MECO Technical Note MTN 0040
What is NMEA 2000
Question
So what is NEMA 2000?
I keep hearing about NMEA 2000, it's essentially how my marine electronics communicate, and thats all I know. What actually is NEMA 2000?
Quick Answer
Short answer A set of standards and protocols that allow marine electronics to communicate with each other reliably.
The standard includes all the protocols required to establish a common electronic language that allows up to 52 devices to automatically introduce themselves on a common bus (a pair of data wires) and self organise who gets to talk and when. It is based on the a proven systems that has been in use in the automotive industry for over 30 years.
Why it matters
In order for electronic devices from different manufacturers to be able to communicate, they need to all speak the same language, with the same dialect, and have the same manners.
Technical explanation
In the digital world of networking, 'language' refers to voltages, data rates, and a common structure of codes, a library of which is held in each device. These are categorised under what are known as 'Parameter Group Numbers' (PGNs) PGNS tell other devices on the bus the category of the data to follow - such as depth, for example, in NMEA 2000 language is PGN 128267. A depth device will, when it gets its turn, identify itself to the other devices on the network as a 'depth source' with this PGN and from that introduction the other devices know how to interpret the data that follows.
Any devices that can use this data will be programmed to recognise it - such as a chart plotter for example, however a VHF radio can not use depth data - so it will not have the means to interpret depth; but it will however be fluent in position data.
All this data is sent in binary - ones and zeros - as a string of voltage blips. This raw data can be seen on some MFDs in Hexadecimal - a sort of short hand for binary.
Data networks at their rawest 'physical layer' still fall under Ohms Law - the relationship between Voltage, current, and resistance, but as data signals (blips of voltage) are such quick pulses the cables have to 'react' quickly. Pulling the bus up in voltage known as a dominant state - is not too difficult, it is letting go of the voltage, know as 'entering a recessive state' is more difficult. Any conductor trying to let go of a voltage quickly without anything to consume will cause reflections or 'ringing' in the cabling. this is like clapping your hands in a large cathedral, the initial clap (the rise) is very clear - however the room (cable) reflects the sound after - during the fall. The same is true of a data cable. To prevent these 'echoes' the data cables have an additional resistance added across them at each end, to give the blips something to do, like padding the walls of an echoey room to absorb the clap, the termination resistors at each end absorb the voltage and dissipate this energy as an infinitesimally small amount of heat.
Common mistakes
NMEA 2000 is almost plug and play, but its simplicity and reliability is also its biggest downfall.
At first glance it looks like a simple case of plugging everything in and seeing if it works, however it is SO reliable that it may well appear to work even though it is just on the cusp of complete failure.
Without proper training and reference to the installation standards NMEA 0400 - it may possible to get a system working whilst in the marina with a battery charger holding the system voltage at 13.7 volts, but as soon as the boat sets off, and the batteries drop to 80% or les, the system shuts down, or some devices stop working altogether.
The problem is that the protocol has some highly specific installation procedures that are absolutely essential to get the full reliability out of the system. Such as power insertion, village drop, interference prevention, termination resistance, instancing, and more.
If you haven't considered the above in a NMEA 200 installation then theres a good chance it's going to either not work - which is not good, or it will work - which is even worse.
Heading out to sea with a data network that is not installed correctly is dangerous. It is a common bus so if there is a problem with the bus itself or a non certified device has been installed anywhere on that bus then the reliability of the entire system is at risk.
The, NMEA 2000 bus - is essentially 2 data wires, and 2 power wires and one ground for interference prevention. If there is any damage to any of the connectors, or the cable itself the whole network can fail. The biggest cause of electronics systems failures is down to an incorrectly installed NMEA 2000 bus.
Standards note
ISO, ABYC
All standards revolve around how electricity behaves in a circuit and assume this knowledge is fully understood.
Technician takeaway
Just because it works - doesn't mean it works properly. There is no real way to see how good the system is without having a snoop around inside with a cheap scope - nothing fancy just looking for clean data pulses with good rise and fall, and no echoes.
You can learn all about this from literally zero electrical knowledge to NMEA 2000 expert level with the MET 3 Masterclass from MECO Training
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