I have a chinese Novusun "NVEM" motion controller, which has a MPG interface including the usual quadrature encoder inputs, axis-selection, step-size selection which are all straight forward discrete interfaces, but also a TxD and RxD pair to support the intriguing "display of MPG information". This appeals to me (I intend to build the MPG interface into the support base of my StarMill) - and throwing a 20x4 display into the base to display coords etc. is appealing - but have no information about what the protocol to display MPG information on a typical MPG interface.

I have scoped the TxD line, but this is held at -5V (RS232/0), so I assume the protocol is command/response with an RS232 drive levels.

Does anyone have any information about what protocol is used in common MPG pendants?