Some of the smaller RadioMaster multi-protocol transmitters are only available with the CC2500 RF chip and not the full 4-in-1 RF chip.
This is fine if you use FrSKY receivers, but you will need a 4-in-1 radio for FlySKY and Spectrum receivers, and all the odd RTF model protocols like V761 etc .
Of course you can add a 4-in-1 external RF module to a CC2500 or ELRS radio for full multi-protocol compatibility
CC2500 RF chip supported protocols
FrSKY (but not ACCESS, TD or Twin)
S-FHSS (Futaba)
RadioLink
Graupner HoTT
Skyartec
4-in-1 RF chip
Includes all the CC2500 protocols and many more, like Flysky (but not ANT) and Spektrum DSMX
Visit the Multi protocol page for the full list
ELRS 2.4 RF chip
Only compatible with ELRS 2.4 CRSF protocol receivers
