For “DIY” people there’s two transmitter modules:
- DJT - The old “shink wrapped” module, supports only PPM input. Doesn’t work with X-series receivers.
- XJT - A real transmitter module that supports D16 FrSky standard (which is required by many SBUS receivers such as X4S, XSR). Supports PPX input too.
PPX = PXX currently allows passing transmit mode (D16, D8 or L), region code (to follow different countries’ radio regulations), model number (for receiver match), failsafe mode and positions, as well as initiating binding and enabling range check mode in addition to up to 16 channels. These are features that are present in non-module based radios but generally not in radios relying on an RF module. (source) PPX protocol at OpenTX source.