Wouldn’t it be nice take your laptop out of it’s waterproof case, plug in one cable, and have all your charts, GPS, and instrument data right there on the screen? Getting your laptop happily integrated with your instruments can be a complex endeavor, but a multiplexer, especially one that just hooks up to one of the computer’s USB ports may be the secret ingredient for success.
To understand the purpose and need for a multiplexer, some background is in order…
In order to communicate with one another, many marine devices speak a simple language called NMEA 0183 (just call it NMEA for short). A GPS, for starters, might want to tell other electronics what the lat/long is, but may not necessarily need to know anything from other instruments, so it would be referred to as a “talker,” and only wants to transmit its info to a “listener.” A VHF radio with DSC location/distress calling, may only wants to listen for the GPS info— making it a “listener, ” as it has nothing to say to the rest of the system. However, an autopilot might be both a listener and a talker, listening for waypoint info from the GPS, but also broadcasting compass bearing info for other instruments to use. The NMEA standard is really a very simple serial interface, with separate pairs of wires for transmit (talking) and receive (listening). In practice, you can have more than one device listening on a line, but you cannot have more than one device transmitting on one connection, since the devices wouldn’t know when another is talking and they would talk over each other, garbling the signal. I.e. a GPS talker and an autopilot talker could not share the same transmit connection. So, to hook up a laptop that will listen to all of the talkers available on your instruments and other electronics, you need to be able to combine the signals from these talkers into a single connection. That’s exactly what a multiplexer does. Hook up several transmitting devices to it, and it will combine all the messages into an orderly queue for any listener that’s interested.
It used to be that computers all had serial ports, so connecting to the NMEA transmit and receive pairs only required a cable with the appropriate plug on the computer end, and bare wires to connect to NMEA devices on the other. Nowadays, serial ports are a rarity on laptops, replaced by the much faster and more flexible USB standard. While there are smart cables that convert USB to serial to use with older devices, these can be problematic for hooking into the boat’s navigation system (your computer may mistakenly detect a phantom mouse, or haplessly try to search for the NMEA devices). A better option is a multiplexer that connects to the USB port directly. Watch this space for sources and features of specific models!
