The difference between SMB model and SMC model.
BUS input to power up the logic board and encoder.
Advantage: Only requires one input power source to operate
Disadvantage: In case of input power failure, the amplifier
will shut down completely including the logic board and encoder.
external 24VDC input to power up the logic board and encoder.
Advantage: As long as the external 24VDC stays on, the logic
board and encoder power will stay alive even if the BUS input
Needs two separate input power sources (external 24VDC & BUS
input) to operate the amplifier