NMEA2000 Primer

Copywrite 2010 by Boat-Project.Com



 

The intent of this article is to familiarize the boatowner with the latest in networking technology. I am writing this article from the boatowner's perspective, and I hope I Can make the non-professional understand the technology. I am retired from the computer/communications/electronics industry rather than working in the Marine industry. However, the NMEA2000 technology was borrowed from the non-marine industry, so the concepts behind this technology is no different than what I have experience with.

 

What is NMEA-2000 anyway? NMEA is the National Marine Electronics Association, an organization that promotes Marine Electronics, and they developed a networking standard so that heterogenous networks (i.e. different vendors) could be created. The idea here is you may want to use a display from manufacturer A, and possibly a sensor from manufacturer B, connect them both to the NMEA-2000 network, and the display will show what the sensor detects. In the ideal network, this should be possible. 2000 is simply the year that the standard was developed. So to say NMEA2000 (or N2K as it is often referred as), is simply stating a modern marine network.

 

Initially, NMEA-2000 was limited to navigation and related electronic equipment. But as time goes on, manufacturers are finding other uses for the network. For instance, switch modules are beginning to show up as NMEA-2000 devices. The idea here is to cluster switches in one area, such as the helm, and control devices on the boat in other areas, such as near the engine. This can be done with a switch unit at the helm, a control unit at the engine, and a single NMEA-2000 connection between them. If done conventionally, separate wires would have to be run to each switch, so using NMEA-2000 modules will reduce the cost and labor of installing copper wiring when the boat is being manufactured.

 

 

 

Prior to the development of NMEA-2000, the boating industry used an older standard for interconnecting equipment; NMEA 0183. This standard was meant to provide a point-to-point connection between two devices, not unlike RS-232 of the computer industry. Obvously the point-to-point scheme is very limited. In contrast, NMEA-200 is a shared network, not unlike Ethernet, whereas many sensors, displays, and control units can be interconnected on one common network.

 

Were not quite there yet, as manufacturers, for whatever reason, have tended to shy away from standardization in NMEA-2000. For instance, some of the first generation NMEA-2000 hardware used different (and incompatible) connectors, depending on the manufacturer. This is incredible, since a connector is defined in the standard. I believe it was an attempt by manufacturers to force their customers to remain with their brand. However, the manufacturers must realize that going down that path is unwise. This has been tried in the past in other industries, especially in the computer industry, and many of the manufacturers that did so are out of business.

An example of the first generation incompatible connectors are the Lowrance "Blue" connectors, which will only connect to a Lowrance NMEA-2000 device. Fortunately, most manufacturers have seen fit to finally embrace the standard, and Lowrance's "Red" connectors are NMEA-2000 standardized, and will interconnect with any other vendor's NMEA-2000 connector.

 

A packet; called a PGN (Parameter Group Number) in NMEA-2000 lingo, is the primary way devices communicate across the NMEA-2000 network. A packet (or PGN) can be thought of as an envelope which contains a piece of data, such as the position of a rudder sent from a sensor to the network. Once the sensor sends this data, it cares less what happens to it. A display device would typically listen on the network for this packet, and if it sees it, will pick it up and display it. The sensor will typically send its information periodically, which constantly updates the display. As more equipment is added to the network, more packets are generated, almost simultaneously. While only one sensor can transmit at a time, the speed of the network allows many hundreds of transmissions per second.

However, there are still some incompatibilities across the various manufacturers as each manufacturer is allowed to develop their own private data packets in addition to the standard (or public) packets defined by the NMEA-2000 standard and used by everybody. As well, not every device understands every packet - even the standard NMEA-2000 defined packets. That rudder packet may be ignored by a certain display if it doesn't have the capability to display rudder information. For that reason, you cannot simply use any display with any sensor. You may need to review the capabilities of the display unit you wish to use to ensure it can read the sensor. Fortunately, most manufacturers do publish this information.

And that old mentality of trying to force customers to buy from a single vendorreturns, at least for the PGNs, as some vendors may purposely use private PGNs (those that only that manufacturer's equipment understands). So a certain device may only be able to be read by that manufacturer's equipment, regardless of whether or not the device has a standard connector.

Also, many sensors must be programmed before use, and this usually requires a display unit of the same manufacturer to program them. However, once programmed - the sensor may be able to be detected by any other manufacturer's product. As of this writing, this incompatibility still exists to some degree, so be aware when mixing products from different manufacturers that you may run across a few problems.

I will not attempt to describe how the underlying NMEA-2000 technology works as this is beyond the need for the typical boatowner. However, you need to be aware of a few points. The NMEA-2000 network is known as a Bus network, which means that a single backbone cable is run through the boat, and all of the devices tap into the bus. If you think of your cold-water pipe in your house, (hopefully) there would be one main line, with all of the faucets connected to the main with Tee fittings. In actual practice, the NMEA-2000 contains both data and power wires. This results in all of the sensors and displays receive their power from the network cable as well.

