Networking Marine Instruments: – 03 | NMEA 0183

NMEA 0183

Introduction to NMEA 0183

Understanding NMEA 0183 network basics is essential for anyone looking to effectively utilize marine electronics.

Installing electronic systems aboard your boat can be a daunting task, especially if you’re new to marine networking standards. One of the most widely used standards is NMEA 0183, which has been around for decades and is known for its simplicity and reliability. This article aims to introduce DIY sailors to the NMEA 0183 standard, summarising its strengths and weaknesses, explaining the necessity of having only one talker connected to multiple listeners, and comparing it to NMEA 2000 networks.

NMEA0183 is a marine networking standard developed by the National Marine Electronics Association (NMEA) of the USA. It allows various marine electronic devices, such as GPS, autopilots, depth sounders, and wind instruments, to communicate with each other using a simple serial data protocol. The standard uses ASCII text sentences to transmit data, making it easy to understand and troubleshoot.

Overview of NMEA 0183 key features

This standard was launched in 1983 and uses twisted copper wire pairs to transmit information in one direction only. Two pairs are needed for bi-directional communications.  This older standard has been somewhat superseded by NMEA2000, however, it remains dominant in commercial shipping, is simple and robust and for a small network, cheaper to install than N2k. The main features of this standard are:

• Devices that transmit are known as talkers.
• Devices that receive are known as listeners. 
• Only one talker can be connected to a twisted pair.
• Multiple listeners can be connected to a single talker.
• Data transmission speed is usually 4800 Baud (very roughly 4800 characters per second) for navigation data or 38400 Baud for AIS and heading data.
• The receive connections are opto-isolated, and the transmit may be depending on usage and the manufacturer. 

Opto-isolation is a method of electrically separating one side of a data circuit from the other. This provides a high level of protection to the individual devices from damaging voltages which may be accidentally applied to the signal wires. More information

Note: Wire Colour Codes – While there may be a standard colour code specified in the NMEA0183 standard, it is not consistently used across manufacturers; therefore, always consult the manual that came with your product. I have used the colours typically used by Actisense, who are a large manufacturer of NMEA interfacing products. 

Strengths and Weaknesses of NMEA0183

Strengths:

• Simplicity: NMEA 0183 uses a straightforward serial communication protocol, which is easy to implement and understand.
• Compatibility: Many N2k devices also support NMEA 0183 connections.
• Cost-Effective: Due to its long-standing presence in the marine industry, NMEA 0183-compatible devices can be more affordable than their NMEA 2000 counterparts. Cable is basic and cheap, especially when compared to N2k factory-built cables.

Weaknesses:

• Limited Data Rate: Vanilla NMEA 0183 has a maximum data rate of 4800 baud, which can be insufficient for transmitting large amounts of data quickly. High-speed NMEA 0183 (38400 Baud) can’t be used on the same interface pair as standard speed data. 
• Single Talker, Multiple Listeners: NMEA0183 requires a single talker (data source) to be connected to multiple listeners (data receivers). This can be a limitation if you need multiple devices to send data simultaneously.
• Wiring Complexity: The standard uses point-to-point wiring, which can become complex and cumbersome in larger installations.

The drawing above shows how you might connect multiple instruments. The Log is sending boat speed to the wind instrument and the fish-finder (ideally, a fish finder requires speed input so that the horizontal scale can be correctly calibrated). 

Using a Multiplexer

Imagine that our GPS is also transmitting position data. We wish to receive the position data on the fish-finder so that we can use the waypoint features of the instrument to mark fish. However, we have already occupied the receive wires with speed data! This is one of the primary limitations of the NMEA0183 standard. 

A multiplexer overcomes this limitation by combining data from several inputs into an output, which can then be distributed as required. In this case, the multiplexer would receive data from the GPS and Log and transmit information containing both speed and position to the fish-finder. 

The diagram above shows the same instruments connected via a multiplexer. The multiplexer is receives data on ports 1 and 2 and transmits the combined output on port 1. Typically, multiplexers can be configured via software running on a PC via a USB connection. Some now have WiFi and Bluetooth capability as well. 

Detailed coverage of multiplexer configuration is outside the scope of this article.

Installation

A future article will delve into the physical installation, choice of cable, connectors, junction boxes, instrument configuration, and such subjects. 

Troubleshooting Common NMEA0183 Issues

Setting up and maintaining an NMEA0183 network can sometimes be challenging, especially when issues arise. Here are some common problems and troubleshooting steps to help you resolve them: 

Remember: Reliable instrument networks require clean and stable power supplies.

No Data Transmission

If your devices are not receiving any data, check the following:

• Power Supply: Ensure all devices are powered on and receiving adequate power.
• Wiring Connections: Verify that the wiring connections are secure and correctly matched (TX+ to RX+, TX- to RX-).
• Device Configuration: Check the configuration settings of your devices to ensure they are set to communicate using NMEA 0183, and that the NMEA 0183 version is identical across all devices.

Intermittent Data Loss

If data transmission is sporadic, consider these steps:

• Cable Quality: Use only marine-grade, twisted pair, shielded cables to reduce interference and signal degradation.
• Connection Integrity: Inspect connections for corrosion or loose wires. Remake connections if there is any doubt.
• Environmental Factors: Ensure that cables are not exposed to extreme temperatures, oil, or moisture, which can affect performance. Yes, I know it’s on a boat, but do your best!

Data Corruption

If the data received is incorrect or garbled, try the following:

• Single Talker Rule: Ensure only one talker is connected to multiple listeners. Multiple talkers can cause signal interference and data corruption.
• Baud Rate: Verify that all devices are set to the same baud rate (typically 4800 baud for NMEA0183).
• Grounding: Check that all devices share a common ground to prevent electrical noise and interference.

Device Compatibility

If certain devices are not communicating, check:

• NMEA 0183 Version: Ensure all devices support the same version of NMEA 0183. Some older devices may not be compatible with newer versions.

Signal Interference

If you suspect signal interference, consider:

• Cable Routing: Route cables away from sources of electromagnetic interference, such as engines, generators, radios or other electronic devices.
• Shielding: Use shielded cables to protect against external interference. Some devices will have a ground terminal, which should be connected to the vessel’s ground point. As a general rule, shielded cables should have the shield connected at the end of the cable nearest to the power supply. In some cases, manufacturers require that the shield be connected at both ends.

Series Navigation

<< Networking Marine Instruments: – 02 | NMEA 2000Networking Marine Instruments: – 04 | NMEA 0183 to NMEA 2000 Conversion >>