Orc

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Why

The PMUN1046A Universal Relay Controller (URC) is a sealed 12 V automotive relay box — ten relay outputs at 15 A each, per-channel fusing, and a busbar power input rated for up to two cables at 60 A each (Motorola's install manual), in a trunnion-mounted weatherproof chassis. Power goes straight to the busbar through cable glands — no special harness needed on the input side. Surplus units are common and cheap. Everything about it is desirable except that it only takes commands from a Motorola APX radio over a proprietary GCAI/USB link.

ORC replaces the controller board outright and makes the box a general-purpose, openly-controlled ten-channel power distribution module. ORC stands for Open Relay Controller.

ORC does not implement, emulate, or interoperate with the GCAI link. The controller board is an independent design that replaces the donor unit's controller board entirely; control is plain CAN or USB.

About the donor unit (PMUN1046A Universal Relay Controller)

Motorola's URC ships as part of the APX mobile radio ecosystem — a sealed relay box meant to switch scene lighting, sirens, and other accessory loads from the radio head. The relay board and chassis are generic 12 V automotive gear; only the controller board and its proprietary GCAI/USB command link to the radio are Motorola-specific. The relay board is entirely passive (relays, fuses, terminal blocks, busbar — no drivers or regulator of its own); a small internal harness carries switched 9 V coil drive between it and the controller board. Power input has no special harness at all — it lands straight on the busbar through the enclosure's cable glands. ORC keeps the relay board and chassis, and replaces the controller board and its proprietary radio link with plain CAN over terminal blocks — no proprietary connector anywhere in this design.

What it is

Category-wise this is a PDM (power distribution module) — the sPOD / Switch-Pros class of device — with an open CAN control interface instead of a proprietary wired link to the radio.

The board

ORC Rev A (green, right) and the OEM control PCB it replaces (blue, left).

ORC Rev A installed in the donor enclosure, alongside the OEM relay board that stays.

Full writeup, build story, and more photos: Open Relay Controller for the Motorola PMUN1046A.

Repo structure

One repo for design inputs, hardware, and firmware — no split-repo overhead for a project this size:

License

Different parts of the repo carry different licenses. Full texts are in LICENSES/; each directory also carries its own LICENSE file.

Hardware and software are licensed separately because they are separate works: under CERN-OHL's own definitions the ESP32-C3 is an Available Component, so a hardware license on the board does not reach the code running on it. See NOTICE for the summary and TRADEMARKS.md for trademark and affiliation.

Status

Real-hardware bring-up complete. Boards were sent to fab 2026-08-02, came back populated, and have been bench-tested since 2026-08-10: the PCA9555/I2C-isolator chain, the full 10-channel relay-driver mapping, and the CAN (RPDO1/TPDO1/TPDO2/Heartbeat) and USB control paths have all been exercised on real hardware. Five real firmware bugs were found and fixed along the way — BUG-001 through BUG-005, see docs/features/LOG.md for the full history — the last of which (a liveness stall under an absent/unpowered PCA9555) was verified on real hardware 2026-09-04. One real mechanical fault (a harness connector shorting against the enclosure chassis) was also found and fixed on the bench — see docs/circuit-draft.md for the follow-up enclosure-clearance item.

No further active development is planned barring a new bug. A couple of feature requests (FR-003, FR-005) remain open in the backlog but aren't being actively pursued right now.

Working conventions for part selection, asset verification, and check gates: docs/hardware-workflow.md. Bench test tooling for talking to a real board over CAN/USB from a PC: tools/.