Standard Horizon CP300i Owner's Manual page 20

2.4.3.1 Connection a Video Camera, FF525, AIS Receiver, VHF and Autopilot
NOTE
Port2 Input and Output is used by the optional FF525. In the diagram above you will notice Port2
Input and Output wires are shown in gray and not used.
Port4, and Port5 Outputs are can be connected to NMEA devices* capable of listening to NMEA-
0183 sentences.
Port4, and Port5 Inputs can be connected to NMEA devices* capable of outputting to NMEA-0183
sentences.
GPS Chart Plotters Page 21