Well first of all you know this is not going to be an easy explanation;-)
The trouble is that that the original standards for serial communications was elaborated upon by PC, printer, and modem manufactures all through the early years if the PC boom from 1980 to 1990 as they tried to innovate and outdo each other – oh, and also a few got tempted to be “plug incompatible” with their competitors!
So for the real experts out there I know this is not perfect but I hope this simple guide will help all the rest of the mystified sailing community.
1. The term “Comms port” is not sufficiently accurate for you to know what type of serial communications are coming in/out of your laptop or being received by the piece of marine electronics that your are trying to connect up. You have to know is your laptop port RS232 or RS422
2. RS232 is older than RS422 and slower and less reliable.
3. RS232 often has large plugs with 25 pins
4. RS422 is likely to have smaller plugs with 9 pins – but they could be RS232…
…click thumbnails for a larger image….
To send commands to a piece of marine electronics like a multiplexer you often have to connect up your laptop or PC and then bare the wires at the other end and stick them into the marine unit as bare wires.
Now look at the pinout diagrams (click on the 3 thumbnails on this post for larger images) and check the wires using the wiring diagrams.
Then you should be able to insert the bare wires into the correct sockets on the multiplexer – or whatever – its not worth making a proper connection – the configuration process is or should be a one-off. Then you connect your actual equipment.
Credit for diagrams to the excellent folk at Zytrax..check out their web site for nice illustrations to help you wire up that pesky cable;-)