Networking Marine Instruments: – 01 | Introduction

Introduction to Marine Electronics Networking: Start here.

This introduction to marine electronics networking will help you master your boat’s electronics.

Let’s say that you have a basic set of instruments on your boat, and let’s assume that it is a sailing vessel of some sort. You might have a wind instrument with an anemometer (measuring wind speed) and a direction transducer at the top of the mast, with a display in the cockpit. You may also have a ‘log’ (boat speed), often combined with a depth transducer mounted in or on the hull, and also with a display in the cockpit.

We have a wind instrument that, as a stand-alone device, can only display ‘Apparent Wind Speed’ (AWS) and ‘Apparent Wind Direction’ (AWD). You now want to dig deeper into the performance of your boat and be able to more accurately predict angles and courses, and need to know the ‘True Wind Speed’ (TWS) and ‘True Wind Angle’ (TWA).

Apparent wind angle and speed are what you would feel if you were standing on the deck of the boat. True wind is apparent wind compensated for the boat’s speed through the water. Therefore, to calculate True Wind, we have to feed the data from the log into the wind instrument; just one reason why we need to network our instruments. 

How is this ‘networking’ done?

Whilst instruments can be networked wirelessly, it is more commonly done using copper wires, and that is what I am going to cover here. 

The information carried over a marine network is encoded into data streams and transmitted in binary format (Ones and Zeros). Generally, the ones and zeros are represented by different voltage levels on the wires, or a wire, connecting the devices. One pair of wires twisted together (a twisted pair) is used in each direction. More advanced systems can use a single pair of wires linking many devices, where each device takes turns to transmit information and listens for information from other devices in the periods between transmissions. Let’s look at a basic example:

Referring to the image above, let’s assume that one wire in the pair is at 0V (signal ground) and that the other wire is used by the log instrument to send the information. The log instrument will send out a stream of codes representing the boat’s speed. It does this by changing the voltage on the signal wire very quickly between 5V and 0V. Circuitry in the wind instrument decodes the signals coming in, converts them to numbers of the correct format, and crunches the numbers to produce the information required. 

The voltages used and the timing of the signals could vary over huge ranges depending on the priorities of the designers; for this reason, many early marine electronics systems could not talk to those made by another manufacturer. This is where ‘standards’ come in. 

NMEA Standards

Most of the marine electronics industry has adopted a set of standards for these interconnections produced by the National Marine Electronics Association (NMEA) of the USA. Or at least closely aligned with those standards. There are two standards in common use with a third being adopted.

NMEA 0183

This standard was launched in 1983 and used twisted pairs to transmit information in one direction only. Two pairs are needed for bi-directional communications.  This is an older standard that has been somewhat superseded by NMEA 2000; 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. 

Note: wire colour codes – Whilst there may be a standard colour code somewhere in the NMEA0183 standard, it is not used across manufacturers; therefore, always check 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. 

NMEA 2000

N2k is a bus network based on the CAN Bus (Controller Area Network) specification, which was originally designed for vehicular use. This is a single cable system that consists of a backbone (or trunk) cable. Instruments and devices connect to the backbone at T-connectors via drop cables. There is an almost standard system of T-connectors and device connectors. The cable, for backbone and drops, contains five conductors. A shield (or drain), a power pair, and a data pair. Power is supplied to the network via a standard T-connector in most cases, although only the two conductors of the power pair are used.

Power is supplied to the network via a standard T-connector. The power supplied through the network often also powers smaller instruments and transducers, but larger instruments such as chart plotters generally need a separate supply. To further confuse things, some instruments can also power the bus. However, your bus is powered; there must only be one power source.

Dive deeper – The electrical interface used in N2k is functionally similar to RS-485, which coincidentally dates back to 1983, just like NMEA 0183.

NMEA OneNet

This is the NMEA version of the common Ethernet, which attempts to standardise the protocols used to transmit the type of data aboard. It supports Gigabit Ethernet (currently the common standard) and opens up the potential for better integration of products from different manufacturers for the end user. It could give end users more opportunities to mix and match devices from various manufacturers, such as a RADAR from Simrad on a Raymarine plotter.

However, whilst an exciting development, it took decades for the big names to start using the same type of connector on their N2k networks and reduce the amount of proprietary messages transmitted; don’t hold your breath.

Summary

Beginners in marine electronics DIY can be bewildered by all the jargon and acronyms, but by breaking the system down into parts, you should be able to get your head around it all. The next article on networking will cover building a simple N2k network and basic testing, if or when it all goes wrong.

Plans for this series

I have now published eight articles in this series. If there is more that you would like to see covered or subjects that you want to see more detail on, please let me know. The same applies if you would like to read a few more basic introductory articles. I have one planned now that will try to, gently, ease the complete noob to boating into the subject. What do you link? Leave a comment or get in touch via the contact page.

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Networking Marine Instruments: – 02 | NMEA 2000 >>