Am I correct in how to connect ELRS receiver to the Pixhawk 6c?

I’m new to ArduPilot so please bear with me. I’ve got a Radiomaster RP1 ELRS receiver which I now want to connect to my Pixhawk 6C. I know CRSF is a different protocol from ELRS, but in the short user manual which came with the RPI receiver it says

Wireless Protocol: ELRS 2.0 pre-installed
Bus interface: CRSF

So if I understand this correct, ELRS is what goes over the air, and CRSF is what the RP1 talks to the flight controller. Right?

In the Ardupilot docs on the Pixhawk 6c I read this:

To allow CRSF and embedded telemetry available in Fport, CRSF, and SRXL2 receivers, a full UART, such as SERIAL5 (UART3) would need to be used for receiver connections. Below are setups using Serial6. For the 6C Mini, SERIAL1 - SERIAL4 would need to be used.

I’m trying to wrap my head around this. So I need a “full UART” such as “SERIAL5 (UART3)” or later they also mention “Serial6”. In the Holybro docs I read that TELEM2 is UART5 and TELEM3 is USART2. I see no mention of SERIAL5, UART3 or Serial6 though. I know UART is a serial connection and as far as I understand USART2 is a more advanced version of UART5.

So since TELEM1 is used for the telemetry radio, am I correct in my assumption that I can connect the Radiomaster RP1 ELRS receiver to either the TELEM2 or the TELEM3 port on the Pixhawk 6c? Is either of them a better choice? Any tips are welcome!

For historical reasons the UART number often doesn’t match the serial port number on these ports. The serial port is what you configure. The UART number only matters if the manufacturer uses it for labeling ports. The 6C has 6 serial ports number 1-6 and any of them can be used with CRSF/ELRS since this is an H7. ELRS is an open source clone implementation of the closed source CRSF protocol from TBS. It refers to both the over-the-air protocol and the protocol used between the radio RX and the flight controller.

Thanks for clearing some things up. I’ve got a couple followup questions:

  • You’re saying the 6C has 6 serial ports. Are those six the 3 TELEM ports, the 2 GPS ports and the I2C port?
  • What is an H7?

Furthermore, in the Holybro docs I read the 6C has:

3 general purpose serial ports

  • Telem1 - Full flow control, separate 1.5A current limit
  • Telem2 - Full flow control
  • Telem3

Since Ardupilot docs say I need “a full UART”, does that mean Telem3 is not “full UART” (since only Telem1 and 2 say it has “Full flown control”)?

Google is your friend here.

You do not need flow control for ELRS so any UART can be used. “full UART” is not really meaningful in this context


I used SERIAL4 for my ELRS and using this web for setting
Ardupilot Setup - ExpressLRS

what’s the meaning of this sentence "RC_OPTIONS turn on Bit 9th which is “Suppress CRSF mode/rate message for ELRS systems”?
Because I cannot to change that parameter.

It just stops mode change spam in your GCS. You can set it through mission planner

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hi andy
could you solve my problem about ELRS ?
Setting ELRS to Pixhawk and Ardupilot Radiomaster TX16S - ArduCopter / Copter 4.3 - ArduPilot Discourse

I also have a problem with connection my radiomaster ER5a to pixhawk 6c
I even don’t understand which socket I have to use. plz help me)))
Binding is ok, but if I connect this receiver to ppm/sbus Rc socket nothing happened.

If you’re using an ER5A by default it will only output PWM signals, as it’s meant for controlling a RC Plane - so that won’t work with the Pixhawk. You can change what that receiver outputs to CRSF, but it involves configuring it differently from the receiver’s Wifi page as well as running master firmware currently. Or you can buy a different ELRS receiver that just outputs CRSF natively (these are much more common than the PWM type you have). Radiomaster RP1,2, or 3, Jumper, Betafpv all make models like this. Then you would wire that CRSF output into a serial port on the Pixhawk, TX to RX, RX to TX, 5V to 5V, GND to GND, etc.

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I unfortunately don’t have an answer to your question @svinozhop , but since I started this thread I do want to say that I did manage to connect my ELRS receiver to the pixhawk 6c I have. In the end I also had a problem with binding, for which I opened another thread here. Finally I concluded in the other thread all the steps that I took to bind and connect the elrs receiver to pixhawk and ardupilot.

Thanks for all your help here as well, and good luck to @svinozhop

Oh man… It was the second wrong receiver I bought…
Ordered RP3, will see.
Am I right I have to connect it to tele3 socket which has TX_RX pins?

Sorry to hear that, ELRS has a bit of a steep learning curve, but once you get it sorted, I find the system to be a very good one. Once you get the recevier, you can choose from multiple serial ports to wire it into that are UARTS. Make sure you’re looking here Pixhawk 6C Ports - Holybro Docs and here Pixhawk 6C 6C Mini Flight Controller — Copter documentation for that information on RC input/ports, etc.