Many long years ago I wrote this protocol (and a 
lot of other Bridgeport internal code).

It was based on the RS-491 protocol and first 
appeared in the V2E3 CNC, and was used thereafter 
in the R2E3, R2E4, V2XT etc. PC-based controls.

The EZ-CAM programming system implemented this as 
well to automate CAM <-> CNC communication (the 
program number could be entered at the control 
and subsequently be sent from the programming 
station).

Entering "-1" for the program number caused the 
CNC to use the "Dumb" subset of the Remote-Load 
protocol; simply send an XON and wait for the 
program. This was included to allow a serial tape 
reader (PAPER tape, that is!) to be used.

I hope this helps those few who are still running 
these old beasts...

Dave Cloud (former Bridgeport Controls System Architect)