Frsky communications intro

Adventures in connecting an RC transmitter to an arduino.

Tables of Overview

Communication Protocols between receiver and MCU

• SBUS

  • SBUS is a bus protocol for receivers to send commands to servos. Unlike PWM, SBUS uses a bus architecture where a single serial line can be connected with up to 16 servos with each receiving a unique command.

  • inverted UART, 100000 8E2 (fast mode 200000 baud)
  • SBUS IN lets you have a backup receiver
• SmartPort / S.Port
  • telemetry
  • inverted UART, 57600 8N1, bidirectional on a single wire
  • receiver polls sensors, sensors respond when poked
• F.Port / FPort
  • Combined control/telemetry
  • "slightly faster than SBUS"
  • Not inverted
  • No documentation
• F.Port V2
  • Came with the ACCESS stuff
  • No documentation
  • Not sure if it's compatible with V1, there is none documentation

Firmware/communication stuff between transmitter and receiver

Firmware on the receiver and transmitter for talking to each other

• ACCST 1.x / ACCST D8(not sure about this name, I feel like 2.x can do 8 channels? Saw it on a forum though)
  • SBUS for 8 or 16 channels
• ACCST 2.x / ACCST D16
  • small fix for some bug, but completely incompatible
• ACCESS
  • New thing people seem to like it better
  • A lot of transmitters that do ACCESS also do ACCST 2.x

Hardware I have

• frsky R-XSR reciever
• frsky taranis x9d-plus running opentx
  • Onboard transmitter can only do ACCST 1.x
  • There is the external radio bay that could do more but eh.

Literature Search for Software Libraries

SBUS

• bolderflight/sbus
• TheDIYGuy999/SBUS
• Sbus converter using Arduino

SmartPort

• jcheger/arduino-frskysp
  • handles teensey or 328P via #ifdef
• zendes/SPort
• DanNixon/FrSky_SPORT_Arduino
• openXsensor/openXsensor
  • multi-protocol, not just S.Port
• sgofferj/arduino-frsky
  • for reading telemetry,
  • It doesn't try to handle serial for you! So much nicer!

In the end I went with

• SPort: jcheger/arduino-frskysp
• SBUS: bolderflight/sbus

The SPort library wanted to use SoftwareSerial and just talk and receive on the same pin, which didn't work too good on the esp32 so I forked it to use HardwareSerial and to swap which pins are which at the right times.

I had to put a 1ms delay before sending a packet, because HardwareSerial doesn't like talking right after a call to setPins. I don't love that, and would like to see it go away or at least get reduced.

In the end, basic kinda working example

mtfurlan/arduino-sbus-test

I don't know how to use the frsky transmitter to view telemetry yet, but you can see that in the telemetry config menu it works.