An open-source EtherCAT motion controller for simracing motion rigs, built around STEPPERONLINE A6-EC servo drives. One shared motion core, two platforms:
- Raspberry Pi (recommended): headless daemon on Debian with the PREEMPT_RT kernel. Deterministic 2-4 kHz control loop, browser-based UI, physical GPIO control panel support (or HID button binds). A dedicated Pi 4B acts as the motion controller; motion software (e.g. SimHub) on the game PC sends telemetry to the Pi via UDP.
- Windows: Qt desktop application using Npcap for raw EtherCAT frames. Fully functional, but Windows is not a real-time OS. See KNOWN_LIMITATIONS before choosing this path.
⚠️ Read SAFETY.md firstThis software commands industrial servo drives that move a simulator with a human in it. A hardware emergency stop wired to the drives, independent of this software, is required, not optional. No warranty; you assume all risk of installation, commissioning, and use. See SAFETY.md.
Pre-1.0, in active development. Runs daily on the author's rig (3x vertical actuators in CSP plus a belt tensioner in CST torque mode) on both platforms. Looking for beta testers with A6-EC hardware. See Community.
- EtherCAT master (SOEM) with DS402 drive management: staged init, DC-sync (SYNC0), fault monitoring/recovery, and provisioning tooling for A6-EC drives.
- Motion: UDP telemetry (e.g. SimHub) → per-axis tracking filter → CSP position drives (heave/pitch/roll) and CST torque drives (belt tensioners), with homing against hardstops, parking, e-stop staging, and telemetry-loss standby.
- Control surface: embedded web UI (status, config, drive cards, HID button bindings) served by the controller itself; optional GPIO panel (buttons + LEDs) on the Pi.
- Drives: STEPPERONLINE A6-EC series (ANCTL AS715N). Other DS402 drives may work but are untested; drive-specific register knowledge (fault codes, runaway-protection tuning) is specific to the A6 series.
- Controller: Raspberry Pi 4 (1GB+) with an additional USB-Ethernet NIC for the Pi-to-PC transport, or a Windows PC with a dedicated Intel NIC + Npcap.
- Telemetry: motion software on the game PC (e.g. SimHub) with custom UDP export.
- Safety hardware (required): a latched mechanical e-stop wired to the drives or a motor-power contactor, independent of this software, plus physical end-stops on every axis. See SAFETY.md.
- Raspberry Pi, blank SD card to running controller (one script): Docs/PI_SETUP.md
- Windows, start to finish (plain language): Docs/FIRST_SETUP_WINDOWS.md
- Build (Pi, on an already-tuned OS):
cmake -S pi -B pi/build && cmake --build pi/build -j2 - Build (Windows): open the top-level CMakeLists with Qt 6 + MSVC; Npcap SDK required.
- Configuration reference: Docs/CONFIG_REFERENCE.md
- Hexapod bring-up (rotary levers or linear struts): Docs/HEXAPOD_SETUP.md
- HTTP/WS command contract: Docs/COMMAND_CONTRACT.md
- Dedicated-controller deployment (NUC/laptop): Docs/NUC_DEPLOYMENT.md
nullCAT is free and will stay free. If you're building the hardware, the CAD, STLs, BOMs, and build guides for the rig it was developed on (actuators, brackets, belt tensioner, button box, enclosures) live on Patreon, which funds servo drives for compatibility testing and continued development.
Questions, build chat, and beta testing: Discord.
Bug reports and issues are welcome, especially hardware compatibility reports (drive model, firmware, what worked). Pull requests aren't open yet while the architecture settles; this will change after 1.0.
Copyright © 2026 Tim Palmgren (Ø Werks) tim@zerowerks.co.nz
GPL-3.0-or-later (see LICENSE). Third-party components: THIRD_PARTY_NOTICES.md.
SOEM is used under its GPLv3 option; corresponding source for every release is this repository at the release tag.
Built by Tim Palmgren (@crossthenoughts).