Hi Darlly01:
I believe you are unclear on how Mach3 and ModBus communicate. There are several pretty good posts regarding this topic, especially those posted by Scott Schaefer.
Basically, Mach3 is continually scanning the "registers" in the ModBus Serial page, and the "registers" each relate to an output from the PLC. This scan is about 10 times a second.
If the PLC output is a panel button changing from 0v to 5v, the register scanned by Mach3 will change from a 0 to a 1.
Mach3 "knows" what to do with this information through a Brain, which is written so Mach3 can take the appropriate action, for instance turning on/off a Mach3 OEM function such as a coolant relay.
Brains can handle digital and analog inputs, like a Feed Rate override potentiometer, or a simple on/off relay switch.
The "Brain" is the method by which Mach3 responds to an external input that is not one of the basic Input channels, which are limited in the basic parallel port.