Electronic instruments are used for communication, navigation, and status monitoring. Small boats usually have fewer instruments installed and those who sail them therefore rely more on handheld instruments. Installed instruments are often wired together so that they can communicate with one another. The interconnection between instruments is called a bus.
On a small boat, the bus might consist of loose wires run directly from instrument to instrument. On a larger vessel, it makes sense to have a central wiring bundle to which the instrument wires attach. This is called a backbone. In either case, a wiring standard specifies the hardware and software protocols that must be followed.
Some instruments use only one protocol but many have separate outputs for different standards. A device that converts from one standard to another is called a bridge. If more than one standard is used, each needs its own separate bus or backbone.
NMEA
0183
NMEA (often pronounced “nema”) is the National Marine Electronics Association (not to be confused with NEMA, pronounced the same, the National Electrical Manufacturers Association, which also promulgates standards). NMEA’s standards are for the wiring and data protocol used in a marine network.The original NMEA 0183 standard defined a 4800-baud, simplex, serial network with collision detection and backoff. It uses six wires: 12-v power, ground, DATA IN + and -, and DATA OUT + and -. When wiring up a NMEA network, power connects only to power (and if there is more than one source they should be isolated), ground connects only to ground, + connects only to + or can be tied to +12v, and - connects only to - or can be tied to ground.
DATA IN and DATA OUT can be single wires (RS-232) or differential + and - pairs (RS-442). If the latter, polarity matters, and when attaching a differential output talker to a single-wire listener, the data is usually driven onto the positive wire and the negative wire can be grounded, but not always. DATA IN connects only to DATA OUT on another device. DATA OUT can connect to DATA IN (talker connects to listener) or DATA OUT (talker joins other talkers). Each device can talk to three or four others but can only listen to one other device. Use of a multiplexer avoids these rules.
NMEA
2000
The more recent NMEA 2000 standard defines a 25-kilobaud, multiplex network based on the Controller Area Network (CAN) technology used in cars (ie, the SAE J1939 standard). NMEA 2000 wiring, like CAN, consists of a backbone with 120-ohm terminating resistors at each end and multiple branches extending up to 6 meters from the backbone to reach devices.Wiring can be mini (ie, thick) cable, often used for the backbone and rated up to 8 amps, or it can be micro (ie, thin) cable rated up to 4 amps. Cable that is NMEA-compliant will connect and handle the load, but more expensive cable that is NMEA-approved will also satisfy additional standards (eg, weather resistance, etc). The network requires one and only one 12-v power source, which should not be the engine starting battery, if possible.