Hi,
Whether GND is needed or not depends on the curcuit and signals..
With a RS485 receiver IC you usually need GND, otherwise the input will float with respect to receiver_GND and thus one can not guarantee input_common mode limits.
With opto_coupled signaling you don't need GND. (Mind the characteristic impedance mismatch)
With transformer coupled signals you don't need GND. But it needs DC_free signaling, otherwise the transformer core saturates. Here comes Manchaster coding into play. It is no interface, no necessarily hardware, no protocol. It's just an additional coding layer to get rid of DC when transmitting data.
what sort of comms protocols woudl you send over a ribbon cable? (RS232, RS485 , SPI, I2C, CAN, MODbus, Manchester coding, TTL?)
This is a mixture of some different things:
* RS232, RS485, TTL .... describes the voltage levels
* SPI (and UART) ... describes an interface, which signals there are, the job of the signals, how data are sent (but both are able to work with RS232, RS485, TTL)
* I2C and MODbus ... is an interface specification, with voltage levels, signal description, timing and protocol
* Manchester just forms one serial data stream into another serial datastream. It's main benefit is, that the datastream becoms DC free and thus enables the use of transformers for electrical isolation.
Klaus