garage door wiring

I have problem opening my garage door using my remote control. All the batteries of the remote controls are all good, so I check the motor. I found out that the 3 wires that connects the transmitter to the main board, the black wire is not connected, the other 2 wires (green and purple) are connected. Where do I connect the black wire? I tested the purple wire, it has 24 volts in it.

Thanks