CAN BUS NMEA 0183

Introduction

In 1980, the NMEA (National Marine Electronics Association) called together a small group of experts at Fish Expo Boston to come up with a solution to make it possible for marine equipment from different manufacturers to share information. The result of that meeting was the NMEA 0180 (for 1980) and was later updated in 1983 to NMEA 0183 (for 1983).

What is NMEA 0183?

The NMEA 0183 is a “nautical” serial communications network used to interconnect marine equipment on various types and sizes of boats with several devices in order to make equipment more useable in various locations in the vessel, connect accessories such as VHF radios to chart plotters for GPS coordinates, or to connect an autopilot to a chart plotter to see heading information, etc. Since the NMEA 0183 serial network is multi-conductor, it is rather difficult to install and maintain and is capable of only one “talker” and multiple “listeners.” That is, only one device may send out a request or response at a time (talker) to every other device on the network (listeners) to complete a task. This is called a “half-duplex” communications protocol…which means data is transferred in only one direction at a time. This also means that it must tie up the entire network to send out and receive a single message for a single device.

The NMEA 0183 standard baud rate is 4.8k bps (bits per second) while the NMEA 0183-HS (high-speed) baud rate is 38.4k bps. These two baud rates use a different electrical interface. NMEA 0183 is an “Asynchronous Serial” interface in that data is transmitted serially (bit by bit) on a single line. The transmission is Asynchronous because no clock signal is transmitted with the data.

The NMEA 0183 wiring consists of 4 conductors (or two pairs) per device. That is, one pair for “in” and one pair for “out.” It should be noted that the “out” from one device needs to connect to the “in” of another. This is easily confused during original installation or maintenance. If wires are not properly labeled, they can easily be wired incorrectly and cause problems that are difficult to diagnose. Cable runs should never exceed 150 feet in length.

Newer Developments

In 2000, because of the need to share more data between a larger number of marine devices, Bosch developed the NMEA 2000 standard (commonly referred to as N2K). This newer and more common N2K system is an easy-to-install “plug and play” system which enables multiple, simultaneous “talkers” and “listeners” on a single network. Unlike the multiple wires required in NMEA 0183 systems, NMEA 2000 networks run on “CAN Bus” (Controller Area Network) and are made up of a single cable in a backbone arrangement with multiple branches called “drops.” The NMEA 2000 (N2K) transmission rate is 250k bps (50 times faster than NMEA 0183) and allows any compatible sensor and display to communicate over an N2K network. It should be noted here that NMEA 2000 is the communications “protocol” and CAN Bus is the “medium” of communication. Obviously, since NMEA 2000 is easier to install and maintain, it should always be the network of choice for new marine network installations.

NOTE: Since an NMEA 2000 network is NOT electrically compatible with an NMEA 0183 network, an interface device (called a “gateway”) is required to send messages between devices on the different types of network. An adapter is also required if NMEA 2000 messages are to be received by or transmitted from a PC.

Is NMEA 0183 Still Used?

Yes. NMEA 0183 is an older standard which usually only appears in older installations. However, it will likely be around for a long time since it is a low-cost, low-capacity, single transmitter/multi-receiver network for interconnecting multiple marine electronic devices. Just because it is older doesn’t mean it should be replaced. We’ve all heard the old saying, “If it aint broke, don’t fix it.” If your NMEA 0183 system works fine and you’re satisfied with its performance, leave it alone. However, if your application requires newer and faster equipment, it may be time to upgrade to an NMEA 2000 network.