I assume you are talking Fanuc?
The communication between the CNC and PMC (PLC) is over an inernal bus called the BMI, Basic Machine Interface.
G registers are used to pass info from PMC to CNC such as operator panel commands, and F Registers are used to pass info from CNC to PMC such as M,S,T codes etc.
The PMC operates like any typical PLC as to programming and I/O.
The results of the PLC logic can be monitored in the on screen Ladder display.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.