What is NMEA?
Over the last 20 years, NMEA has slowly become the standard method by which marine electronics devices talk to one another. The standard specifies both the hardware connections that make up an NMEA system and the format of the data sentences that carry the NMEA information. The NMEA 0183 standard is a digital data transmission method, using ‘1’s and ‘0’s in a binary format, to communicate a digital representation of the required information (depth, speed etc.) to a connected instruments. All NMEA data is sent in the form of text sentences, each beginning with a $ symbol in plain “ASCII” text. We hope to monitor the installation of our network by viewing this data realtime in a Terminal application on our laptop.
Connecting NMEA devices together
NMEA data is transmitted from an information source such as GPS, depth sounder etc. These data sending devices are called “talkers” Equipment receiving this information such as a chart-plotter is called the “listener”. You cannot connect a number of talkers onto a single NMEA line, as this is not allowed – they are not synchronised, and will attempt to talk at the same time, resulting in corruption of your data, and possible disaster if valuable data such as navigation information is lost or worse, corrupted so that it is incorrect. This was the mistake we made on Mystere. We had too many talkers and listeners, everything had something it wanted to say. To help us overcome the problem we come to our investment in the Miniplex-41.
The MiniPlex-41 reads NMEA sentences from the listener ports and stores them in a buffer, one for each input. The sentences are read from the buffers in a round robin fashion, one sentence at a time, giving each listener port equal priority. Each sentence is then sent to the talker ports and the RS-232 port. This will isolate the talkers from each other and hopefully stop the collision and corruption of our navigation data.
On Mystere, when we install the multiplexer, we are aiming to have a configuration as shown in the high quality technical schematic above. Please note that this high quality technical schematic is copyright… It took a lot of man-minutes to complete! Of course we are a dynamic bunch and this plan is very likely to change a few times during installation if we think we find a better way of doing it!!
A nice benefit of this system is that when we upgrade our instruments in the future, we will be able to easily add another device into the network with the knowledge that all should be well with the sharing of data. The diagram shows a future purchase being a ‘TackTick’ wireless windex. I imagine that this will be the next upgrade purchased so we want to make sure it will go in without a hitch. Theory states that this will be the case!
If you have found this introduction interesting and you want to see how we turn a soldered blob of cables into a cohesive structure of interconnected devices… check back regularly… we will install the new box as soon as it arrives!
February 9th, 2005 at 6:53 pm
Nice article. We must take a camera down to the boat on Saturday and get some pics of the work we did and other parts of the boat though to put on here.
February 9th, 2005 at 7:02 pm
Indeed… could maybe get some out whilst racing!! Could bring along the water proof case!!
February 10th, 2005 at 10:55 am
An interesting selection of electrickery, good luck! Agree some pics of what you’re doing and Mystere in general would be interesting.
October 26th, 2005 at 7:55 am
One basic problem with your design, if the MUX goes down (this is quite common), so does everything else. Hybrid redundent solution of some direct and some multiplexed is better. Typically we amp up GPS and gyro output signals and direct connect that to all the listeners, and mux everything else.
Sure, more wire and needs at least multiple input ports on other kit.
October 26th, 2005 at 8:37 am
Good point though… hadn’t really considered that before!!
July 5th, 2008 at 1:29 am
Wow these use electronics in everything these days, haha hope it is water proof haha