Pixracer Serial 4 / FrSky port configured for GPS

My question is: can Serial 4, ie the FrSky, port on Pixracer be configured to accept a GPS unit?

I’m running ArduCopter v3.5.7 and cannot connect a GPS to Serial 4 although the serial mapper indicates I can configure Serial 4 to be a MavLink port, GPS, FrSky, etc.

I’m following the pinout for Serial 4 from

  • Jade

I don’t think so, I think it has a hardware inverter built-in.

Thanks fnoop! It does. From the data sheet for the inverter http://www.ti.com/lit/ds/symlink/sn74lvc2g86.pdf it looks like if I set the “FrSky INV” signal low from the STM chip, then the inverter should hopefully act as a pass through. I’m going to see if I can track down where that pin is set in the software -

Actually looks like @tridge has just added the option to the chibios build:

Even better - thanks!

We have a system that works on an older version of Ardupilot (v.3.5), so I had to figure out how to set the “FrSky INV” signal low from the STM chip.

If anyone is interested, this is how you do it.

  • add below line to /modules/PX4Firmware/src/drivers/boards/px4fmu-v4/board_config.h
  • add below two lines to int nsh_archinitialize(void) in /modules/PX4Firmware/src/drivers/boards/px4fmu-v4/px4fmu_init.c
    stm32_gpiowrite(GPIO_FRSKY_INV, 0);