# Serial Port Mapping

This topic shows how to determine the mapping between serial ports (UART), device (e.g. "ttyS0") and specific functions enabled by PX4 (e.g. TELEM1, TELEM2, GPS1, RC SBUS, Debug console).

Note

The instructions are used to generate serial port mapping tables in flight controller documentation. For example: Pixhawk 4 > Serial Port Mapping.

# NuttX on STMxxyyy

This section shows how to get the mappings for NuttX builds on STMxxyyy architectures by inspecting the board configuration files. The instructions use FMUv5, but can similarly be extended for other FMU versions/NuttX boards.

# default.cmake

The default.cmake lists a number of serial port mappings (search for the text "SERIAL_PORTS").

From /boards/px4/fmu-v5/default.cmake (opens new window):

SERIAL_PORTS
	GPS1:/dev/ttyS0
	TEL1:/dev/ttyS1
	TEL2:/dev/ttyS2
	TEL4:/dev/ttyS3

# nsh/defconfig

The nsh/defconfig allows you to determine which ports are defined, whether they are UART or USARTs, and the mapping between USART/UART and device. You can also determine which port is used for the serial/debug console.

Open the board's defconfig file, for example: /boards/px4/fmu-v5/nuttx-config/nsh/defconfig (opens new window)

Search for the text "ART" until you find a section like with entries formatted like CONFIG_STM32xx_USARTn=y (where xx is a processor type and n is a port number). For example:

CONFIG_STM32F7_UART4=y
CONFIG_STM32F7_UART7=y
CONFIG_STM32F7_UART8=y
CONFIG_STM32F7_USART1=y
CONFIG_STM32F7_USART2=y
CONFIG_STM32F7_USART3=y
CONFIG_STM32F7_USART6=y

The entries tell you which ports are defined, and whether they are UART or USART.

Copy the section above and reorder numerically by "n". Increment the device number ttySn alongside (zero based) to get the device-to-serial-port mapping.

ttyS0 CONFIG_STM32F7_USART1=y
ttyS1 CONFIG_STM32F7_USART2=y
ttyS2 CONFIG_STM32F7_USART3=y
ttyS3 CONFIG_STM32F7_UART4=y
ttyS4 CONFIG_STM32F7_USART6=y
ttyS5 CONFIG_STM32F7_UART7=y
ttyS6 CONFIG_STM32F7_UART8=y

To get the DEBUG console mapping we search the defconfig file (opens new window) for SERIAL_CONSOLE. Below we see that the console is on UART7:

CONFIG_UART7_SERIAL_CONSOLE=y

# board_config.h

For flight controllers that have an IO board, determine the PX4IO connection from board_config.h by searching for PX4IO_SERIAL_DEVICE.

For example, /boards/px4/fmu-v5/src/board_config.h (opens new window):

#define PX4IO_SERIAL_DEVICE            "/dev/ttyS6"
#define PX4IO_SERIAL_TX_GPIO           GPIO_UART8_TX
#define PX4IO_SERIAL_RX_GPIO           GPIO_UART8_RX
#define PX4IO_SERIAL_BASE              STM32_UART8_BASE

So the PX4IO is on ttyS6 (we can also see that this maps to UART8, which we already knew from the preceding section).

# Putting it all together

The final mapping is:

ttyS0 CONFIG_STM32F7_USART1=y GPS1
ttyS1 CONFIG_STM32F7_USART2=y TEL1
ttyS2 CONFIG_STM32F7_USART3=y TEL2
ttyS3 CONFIG_STM32F7_UART4=y TEL4
ttyS4 CONFIG_STM32F7_USART6=y
ttyS5 CONFIG_STM32F7_UART7=y DEBUG
ttyS6 CONFIG_STM32F7_UART8=y PX4IO

In the flight controller docs the resulting table is:

UART	Device	Port
UART1	/dev/ttyS0	GPS
USART2	/dev/ttyS1	TELEM1 (flow control)
USART3	/dev/ttyS2	TELEM2 (flow control)
UART4	/dev/ttyS3	TELEM4
USART6	/dev/ttyS4	RC SBUS
UART7	/dev/ttyS5	Debug Console
UART8	/dev/ttyS6	PX4IO

# Other Architectures

Note

Contributions welcome!

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