Omnibus F4 v5 serial ports

I have ArduCopter 4.0.4 running on an Omnibus F4 v5 (and also on 3 Pixracers so I am fairly familiar with the overall setup and functionality).

When installing the GPS I first tried the TX6/RX6 pins which seems to be the Serial3 port and which also is labelled GPS1 in the Arducopter docs. Unfortunately I never got it running.

When moving the GPS to SERIAL1 and connecting the compass to the SCL/SDA pins everything works fine.

By now I have read thoroughly through all info I can find and to me it looks like the TX6/RX6 pins are connected to an inverted port and this FC has no functions to change this using the software “Invert” settings for the port.

My next thought thus was to use TX6/RX6 for the FrSky Smartport Telemetry because the normal connectivity for this is to an inverted port. So far I have had no luck getting this to work on TX6/RX with any settings.

By setting the BRD_ALT_CONFIG parameter to 2 (or 3) and connect the Smartport telemetry to the RSSI pin (SERIAL4 with settings 57,4,10) and soldering the cable to an uninverted connection of the R-XSR receiver the FrSky telemetry works like a charm.

So I have both telemetry and GPS/Compass working well but would like to use also the SERIAL3 port (TX6/RX6). But as already said it does not work as a normal serial connection and it does not work as an inverted connection for Smartport telemetry.

The Omnibus board works surprisingly well for everything else so I have not so far suspected that it is faulty. My question thus is if anyone has been able to use this port (SERIAL3/USART6) for anything useful and if yes, what is the setup?

After more experiments and readings I can answer my own question which might be helpful for others (who like me are no experts when it comes to flight controllers)

For the F4 AIO setup UART6 is used for more than the Tx6/Rx6 of SERIAL3, it is also used for the SBUS/PWM communication which means that SERIAL3 cannot be used when the radio is connected to the SBUS/PWM pin.

But after disconnecting the radio the SERIAL3 (Tx6/Rx6) works perfectly well for the GPS serial connection as mentioned in the docs.

Some posts I have read suggests that the hwdef.dat file for Omnibus F4 AIO must be modified for this to work but that is not the case. What is described above works without any modification (at least for me).