Is it possible? If so, do I only need to hook up the ground and the NMEA2 input from the plotter to the serial port? And if so which of the many terminals on the serial port plug do I connect them to?
Sure you can.
A 9 pin serial cable uses pins 2 and 3 to receive and send data. You can connect those up and receive the GPS signal to the laptop and send way points alone or routes to the plotter. You need to consult the plotter manual for how you enter the plotter into receive mode.
I have no idea what so ever about NMEA
1 and NMEA
2 it's not what they are normally called. One is data out of the GPS and the other is data IN. I'm guessing the one connected to the auto pilot is NMEA OUT.
For a DB 9 serial connection (25 pin serial is backward of this!)
NMEA In Computer Pin 3 TxD
NMEA Out Computer Pin 2 RxD
Now for the bad news. Serial cables
don't use standard colors for the wires so you need to look at the female end of the cable and you can see the pin numbers molded into the cable connector. Looking at the female end with the 5 pins at at the top it is the 2nd and 3rd from the right on the top row. Take a voltmeter and measure resistance from the hole in pin 2 to find the color of the wire it goes to and do the same for pin 3. If you have an old mouse you can cut off the mouse and keep the cable. You need to find a good way to attach the tiny thin wires to the thicker wires so the wires don't pull apart.
Set the serial port properties to 4800 for speed, 8 data bits and no parity with one stop bit aka "8, none and 1". Most software
has a place to set this up.
NMEA OUT is the gps signal you want coming into the computer and that is pin 3 the other pin 2 is for sending data to the chart plotter. Take your USB GPS and save it some place as a back because you don't need it for this setup. You can use the chartplotter
GPS to watch the boat move over the map. You need to create the route
on the computer first then set the plotter to receive, then set the softeware to send then load the route, then switch back to chartplotter
mode and set the software
back to receive. Now you can select the route on the chart plotter and watch it real time on the computer too. You software should have special instructions for loading routes as well as the chart plotter.
products need to switch the communications
mode to receive routes so you may need to change the communications
parameters before and after transfers on each end. Once you do it it is not so complicated.