 

While some manufacturers include a few NMEA-2000 networking components such as backbone cable, terminators, and tee fittings when you purchase a display unit, I use Maretron NMEA-2000 products for the network in my boat, as they are certified 100% NMEA-2000 standard, and they are high quality components. While they might cost a bit more, any cabling problems should be all but eliminated. As well, Maretron provides field installable connectors that you can use to correct the early manufactured non-standard connections. For instance, I have a few Lowrance "Blue" devices that I cut the connector off and installed a Maretron field installable connector to make them compatible with my network.


Building the Network

NMEA-2000 initially provided for two network cabling schemes; called Mini and Micro. Maretron now offers a third scheme called Mid. So the first question to answer is which one do I use? While each cabling scheme is identical in their bus configuration, the major difference is the capacity each network will provide. They are:

Cable SchemeMax LengthAmp CapacityPower AWGConnectorsMaximum DevicesLoad Equivalence Number (LEN)Drop LengthCost
Mini250 Meters8 Amps15 AWGMini501606 Meters$$$
Mid250 Meters4 Amps16 AWGMicro50806 Meters$$
MicroMax 100 Meters4 Amps22 AWGMicro50806 Meters$

Load Equivalency Number (LEN).

It was mentioned above that all devices on the NMEA-2000 network are powered by the network itself. Since each device on the network requires a diffierent amount of power (i.e. a display unit is likely to demand more power than a sensor), some thought must be given to how much total power the various devices require. NMEA-2000 includes a LEN specification that every NMEA-2000 device must provide:

1 LEN = 50mA.

Therefore, if a sensor states that it has a LEN of 5, you know that it's current requirement is 250mA. The idea of the LEN is to add them all up, and they should be less than the maximum LEN of the cable. From the chart, essentially the decision of which cable to use is determined by the length of the network you want to install, and the number of devices you need to support, and their total current requirement. The logical choice for a network for the typical pleasure boat will be the Micro, and most networks are of this type.

One issue that must be mentioned is that old nemisis, DC Voltage Drop vs. Cable Distance, especially for sensitive electronic equipment. If you need a refresher on this topic, review my paper on DC Voltage Drop:

DC Voltage Drop Tutorial

Excessive voltage drop can be an issue when using Micro cable, due to the small size of the power wires within the backbone. Especially on long cable runs you can have excessive voltage drop, even if the total power required by all of the devices is less than the LEN rating of the cable. To determine the voltage drop, an easy formula can be used:

Voltage Drop = LEN x Cable Length x Cable Resistance/100

To calculate voltage drop, assume you have a backbone cable length of 3 Meters, and by checking specifications, you have found the cable resistance is 6 Ohms per 100 meters, and when you add up the LEN of all of the devices on your network, you end up with a LEN of 10.

Voltage Drop = 10 x 3 x 6/100

The result is 1.8VDC voltage loss. If you recall from my DC voltage drop tutorial referenced above, electronic equipment should never be powered by cabling having more than a 3% voltage drop, which at 12V, would be a loss of 0.36VDC. Therefore, a loss of 1.8VDC is way too much, and it would be expected there would be some problems with this network. In this situation, the options are:

If we exchanged the Micro network backbone for a Mid cable, its resistance is 1 Ohms per 100 Meters, so the resulting voltage drop would be 0.3VDC, which is less than a 3% voltage drop, which is acceptable.

The beauty of using a Micro network with a Mid backbone cable is essentially the only difference is that the Mid cable uses 16 AWG power wires, where the Micro cable uses 22 AWG. Otherwise, all of the connectors and terminators are identical. This is the most cost-effective solution as Mid cables do not really cost much more than a Micro cable, and you can still use the less expensive Micro connectors and drop cables.


Legal Issues

It is actually a violation of USCG regulation 33CFR183 to use NMEA-2000 micro cable! The regulation requires a minimum of 16 AWG (18AWG bundled) cabling for any cable carrying current, while micro cable is 22AWG. While there is action being taken between NMEA and the USCG to remedy this situation, the current status can be summed up from the following excerpt found in NMEA2000 cable status Update:

The NMEA through independent third party engineering testing has proven that the so called “NMEA 2000 Light Cable Backbone” exceeds the safety requirements as per the 33CFR 183.425. This empirical data had been provided to the U.S. Coast Guard Office of Ship Design and Engineering Standards, Systems Engineering Division and to the Office of Auxiliary and Boating Safety. The NMEA 2000 Light Cable meets the requirements in the 46 CFR but did not meet the now thirty year old requirements in the 33 CFR.

