UARTs

The 305ap exposes 7 hardware UARTs. Each maps to a NuttX ttyS device and a PX4 serial port name.

Port Map

UART	ttyS	PX4 Port	Default Function	Baud
USART1	ttyS0	—	Motor Telemetry RX	57600
USART2	ttyS1	TEL3	VTX	57600
USART3	ttyS2	—	VTX SBUS output	—
UART4	ttyS3	TEL1	Telemetry 1 / MAVLink	57600
UART5	ttyS4	RC	RC Input	—
USART6	ttyS5	GPS1	GPS	Auto
UART7	ttyS6	TEL2	Telemetry 2	57600

Port Details

USART1 — Motor Telemetry RX (ttyS0)

Mode: SINGLEWIRE + SWAP

The physical TX pin (PA9) is reconfigured as the RX line. This is a receive-only port used to collect ESC telemetry (e.g. BLHeli32/AM32 serial telemetry).

• RX pin: PA9
• PA10 is repurposed as the FDCAN1 standby (STB) enable and is not available as USART1 RX/TX

USART2 — VTX / TEL3 (ttyS1)

Standard full-duplex UART. Assigned as TEL3 by default.

• TX: PD5, RX: PD6

USART3 — VTX SBUS / Debug Console (ttyS2)

TX-only port. PD9 (the red LED) occupies the physical RX pin position, so no RX is possible.

• TX: PD8
• Used as the NuttX debug console (boot messages appear here)
• Can output SBUS to a VTX/OSD if reconfigured

UART4 — Telem 1 (ttyS3)

Primary telemetry port. Supports hardware flow control.

• TX: PA0, RX: PA1
• RTS: PB14, CTS: PB0
• Hardware flow control can be toggled from QGroundControl via MAV_0_FLOW_CTRL (no rebuild required)

UART5 — RC Input (ttyS4)

Mode: SINGLEWIRE + SWAP

Single-wire half-duplex RC input port. The physical TX pin (PB13) is the signal line. PB12 is the green LED and is not available.

• Signal pin: PB13
• Supports SBUS (inverted), CRSF, and other single-wire protocols via PX4's RC driver

CRSF/ELRS on Telem 1

For CRSF/ELRS receivers that need a full-duplex UART, connect to UART4 (Telem 1) instead and set SER_UART4_FUNC = RC Input (13) in QGroundControl.

USART6 — GPS (ttyS5)

Standard full-duplex UART. Paired with I2C4 on the GPS connector for combined GPS+compass modules.

• TX: PC6, RX: PC7
• I2C4 (PD12/PD13) shares the same connector

UART7 — Telem 2 (ttyS6)

Secondary telemetry port with hardware flow control enabled by default. Intended for companion computers.

• TX: PE8, RX: PE7
• RTS: PE9, CTS: PE10

Changing Port Functions

Port functions are configurable from QGroundControl without a firmware rebuild:

1. Go to Vehicle Setup → Parameters
2. Search for SER_ to find serial port assignments
3. Change the function (e.g. set SER_UART4_FUNC to RC Input for CRSF on Telem 1)
4. Reboot. Additional parameters for that function will appear.