I have heard this question quite a bit recently, so I thought I would make a post about it. To completely answer this question would be a very long post, so I will target specifically to those with Ag Leader equipment as that is the largest percentage of my readers according to my poll.
Ag Leader uses a non-standard 9 pin connection. This means that unless you get an Ag Leader GPS, you will need an adapter cable, or more commonly called a null modem cable (PN 2000819). The reason for this is because Ag Leader powers their own GPS units through the same 9 pin connector, so to hook up another brand of GPS it would send 12 volts of power to the unit possibly causing damage to it.
So the bottom line is, if you are using an Ag Leader monitor and Ag Leader GPS that does not have a seperate power cable, no adapter cables should be necessary – they should plug right together. If you are using a lightbar (even the ones that Ag Leader sells) or third party GPS, these are already powered on their own and do not need power from the monitor. This is where you will use the null modem cable.