NMEA has received a memorandum from the USCG Office of Ship Design and Engineering Standards, Systems Engineering Division. The NMEA 2000 Light Cable meets the Title Code 46 Code of Federal Regulations Chapter 1 Subchapter T Part C Regulation 183.340 “Cable and Wiring Requirements” and 183.130 “Alternative Standards.” The memo states: “...cables constructed to NMEA 2000 are acceptable for use in certain applications on small passenger vessels subject to 46 CFR 175.110. Cable constructed under NMEA 2000 (Light Cable) may only be used in direct current applications under 5 amps. It must be used only in integrated power supply applications for networking, control and communications systems designed to NMEA 2000.”

In a letter to NMEA, the USCG Office of Auxiliary and Boating Safety writes the following; “...even though there may be no significant safety risk associated with the use of cabling that employs 22AWG conductor under these circumstances the recreational boat manufacturing standards and regulations provide no flexibility for interpretation. 33CFR 183.425, explicitly prohibits power conductors that are not at least 18AWG. In the interim, a boat manufacturer who wishes to utilize the NMEA light cable in a boat to which this standard (33CFR183.425) applies may apply to this office for an exemption.”

So one office of the USCG (USCG Office of Ship Design and Engineering Standards, Systems Engineering Division) has endorsed the use of Micro cable, while another office of the USCG (USCG Office of Auxiliary and Boating Safety) has denied its use. Confused yet? This is all the more reason to use mid cable, notwithstanding the added benefit of fewer potential voltage problems.

Network Backbone Best Practice

Use high quality network components, such as those by Maretron. And construct a network with a Mid backbone. Finally, connect the power to the network in the center, which will minimize any voltage drop issues at the ends of the network.


Powering the Network

The network can be powered in several ways; at one end, in the center, or multiple taps. Powering the network using multiple taps is another effective way to reduce the voltage drop, as each segment is independantly calculated.

Caution

Each segment of the network backbone must only be powered at one point.
Also, the entire network backbone must only be grounded at one point.

Terminator: The network backbone must also be terminated at each end with a 120 Ohm resistor. These terminators prevent interference in the cable due to reflection of spirious signals from each end of the backbone. Special connectors with terminating resistors already built in are available from Maretron and other vendors. Some NMEA-2000 display units may come with combination power cable/termination resistors that you can use, but this may limit any future network configuation, and I would recommend avoiding those. Go with the standard stuff.


End powered network

 

         

Some manufacturers integrate several functions, such as this Lowrance combo power tap and terminator. Used for an end-powered network only, it is useful for small networks.


Center powered network


Simple NMEA-2000 Network

The network shown below is the minimum operational network. Note that the power is center-feed, and overcurrent protected with fuses. The rudder sensor places packets on the network and read by the rudder display. The power source should be switched so that the battery does not drain when the boat is not used. Note that although not shown, the network should be grounded to the boat's ground by the ground connector on the power tee. However, tees must not be cascaded or placed on the drop lines to the sensors.

It should also be noted that the tee fittings could be anywhere along the backbone, and the display tee could be to the left of the sensor. It does not matter where the tees are placed, as long as they are placed on the backbone.

Expanding the network is simple. To add a component, such as the Fuel Tank Sensor, simply add a Tee fitting to the BACKBONE as shown. Of course, some method should be available to read the data, and in this example, the display unit would have a multi-screen capability to allow the viewing of both sensors.

When expanding the network, all tees must be located on the network backbone. Tees should never be cascaded or located anywhere but the backbone.

One exception is a Multi-Port box, available from some manufacturers. This allows expansion of network devices at a minimum of cost.


One important item that needs repeating is voltage drop. By now, you should have a good foundation on minimizing voltage drop, however, attention must also be given to the feed wires. The wiring route from the battery to the NMEA-2000 power tee should be minimized to reduce voltage loss. This is sometimes difficult as often, the power switch for the network logically belongs at the helm, however (especially with a center-feed network), the power tee may be located some distance from the helm. One solution is to install a relay near the power tap, which is switched at the helm. The relay would allow a remote switch at the helm to turn the network on or off with minimum voltage loss.

 

Lowrance "RED" vs. "BLUE" NMEA 2000 networks. In the beginning of NMEA 2000, Lowrance had the crazy idea of making their connectors proprietary, which defeats the whole purpose of NMEA 2000. Well they seen the light soon after and begain making compatible connectors. Lowrance "RED" connectors are the original proprietary version, while the BLUE connectors are standard (and interchangeable) NMEA 2000 connectors. I would highly discourage installing a Lowrance RED network. If you go Lowrance, use the BLUE system.

For those of you that have boats wired with the RED cables, there is no reason to rip it out, as you can get a RED to BLUE adapter if you hae need to expand your network with different-manufacturer sensors.

 

Maretron NMEA-2000 Network Components

           

           

 

Garmin NMEA-2000 Network Components

           

 

Lowrance NMEA-2000 Network Components

           

 


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