248 (62 slaves × 4 inputs)
- Maximum Binary Outputs: 248 (62 slaves × 4 outputs)
- AS-i Cable: Unshielded flat 2-wire (yellow for standard, black for auxiliary power)
- Maximum Cable Length: 100 m per segment (extendable to 300 m with repeaters)
- Built-in AS-i Power Supply: Yes (integrated, up to 8 A)
- Extension Bus Occupation: 32 I/O points
- Current Consumption (FX3U bus): 0.15 A at 5 VDC
- Operating Temperature: 0°C to 55°C
- Weight: Approx. 0.22 kg
Typical Industrial Applications
The FX3U-128ASL-M is deployed in applications where large numbers of binary field devices must be connected with minimal field wiring infrastructure. In assembly line conveyor systems with 30–60 workstations, each workstation equipped with a call button, status lamp, and presence sensor, the FX3U-128ASL-M connects all devices via a single AS-i cable loop running alongside the conveyor — replacing 90–180 individual signal wires between each device and the PLC cabinet with a single 2-wire cable. The total field wiring installation time reduction in such applications is typically 60–70% compared to conventional discrete wiring, with corresponding reduction in panel termination blocks and cable trays. In robotic welding cells, the FX3U-128ASL-M connects weld tip presence sensors, fixture clamp position sensors, part presence sensors, and status indicator columns at each fixture location via AS-i cable — the ability to mix inputs and outputs on the same AS-i slave module at each fixture point eliminates the need to run separate input and output cables to each fixture. Vending machine and packaging machine operators’ panels frequently use AS-i for pushbutton and indicator wiring — a panel with 30 pushbuttons and 30 indicator lamps can be wired with 8 AS-i slaves and a single cable loop rather than 60 individual signal wires, and AS-i’s flat yellow cable snaps directly into the back of IP67-rated slave modules without connectors, enabling hygienic panel designs for food industry applications.
Installation Advice
AS-i cable installation does not require shielding, conduit, or careful impedance matching — the flat yellow 2-wire AS-i cable can be cut to length with standard cable cutters and the slave modules pierce the insulation to make contact (insulation displacement connection, IDC) when snapped onto the cable, without terminal blocks or connectors. However, several installation practices are critical for reliable network operation: ensure that the AS-i cable is oriented consistently (profile keys on the cable and slave connector must align) to prevent reverse polarity connection — the flat cable’s asymmetric profile makes reverse insertion physically difficult but verify with a continuity check before commissioning. For cable runs passing through damp environments (wash-down areas, outdoor routes), use the IP67-rated version of AS-i flat cable with hermetically sealed slave modules to prevent moisture ingress at the IDC connection points — moisture at the IDC connection causes gradual increase in contact resistance that manifests as intermittent slave communication errors. Limit the total AS-i cable length to 100 m per segment for reliable operation — cable runs exceeding 100 m cause increased signal attenuation and rise time degradation that the FX3U-128ASL-M’s receiver may not reliably detect, resulting in intermittent slave errors. For installations requiring more than 100 m of cable, use an AS-i repeater (available from Bihl+Wiedemann, Pepperl+Fuchs, and other AS-i component manufacturers) to regenerate the signal and extend the network to a second 100 m segment.
Frequently Asked Questions
Q: Can AS-i Safety slaves (for emergency stop monitoring) be connected to the FX3U-128ASL-M?
A: The FX3U-128ASL-M supports standard AS-i binary I/O slaves only. AS-i Safety at Work (SaW) slaves — which implement a safety-rated communication protocol for emergency stop buttons and safety gate switches — require a dedicated AS-i Safety monitor module to process the safety telegram. Standard AS-i safety monitors from Bihl+Wiedemann (ASM Safety Monitor) or Pilz can be installed alongside the FX3U-128ASL-M master on the same AS-i cable, with the safety monitor evaluating the SaW slave data independently for safety functions while the FX3U-128ASL-M handles the standard I/O data.
Q: What happens to AS-i slave output states if the FX3U CPU enters STOP mode?
A: When the FX3U CPU enters STOP mode, the AS-i slave output data in the auto-refresh device addresses is cleared to 0 (all outputs OFF) by the FX3U system, and the FX3U-128ASL-M transmits the zeroed output data to all slaves in the next AS-i cycle — deactivating all slave outputs. This behaviour is appropriate for most machine safety applications where outputs should return to their safe (OFF) state during CPU STOP. If specific outputs must maintain their state during CPU STOP, the FX3U-128ASL-M’s output hold function can be configured to retain the last output values during CPU STOP — configure this via the module parameter settings in GX Works2.
Q: Can the FX3U-128ASL-M be installed alongside a FX3U-16CCL-M in the same FX3U system?
A: Yes. The FX3U-128ASL-M and FX3U-16CCL-M are both FX3U right-side extension bus modules and can coexist in the same FX3U extension bus, each occupying 32 I/O points of the FX3U address space. This configuration enables an FX3U system to simultaneously manage AS-i binary field devices locally (via FX3U-128ASL-M) and communicate with remote CC-Link I/O stations and intelligent devices (via FX3U-16CCL-M) — a dual-network architecture providing comprehensive distributed I/O coverage from a single FX3U controller.
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