Is there a Gcode type command that could be added at the beginning of the program - Wait for input to go true and if that input went true then start running and executing the Gcode program again.
If so, can you assign the start to a input in mach3?
Miscellaneous functions, called M codes, are used by
the CNC to command on/off signals to the machine
functions.
i.e.
MØ3 - spindle forward (CW),
MØ5 - spindle stop stop, etc.....
M07 - mist coolant on
M08 - Flood coolant on
M09 - All coolant off
You would note that M## codes normally output a signal to a pin of a parallel port that can be used in the Break Out Board (BOB) to control a RELAY on or off.
So the RELAY is what will "push the button" to on your extruder or conveyor belt to load material etc
Attached is the MACH3 std M-codes list.
Std M codes can be edited (modified) and you can also create your own (higher number) M codes too.
But for your requirements, you can for simplicity (& proof of concept) initially adapt
M03 or M04 to on and M5 to off
M07 or M08 to on and M9 to off
YOUR extruder machine.
The PWM or 0-10V output can be used to control the flow rate of the extruder material
You could use G4 to wait
To further assist you, it would be good that you let us know your motion controller (parallel port or ext USB or ext Ethernet) and BOB brand and model.