Understand the basics of NMEA 0183

Firstö the basics of NMEA 0183
NMEA developed a standard over 20 years ago that defines the interface between various pieces of marine electronic equipment and navigation computers. Let them talk together and share important information. The NMEA 0183 standard slowly became the common method by which marine electronic devices could talk to each other.

The standard specifies both the electrical connections that make up a NMEA system, the communication method for ötransfer the data and the format of the the sentences that carry NMEA data.

The NMEA 0183 standard is a purely digital data transfer method. Use a binary format on; '1' and '0' to communicate a digital representation of the necessary data to a connected device.

NMEA 0183 has evolved from the previous standards (NMEA 0180 and 0182) . However, since they differ in baud rate and transmission parameters, they are not compatible with NMEA 0183. På likewise, the "newer" NMEA 2000 standard is quite different. They twoå The networks cannot be mixed without a specialized gateway device between converting the two; the standards, such as the Actisense NGW-1.

The Basics
The NMEA standard defines the electrical signaling, data protocol and sentence format for the abaud serial data bus. NMEA data is transferred from a source such as GPS, sonar, compass etc...

These devices are called "Talkers". Equipment that receives this data, such as a chart plotter, radar, PC, NMEA display is called a "Listener". Each NMEA 0183 connection/bus will have one speaker, but is capable of multiple listeners.

The latest specifications for NMEA 0183 (version 2 and up) bö ;r as a minimum meet the requirements of the computer standard "RS422" (Standard EIA-422-A). They use +5 / 0 volt signaling, which is low voltage and easy to connect to computer equipment.

Voltage levels available; however, a NMEA 0183 network can be much higher – up to +/- 15 volts. Especially when older equipment is used, as the original version 1 specification used +/- 12-15 volt signaling. Thus, all NMEA 0183 inputs compliant with v2.0 and above should be able to accept +/- 15 volt differential signals without damage.

NMEA 0183- the specification also requires that all receiving equipment must be opto-isolated. This requirement for optoisolation reduces the risk of interference and removes the problem of ground loop effects.

All connections must be made with twisted pair cables, with a shield. To prevent ground loops, the shield must only be connected at one end – NMEA specifies that the display should be connected to the Talker chassis.

Communication Specification
All data is sent in printable ASCII form (data bit 7 is always zero in NMEA 0183 data transfers, and all characters must be between ASCII HEX 20 and HEX 7E). This means that it can be viewed directly on; a "terminal" type PC program (such as "HyperTerminal") - although what is seen may not be correct; meaningful.

Note: NMEA 0183-HS or NMEA 0183 v3 is not supported. Something related to the new NMEA 2000 standard (wrongly proposed by some manufacturers). It uses a completely different electrical connection standard that is not compatible with NMEA 0183 on Something sweet.

If you need the ability to convert between NMEA 0183 and the new NMEA 2000 network, the Actisense NGW-1 Gateway will allow full bi-directional conversion between the twoå the different standards.

The different versions of the 0183 standard
As the NMEA 0183 specification has slowly evolved over the years, connecting one device to another is not always a simple matter . The situation is further complicated, as many manufacturers still use the old (“single ended”) method of connection because it is cheaper to implement.

So how can an older type NMEA device be connected to a newer device type? Care is needed – it is possible to damage or overload the output of a newer differential device if it is incorrectly connected to an older device.

This is because the older devices used ground as the return, whereas the newer devices actually drive the NMEA “-/B” line between 5v and 0v. Thus, connecting this output to ground will result in high currents being drawn by the driver instrument, resulting in potential overheating and damage to the driver circuits.

We present Actisense ISO-Drive
To solve all the above basic interface problems, Actisense implements ISO-Drive outputs on; all new products. These outputs provide the essential isolation that will keep a system safe when mixing products from different manufacturers on; a complex marine bus. If the ISO-Drive "B" wire is connected to ground, no current will flow and the system is safe.