NMEA for dummies?

colind3782

colind3782

Well-Known Member
Joined
18 Jan 2011
Messages
4,510
Location
Shropshire/Empuriabrava
Visit site
Can anyone recommend a good book to demystify the black art that is NMEA? I've looked through all the marine electrics books I have on board by Calder et al but they only mention it in passing. I guess they think it's all magic too! :)
 
In simplistic terms, NMEA0183 is basically just a data transmission system. Data is sent out by a device, using 2 wires, and received by another device. If you think of it as a one-way process, it explains why some devices have 2 wires for NMEA Out and 2 wires for NMEA In. The second wire in each case can sometimes simply be the ground connection (12v negative).

Also in simplistic terms, NMEA2000 is similar, but has a power feed in the wiring too.
 
colind3782 said:
Can anyone recommend a good book to demystify the black art that is NMEA? I've looked through all the marine electrics books I have on board by Calder et al but they only mention it in passing. I guess they think it's all magic too! :)
Click to expand...

Short answer, there isn't... As mentioned by pvb, the system itself is very straightforward... In Theory... When it comes to connecting different makes of kit and reading their instructions, they all use different terms - Data In and out, transmit and receive + and -, and some (namely Garmin) use a "single wire" system where the data return comes via the instrument -ive... There is only one way, and that's to start simple connecting stuff together. If you're ONLY connecting data, you can get it wrong and not do any damage... You still have to mark sure your power supply stuff is correct - obviously
 
pvb said:
In simplistic terms, NMEA0183 is basically just a data transmission system. Data is sent out by a device, using 2 wires, and received by another device. If you think of it as a one-way process, it explains why some devices have 2 wires for NMEA Out and 2 wires for NMEA In. The second wire in each case can sometimes simply be the ground connection (12v negative).

Also in simplistic terms, NMEA2000 is similar, but has a power feed in the wiring too.
Click to expand...

Just to clarify for the OP's benefit, the two data wires (blue & white) in a Nmea 2000 (N2K) cable are not transmit and receive, they are a balanced pair (CAN high & the opposite phase, CAN low) which transfer data in both directions.. (CAN = Controller Area Network)

No one has yet mentioned the NMEA 0183 transmission speed - the Baud rate. The sender and receiver must be set to the same baud rate, which is either 4800bits per sec, or 38,400 for AIS data.

N2K data rate (125k baud) is the same for all makes and devices, so there is no possibly of a mis match.
 
NMEA is also the format of the data transmitted, not just the hardware protocol (which is EIA-422, for NMEA 0183). Unfortunately, the details of the format are proprietary, so they aren't available in one place, and someone publishing them would be in breach of NMEA's copyright. NMEA charge a high price for the full definition - I've seen $340. Further, equipment manufacturers often implement a few sentences of their own devising, which should be ignored by devices from other makers, but which provide extra functionality within a particular manufacturer's ecosystem. Documenting all that would require a) paying a vast fee to NMEA and b) access to EVERY equipment maker's list of special codes.

Don't hold your breath!
 
AntarcticPilot said:
NMEA is also the format of the data transmitted, not just the hardware protocol (which is EIA-422, for NMEA 0183). Unfortunately, the details of the format are proprietary, so they aren't available in one place, and someone publishing them would be in breach of NMEA's copyright. NMEA charge a high price for the full definition - I've seen $340. Further, equipment manufacturers often implement a few sentences of their own devising, which should be ignored by devices from other makers, but which provide extra functionality within a particular manufacturer's ecosystem. Documenting all that would require a) paying a vast fee to NMEA and b) access to EVERY equipment maker's list of special codes.

Don't hold your breath!
Click to expand...

The sentence formats are are easy to find on the 'net and no fee is needed, the hardware layer is not NMEA's property so anyone can design and manufacturer a NMEA product.

The costs are incurred because 'NMEA' is a registered trademark so you cannot use it unless you join NMEA.org and pay the relevant fee.

However, all of this is not relevant to users of NMEA equipment.
 
ianj99 said:
The sentence formats are are easy to find on the 'net and no fee is needed, the hardware layer is not NMEA's property so anyone can design and manufacturer a NMEA product.

The costs are incurred because 'NMEA' is a registered trademark so you cannot use it unless you join NMEA.org and pay the relevant fee.

However, all of this is not relevant to users of NMEA equipment.
Click to expand...

As you say, the hardware layer is a standard system; I haven't chased up the details but I would imagine that EIA-422 is freely available. But EIA-422 is not the most important bit of NMEA-0183 (or later), and in any case, as has often been pointed out on these forums, has sufficient "slack" in it to allow interfacing with similar serial formats (e.g. EIA-232 AKA RS-232 - I wish I had a pound for every dodgy RS-232 interface I've worked with back when I was involved with prototype electronics!). The vital bit of NMEA is the sentence structures, and if you look at nmea.org, you'll see that NMEA certainly regard these as proprietary information, for which they charge a lot, whether or not they are widely available on the web. I would imagine that a resource that published a large proportion of the sentences would be pursued by NMEA for violation of their IPR.

Worth remembering that a lot of the information "out there" is only for a sub-set of sentences; usually those output by GPS systems. I guess that's less than 10% of the total set, and so OK within "reasonable use". Also, some of the information "out there" is reverse engineered, and may not have all the details of more obscure fields.
 
You must log in or register to reply here.

Share this page

Top