mohan-bee/rc-link-breakaway-panel

manufacturing/POWER-AND-WIRING.md

# Power and wiring

This is a prototype requiring first-article electrical, thermal and radio testing. The PCB audits do not qualify firmware, RF range or aircraft operation.

## Receiver

- Supply regulated **5 V** to J_RX_POWER or J_RX_LINK / J_RX_SERIAL pin 1; pin 2 is ground. Use one logic supply, avoiding backfeeding another BEC or FC regulator. Never connect a 3S battery directly.
- Optional servos: separate regulated **5 V** BEC to J_RX_SERVO_POWER pin 1, ground to pin 2. The servo positive rail is separate from logic positive; grounds are common. The BEC must tolerate the aggregate servo stalls/transients, with **2 A total maximum for this design**. Use external distribution if more current is needed.
- J_RX_PWM1–6: pin 1 signal, pin 2 servo +5 V, pin 3 ground. Check pin numbering against the PCB view; do not infer polarity from cable colour alone.
- J_RX_LINK: pins 1=5 V, 2=GND, 3=RC_OUT, 4=SER_RX. For the existing FC serial link, connect pins 1, 2 and 3 to FC J_RX pins 1, 2 and 3 respectively (5 V, GND and UART3 RX). Leave pin 4 open on both boards: SER_RX is PA10/USART1 RX, whereas RC_OUT is PB3/USART2 TX, so pin 4 is not a CRSF telemetry return. Use the serial receiver firmware and the FC custom-receiver.cli profile.
- J_RX_SERIAL: pins 1=5 V, 2=GND, 3=SER_TX, 4=SER_RX. UART signals are MCU 3.3 V logic, not RS-232. Do not connect competing TX outputs together.
- J_RX_SWD: pins 1=3V3 reference, 2=GND, 3=SWDIO, 4=SWCLK, 5=NRST. Program with ST-Link; CH341A cannot program this SWD interface. Reference is not permission to backfeed the 3.3 V regulator.

Use 2.54 mm single-row male headers and matching crimp contacts rated at least 3 A per contact for the servo input and outputs. Example verified male headers: [Harwin M20-9990246, 2 pins](https://www.harwin.com/products/M20-9990246) and [M20-9990346, 3 pins](https://www.harwin.com/products/M20-9990346). These are hand-installed parts, outside the SMT BOM. Select the mating harness to the same current rating; unspecified hobby jumper leads are not a qualified high-current harness.

The dedicated supply and return copper is >=1.5 mm to all six outputs. The analytical worst-case PCB loop drop is approximately 0.106 V at 2 A, excluding contacts and cables. Measure voltage at the furthest loaded output and inspect temperature at connectors, plated holes and feed traces before accepting that load. No particular servo/BEC model or measured continuous-current rating is claimed.

## Transmitter

Install the XIAO RP2040 and sockets separately, with the orientation shown in the source/assembly view. Power through its USB-C connector. The XIAO USB firmware and the STM32 radio firmware are separate programming steps; the STM32 uses J_TX_SWD (3V3 reference, GND, SWDIO, SWCLK, NRST). Match transmitter and receiver protocol, pin maps and failsafe settings. Do not link TX and RX positive supplies across the separated boards.

## First article

Clip and deburr all breakaways with the boards supported. Inspect assembly polarity and bridges, check unpowered supply-to-ground resistance, then bring up each board using a current-limited supply. Confirm 5 V and 3.3 V before programming. Test binding, channel order/direction, HOLD/STOP, USB disconnect, lost-radio failsafe and ground range before attaching loads. Verify the servo rail with an electronic load and then the selected servos, including stalls, before use. Flight approval requires separate FC/ESC/motor and failsafe validation.