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Setting up a Pixhawk 4 from Holybro for Frsky X8R receiver telemetry link

Hello! I have a brand new pixhawk 4, with the DSM/SBUS RC connected to a Frsky X8R receiver.
My tx is a Horus X10 running OpenTX with Yaapu script installed.
I used the cable included in the package from Holybro, according to them a TTL-to-RS232 converter is not needed.
So it looks like I have to set this according to the manual:
SERIALx_PROTOCOL =10
SERIALx_OPTIONS = 4 (Half Duplex)
If I am not mistaken, the x stands for a number.
After some research on the internet this looks to be:
USART6 /dev/ttyS4 RC SBUS
I think that the serial number should be 4.
Is this correct?

At least the documentation for a Sik radio is not puzzling, that appears to use
USART2 /dev/ttyS1 TELEM1 (flow control).
It apparently requires no settings in the pixhawk 4, but mission planner needs to be configured to use it.
The documentation for that is easy to find, and is not puzzling like the instance for Frsky / Yaapu script.

You need buy this https://www.banggood.com/FrSky-Yaapu-Telemetry-Converter-Cable-Pixhawk-to-Horus-X10S-X12S-Taranis-X9DP-QX7-or-X7S-Smart-Port-Receiver-X8R-X4R-XSR-RS8R-p-1535564.html

Servers by jDrones