The FX3U-20SSC-H is Mitsubishi Electric’s SSCNET III/H servo network interface adapter for the MELSEC FX3U compact PLC series, providing 2-axis closed-loop servo control via Mitsubishi’s high-speed optical SSCNET III/H servo network — enabling direct connection of up to two Mitsubishi MR-J4 or MR-JE series servo amplifiers to any FX3U base unit through a fibre optic link, with full servo loop closure, encoder feedback, and advanced motion functions including linear interpolation, synchronous control, and cam control that are not available through pulse train interfaces. As the premium motion control adapter for the FX3U platform, the FX3U-20SSC-H bridges the gap between the FX3U compact PLC and Mitsubishi’s full MELSERVO servo ecosystem, delivering MELSEC-Q-class servo performance in a compact PLC format. Available now at Atlantech Drives with worldwide express delivery and 12-month warranty — complete our quote form for pricing and availability.
What Is the FX3U-20SSC-H?
The FX3U-20SSC-H is a MELSEC FX3U series special function adapter module implementing the SSCNET III/H (Servo System Controller Network III High-speed) protocol — Mitsubishi’s dedicated 150 Mbps full-duplex fibre optic servo communication standard. The adapter connects to the FX3U base unit’s left-side special adapter interface and communicates with up to two MR-J4 or MR-JE series servo amplifiers connected in a SSCNET III/H chain via optical fibre patch cables. Unlike pulse train positioning (FX3U-1PG, FX3U-2PG), the FX3U-20SSC-H provides full closed-loop servo control — the adapter sends position commands to the servo amplifiers at 0.888 ms communication cycle time, and the servo amplifiers execute the position control loop using the servo motor’s built-in encoder feedback, with actual position data returned to the adapter each cycle for monitoring and synchronisation. This closed-loop architecture enables the FX3U-20SSC-H to support linear interpolation between the two axes, electronic gearing synchronisation, and cam profile following — motion functions requiring tight axis coordination that open-loop pulse systems cannot reliably provide.
Key Technical Specifications
- Model: FX3U-20SSC-H
- Controlled Axes: 2 (SSCNET III/H servo amplifiers)
- Communication Protocol: SSCNET III/H (150 Mbps full-duplex, optical fibre)
- Compatible Servo Amplifiers: MR-J4-A/B series, MR-JE-A/B series (SSCNET III/H models)
- Communication Cycle Time: 0.888 ms
- Maximum Fibre Cable Length (per segment): 50 m (plastic optical fibre)
- Motion Functions: Linear interpolation, home return, JOG, speed control, synchronous control, cam control
- Positioning Patterns per Axis: 600
- Position Data Range: -2,147,483,648 to +2,147,483,647 command units
- Connection Interface: FX3U left-side special adapter bus
- Current Consumption (adapter bus): 0.20 A at 5 VDC
- Operating Temperature: 0°C to 55°C
- Weight: Approx. 0.20 kg
Architecture & Design Overview
The FX3U-20SSC-H implements a master-slave SSCNET III/H architecture in which the adapter acts as the network master, initiating communication with each connected servo amplifier in sequence at 0.888 ms intervals. The optical fibre medium provides complete galvanic isolation between the FX3U controller and the servo amplifiers, eliminating ground loop interference that affects pulse train connections in installations with multiple servo drives sharing a common cabinet ground reference. The 150 Mbps bidirectional communication bandwidth supports transmission of position commands, velocity feed-forward, torque limiting, and gain switching commands from the adapter to each servo amplifier, while returning actual position, actual velocity, torque, alarm status, and I/O state data from each amplifier in every communication cycle. The adapter’s internal motion processor executes the trajectory generation (position profile calculation, interpolation, synchronisation) autonomously at the SSCNET III/H cycle rate, independent of the FX3U CPU scan time — this separation of motion execution from PLC scan ensures that servo position control timing is deterministic regardless of CPU scan time variability caused by complex ladder programs or communication processing. The FX3U program interacts with the adapter via FROM/TO buffer memory instructions, initiating motion commands, reading axis status, and updating motion parameters without time-critical programming constraints.
Installation Advice
SSCNET III/H optical fibre installation for the FX3U-20SSC-H uses plastic optical fibre (POF) with push-pull SH connector terminations — significantly easier to handle than glass fibre, with a minimum bend radius of 25 mm and tolerance for casual routing in cable trays alongside power cables. However, plastic optical fibre degrades in ultraviolet light exposure — if cable routing passes through areas with strong UV light sources (such as UV curing ovens or high-intensity lighting), use UV-resistant conduit to protect the fibre cable. Ensure the SSCNET III/H fibre cables are correctly connected in the chain order: the adapter’s CN1 port connects to servo amplifier 1’s CN1A port, and servo amplifier 1’s CN1B port connects to servo amplifier 2’s CN1A port — incorrect chain order or reversed connections prevent network initialisation. After completing the hardware installation and before commissioning the motion program, perform the servo amplifier gain tuning via MR Configurator2 software connected to each amplifier’s USB port — the default servo gains are conservative and typically require adjustment for the actual mechanical system (inertia, friction, stiffness) to achieve optimal positioning performance. Verify that the FX3U-20SSC-H adapter is the leftmost special adapter in the chain (position 1, directly adjacent to the FX3U base unit) when installed alongside other FX3U special adapters — the SSCNET III/H communication timing requires position 1 for the 20SSC-H adapter to function correctly.
Frequently Asked Questions
Q: Can the FX3U-20SSC-H control MR-J4-A servo amplifiers in pulse train input mode instead of SSCNET III/H mode?
A: No. The FX3U-20SSC-H exclusively uses SSCNET III/H optical communication and cannot interface with servo amplifiers via pulse train. For MR-J4-A amplifiers configured in pulse train input mode, use the FX3U-2PG adapter or the FX3U base unit’s built-in Y0/Y1 pulse outputs instead. MR-J4-B amplifiers (SSCNET III/H dedicated) are the compatible model for the FX3U-20SSC-H — verify the servo amplifier model suffix before specifying the connection interface.
Q: Does the FX3U-20SSC-H support 2-axis linear interpolation between its two servo axes?
A: Yes. The FX3U-20SSC-H supports 2-axis linear interpolation, generating coordinated position commands on both servo axes simultaneously to move the controlled mechanism along a straight-line path between two points at a configured composite speed. The interpolation is executed by the adapter’s internal motion processor at SSCNET III/H cycle rate, providing smooth coordinated motion without CPU scan time dependency — a key advantage over pulse train interpolation implementations that are constrained by FX3U scan time timing accuracy.
Q: What is the maximum servo motor power that can be controlled by the FX3U-20SSC-H?
A: The FX3U-20SSC-H controls MR-J4-B or MR-JE-B servo amplifiers of any power rating within the SSCNET III/H compatible range — from MR-J4-10B (100W) up to MR-J4-22KB (22 kW) and larger. The adapter itself has no power limitation — the maximum controllable servo power is determined by the servo amplifier and motor specifications, not by the FX3U-20SSC-H adapter.
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