FrSky SPort protocol specs

from https://www.ordinoscope.net/static/frsky-arduino/FrskySP/doc/html/index.html

It's a master/slave protocol. The RX sends a poll packet every 12ms (4bytes)

2bytes	description
0x7E	poll header
ID	physical ID (1-28) computed with a CRC

The receiver will poll the IDs in sequence to find which one is present. If only one physical ID is found, the receiver will alternate the sensor polling and the search (present sensor, next ID to search, present sensor, next ID and so on). If more sensors are found, the poll sequence returns almost to a normal search pattern.

Sensor behavior

The sensor answers to every pool on its physical ID to announce its presence. If no data can be transmitted (no refresh), the sensor answers by an empty packet and a false CRC (type 0x00, ID 0x0000, value 0x00000000, CRC 0xFF).

byte(s) descrption 1 type (only 0x10 at now) 2 sensor logical ID 4 value 1 CRC