New Build 1000x600x200mm (newbie)

[JAZZCNC]

Ok well the need for a relay is to act as the switch.
The vfd will provide 24V source which you take to which ever input you want to use. The relay acts as a switch and keeps the digital input using the same potential as the source.
If you use an external voltage it won't be at the same potential.!!!!! . . . . Hey what the hell is the Potential he's asking him self.?

Basicly it's ground and all it means is the digital input needs to use the same ground as the source voltage. (Or often other way around and Digital input provides 24V which returns to Com ground)
Either way there must be a path back to ground for the cicuit to be made.

Relays: Well from your statement then I don't think you do fully understand how a relay works.? A relay is just a electro mechanical switch and doesn't output any voltage.
The coils of the relay are controlled by voltage you or the device controling the relay apply. The Contacts of the relay are the switches which open or close depending on type. NC or NO.

In the case of the C6 board the Coils are controlled thru the BOB using "M" commands in g-code. The contacts you just use to make or break a circuit.
IE: M3 would turn on the relay coils closing the NO contact which you run the wire from 24V source on VFD thru one side of contact and back out the other to Digital input on VFD.