UNDERSTANDING NMEA AND INSTALLATION ON YOUR BOAT

The Actisense NMEA 2000 Network Starter Kit contains everything you need to make an initial NMEA 2000 network

Phil Whitehurst is the CEO of NMEA specialist Actisense, a marine electronics manufacturer based in Poole.

NMEA, which stands for National Marine Electronics Association, is a not for profit organisation based in the USA. Its mission is to improve communications between marine electronics manufacturers.

The latest standard is known as NMEA 2000 or N2K and is intended as the eventual replacement for the much older NMEA 0183. NMEA 2000 is a plug-and-play communications standard used for connecting marine sensors and display units within ships and boats.

So why upgrade?

NMEA 0183 is limited and is becoming outdated. NMEA 2000 continues to grow with new features from control over the on board entertainment system to engine monitoring.

Geek Mode ON: In the latest version of NMEA 0183 released in June 2012, there are 148 sentences (messages) with between 550 and 570 data fields. In total contrast, in the latest version of NMEA 2000 released in February 2015, there are 163 PGN messages and 1,569 data fields.

Geek Mode OFF: Other reasons why you should consider upgrading to NMEA 2000 with your next marine electronics purchase are:

  • It is easier to install and expand
  • The resultant network is lighter
  • Cabling uses far less space

But I cannot afford to upgrade all my kit…

The good news is that you do not have to. Conversion gateways are available so that you can keep your old NMEA 0183 devices and integrate them with the NMEA 20000 network. This means that you can upgrade only what you need or want to.

I am converted, but what do I need?

Every NMEA 2000 installation, however small, must consist of a network. Devices cannot connect directly together. The illustration shows what the beginnings of an NMEA 2000 network may look like when upgrading from NMEA 0183.

As a minimum, every NMEA 2000 network must have:

  • 2 x termination resistors
  • 2x T-pieces: one per device. Without two devices sharing information there is not a network
  • Power T-piece: just like a normal T-piece but used for giving power to the network

Extra cabling is only required if you need to extend the distance between T-pieces for the backbone, or if your NMEA 2000 device does not come with a cable for connecting in to the network, known as an instrument drop.

Starter kits are a good way to make sure you have the minimum requirements to install your first NMEA 2000 network. Different manufacturers may include additional items with their starter kits.  Check the contents to see if you need anything else and make sure that what it does include is fit for purpose. For example, we recommend the cable on the Power T is an absolute minimum of two metres.

Planning

It is best to start with planning where to insert the power. Power T-pieces with a twin pair of wires will help you distribute power more efficiently than those with only a single pair.

Keep the power demands even on each half of the Power T. An easy way to figure this out is to find the Load Equivalency Number or LEN for each device that will be installed on your network.  1 LEN = 50mA.

Insert the Power T at a point along the backbone where the LEN will be approximately the same on both sides.  This may mean you have more devices on one side of the Power T than the other but this will not matter. It is more important that the network is electrically balanced to prevent a significant volt drop on one side.

All that is left to do from here is connect additonal T-pieces and cabling when/if required and ensure that there is only ever two termination resistors installed and that these are at opposite ends of the network.

NMEA, which stands for National Marine Electronics Association, is a not for profit organisation based in the USA. Its mission is to improve communications between marine electronics manufacturers.

The latest standard is known as NMEA 2000 or N2K and is intended as the eventual replacement for the much older NMEA 0183. NMEA 2000 is a plug-and-play communications standard used for connecting marine sensors and display units within ships and boats.

So why upgrade?

NMEA 0183 is limited and is becoming outdated. NMEA 2000 continues to grow with new features from control over the on board entertainment system to engine monitoring.

Geek Mode ON: In the latest version of NMEA 0183 released in June 2012, there are 148 sentences (messages) with between 550 and 570 data fields. In total contrast, in the latest version of NMEA 2000 released in February 2015, there are 163 PGN messages and 1,569 data fields.

Geek Mode OFF: Other reasons why you should consider upgrading to NMEA 2000 with your next marine electronics purchase are:

  • It is easier to install and expand
  • The resultant network is lighter
  • Cabling uses far less space

But I cannot afford to upgrade all my kit…

The good news is that you do not have to. Conversion gateways are available so that you can keep your old NMEA 0183 devices and integrate them with the NMEA 20000 network. This means that you can upgrade only what you need or want to.

 

I am converted, but what do I need?

Every NMEA 2000 installation, however small, must consist of a network. Devices cannot connect directly together. The illustration shows what the beginnings of an NMEA 2000 network may look like when upgrading from NMEA 0183.

As a minimum, every NMEA 2000 network must have:

  • 2 x termination resistors
  • 2x T-pieces: one per device. Without two devices sharing information there is not a network
  • Power T-piece: just like a normal T-piece but used for giving power to the network

Extra cabling is only required if you need to extend the distance between T-pieces for the backbone, or if your NMEA 2000 device does not come with a cable for connecting in to the network, known as an instrument drop.

Starter kits are a good way to make sure you have the minimum requirements to install your first NMEA 2000 network. Different manufacturers may include additional items with their starter kits.  Check the contents to see if you need anything else and make sure that what it does include is fit for purpose. For example, we recommend the cable on the Power T is an absolute minimum of two metres.

Planning

It is best to start with planning where to insert the power. Power T-pieces with a twin pair of wires will help you distribute power more efficiently than those with only a single pair.

Keep the power demands even on each half of the Power T. An easy way to figure this out is to find the Load Equivalency Number or LEN for each device that will be installed on your network.  1 LEN = 50mA.

Insert the Power T at a point along the backbone where the LEN will be approximately the same on both sides.  This may mean you have more devices on one side of the Power T than the other but this will not matter. It is more important that the network is electrically balanced to prevent a significant volt drop on one side.

All that is left to do from here is connect additonal T-pieces and cabling when/if required and ensure that there is only ever two termination resistors installed and that these are at opposite ends of the network.

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