The TMCC protocol and the Legacy TMCC2 protocols and neither 7-bit, 8-bit, or 9-bit.
Both protocols use a 24-bit word (three bytes). The TMCC2 protocol has two formats - single-word commands, and three-word commands. The TMCC2 three-word commands allow the protocol to be extended for additional Legacy features.
TMCC1 includes engine, train, accessory, and switch commands.
TMCC2 has engine, train, and Legacy sound, lighting, and effects commands only.
For single-word commands to engines and trains, TMCC2 mirrors the TMCC1 commands. The mapping of bits within the 24-bit word is different between the two protocols. Address bits remain at 7 (1 to 99). Data bit mapping allows expansion of some data values to 8 bits. Most importantly, absolute speed control increases from 5 bits (32 steps) in TMCC1 to 8 bits (200 steps) in TMCC2 which allows precise speed control of Legacy engines.
The TMCC2 three-word commands are used to command Legacy sound cars, station announcements, lighting, and special effects and features available on engines and cars.
When you are controlling your Legacy layout with the Cab2, you are using both protocols. You have more precise speed control, quilling whistles, special sounds and lighting, and control of unique features like pantographs.
