Amstrad released two RS232 interfaces, both using the same input/output addresses - thereby establishing the standard for serial interfaces - but with significantly different hardware.
Original serial interface
Amstrad's first serial interface was one of the original peripherals released for the CPC. It was remarkable (in a bad way) for the necessity for an external, brick-like power supply.
Software was supplied on ROM, mostly comprising utility RSXs for hobbyists rather than a comms application per se.
Pace serial interface
Commstar Software is supplied on rom!
Not much is know about this interface please add!