Bind mode. There is a "bind plug" that goes into the "bind port". The bind port is any of the ports. The bind plug puts a ground (logical low) in the signal path (there are three paths: + - and signal. The + and - are obvlous.. what's left is the signal line because there are only three.

So, the bind plug puts a logical low that forces the receiver into the bind mode. Then when you turn on the transmitter, the receiver recognizes the transmitter and "binds" to it.

CGr.