MPGs - any standard protocols?

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?

The protocol standard used will be to do with serial data transmission to display units and will not be unique to MPGs, without the specification of their display you're guessing, especially if it's a display which is programmed.
Do you know what display unit they use?

Ta, the motion controller just presents this as an MPG interface, there's no indication of what MPG it actually connects to - I was guessing there'd be a common interface standard. Shortly after posting this I'm reading about modbus, and thinking I could probably achieve what I need/want with a simple ethernet/arduino controller and suitable modbus/brain setup. It's something to play with over the winter months, anyway!