SerialTap

SerialTap

An open-hardware board that puts an appliance's 5 V serial service port on Home Assistant. An ESP32-C3 with level translation, powered by the appliance itself: one 4-wire cable carries 5 V, GND, TX and RX.

The reference target is a Haier AS50QDFHRA split-system air conditioner, whose service port speaks GE Appliances GEA3 at 230400 8N1 (docs/gea3.md). The board is not specialised to it: any appliance exposing 5 V and a UART on a service connector is in scope.

Status

Version 1.0 has been sent to PCBWay; no board has been built yet. The v1.0 tag marks the sources the order was built from.

Every design stage is done: power-path simulation, parts, library, schematic, layout and three review gates. What is still owed is measurement on the real appliance: the rail's current has so far been inferred, not measured (ADR 0007).

The board

• 22 × 54 mm, 4-layer, single-sided assembly, no mounting holes. It keeps the OEM module's outline and sits in its slot.
• ESP32-C3-MINI-1 module. Programming and logs go over its built-in USB on a USB-C connector, so the appliance UART is never shared with logging.
• Fixed-direction level translation (TXU0204) between the 3.3 V module and the 5 V port, behind ESD protection and series resistors.
• Power from either the appliance or USB-C, ideal-diode OR-ed, through a current-limited eFuse and a 470 µF polymer bulk capacitor into a synchronous buck (TPS62162).

Service connector, as fitted to the reference appliance:

Where to start

Repository layout

pcb/     KiCad 10 project: serialtap.kicad_sch / .kicad_pcb / .kicad_pro
lib/     the project's own symbols, footprints, 3D models and datasheets
sim/     LTspice power-path decks and their results
bom/     bill of materials and sourcing notes
docs/    design document, ADRs, protocol, layout and bench notes
bench/   capture and analysis scripts, and the ESPHome config used on the bench

Opening and checking the design

Open pcb/serialtap.kicad_pro in KiCad 10. Everything it references is in lib/ through ${KIPRJMOD}-relative paths, so it needs no global library.

CLI=/Applications/KiCad/KiCad.app/Contents/MacOS/kicad-cli   # macOS path
"$CLI" sch erc pcb/serialtap.kicad_sch
"$CLI" pcb drc --schematic-parity pcb/serialtap.kicad_pcb

The simulations need LTspice and vendor models that are not redistributed here; sim/README.md explains how to get them.

Fabrication outputs are regenerated rather than committed. Each fab order is tagged, so a physical board traces back to its sources.

Licence

Hardware under CERN-OHL-P v2; firmware and documentation under MIT. Both are permissive. NOTICE sets out which files fall under which, and the third-party material that falls under neither.