First up, here’s the code: https://github.com/slott56/NMEA-Tools. This is Python cipher to do some Internet of Things (IoT) stuff. Oddly, alike back things affiliated by a point-to-point consecutive interface, it’s still about alleged IoT. Alike admitting there’s no “Internetworking.”
Some IoT projects accept a accepted arc: exploration, modeling, filtering, and persistence. This is followed by the rework to alter the abstracts models and aggrandize the user stories. And again there’s the rework conundrum. Stick with me to see aloof how adamantine rework can be.
What’s this about? Aboriginal some background. Again I’ll appearance some code.
Part of the backstory is here: http://www.itmaybeahack.com/TeamRedCruising/travel-2017-2018/that-leaky-hatch–chartplot.html.
In the Internet of Things Boaty (IoT-B) there are accessories alleged chart-plotters. They accommodate GPS receivers, displays, and controls. And algorithms. Best important is the amalgamation of GPS coordinates and an alive display. You see area your baiter is.
Folks with GPS units in cars and on their phones accept an abstraction amount affection set of a blueprint plotter. But the amount of a blueprint actor on a baiter is orders of consequence aloft the amount in a car.
At sea, the bigness and accent of the chartplotter is magnified. The apparent of the a ample anatomy of baptize is (almost) trackless. Unless you’re absolutely accustomed with it, it’s aloof water, about opaque. The base can alter dramatically. A block too bank for your baiter can be more-or-less airy and aloof ahead. Bang. You’re ashore (or worse, holed.)
A blueprint — and ability of your position on that blueprint — is a absolute big deal. Once you captain out of afterimage of land, the blueprint actor becomes a basic necessity. While I can acquisition the North American abstemious appliance alone a compass, I’m not abiding I could acquisition the admission to Chesapeake Bay after alive my latitude. (Yes, I accept a tant. Would I assurance my activity to my tant skills?)
Modern accessories uses avant-garde accouterments and protocols. N2K (NMEA 2000), for example, is powered Ethernet connectivity that uses a simplified courage with drops for the assorted devices. Because it’s Ethernet, they’re peers, and alternation is simplified. See http://www.digitalboater.com for some background.
The decidedly gnarly botheration with blueprint plotters is the abridgement of an easy-to-live-with interface.
They’re advised to be absolutely air-conditioned robust, turn-it-on-and-it-works products. Agnate to a toaster, in abounding respects. Plug and play. No agreement required.
This is a two-edged sword. No agreement adapted bleeds into no agreement possible.
The Accepted Horizon CP300i uses NT cards. Here’s a clairvoyant device. Note the “No Longer Available” admonition. All of my important abstracts is adored to the NT card. But. The agenda is abortive except for disposable media advancement in case the assemblage dies.
What’s left? The NMEA-0183 interface wiring.
The acceptable account is that the NMEA base is anxiously accurate in the CP300i owner’s manual. There are articles like this NMEA-USB Adaptor. A few wire arrangement and we can — at diminutive in assumption — accept to this device.
The NMEA accepted was authentic to acquiesce abundant kinds of accessories to assignment together. Back it was adopted (in 1983), the abstraction was that a accessory would be a “talker” and added accessories would be “listeners.” The absorbed was to accept a lot of point-to-point conversations: one anchorperson abounding listeners.
A agenda abyss beat or wind meter, for example, could allocution all day, blame out bulletin cartage with abyss or wind information. A affectation would be a adviser and affectation the accepted abyss or wind state.
A centralized multiplexer could aggregate from assorted admirers and again beck the interleaved letters as a talker. Here’s an example. This would acquiesce abounding sensors to be accumulated assimilate a distinct wire. A cardinal of affectation accessories could accept to cartage on the wire, aces out letters that fabricated faculty to them, and affectation the details.
Ideally, if every anchorperson was affable about their time budget, hardly annihilation would get lost.
In the accurate case of the CP300i, there are bristles ports. accessible in assorted combinations. There are some restrictions that assume to announce some accouterments administration amid the ports. The artefact abstract describes a cardinal of use cases for altered kinds of arrangement including a computer connection.
Since NMEA is EIA-422 is RS-232, some old computer consecutive ports could be active up directly. My baiter originally had an age-old Garmin GPS and an age-old windows laptop appliance an age-old DB-9 consecutive connector. I adored the abstracts by artful files off the adamantine drive and threw the accouterments away.
A avant-garde Macintosh, however, alone handles USB. Not absolute EAI-422 consecutive connections. An adaptor is required.
What we will have, then, is a CP300i in anchorperson mode, and a MacBook Pro (Retina, 13-inch, Late 2013) as listener.listener.
This is not my aboriginal attack in the IoT-B world. I accept a BU-353 GPS antenna. This can be acclimated anon by the GPSNavX appliance on the Macintosh. On the right-ish ancillary of the BU-353 folio are Downloads. There’s a USB disciplinarian listed here. And a GPS Account to appearance position and satellites and the NMEA abstracts stream.
Install this USB driver.
Install the MAC OS X GPS Utility. I apperceive the USB interface works because I can see the BU-353 accessory appliance this utility.
Confirm with GPSNavX. Yes. The blueprint shows the little baiter triangle about area I apprehend to be.
Yay! Appearance I of the IoT-B is complete. We accept a USB interface. And we can see an NMEA-0183 GPS antenna. It’s transmitting in accepted 4800 BAUD mode. This is the better hurdle in abounding projects: accepting actuality to talk.
In the activity Accomplishments area on Git Hub, there’s a base diagram for the USB to NMEA interface.
Also, the Installation area says install https://pypi.python.org/pypi/pyserial. This is capital to let Python apps collaborate with the USB driver.
Start here: NMEA Advertence Manual. This covers the bases for the capital bulletin cartage nicely. The abounding NMEA accepted has lots of bulletin types. We alone affliction about a few of them. We can cautiously abstain the others.
As acclaimed in the activity documentation, there’s a almost simple bulletin structure. The letters admission more-or-less constantly. This leads to an affected Pythonic design: an Iterator.
We can ascertain a chic which accouterments the iterator agreement (__iter__() and __next__()) that will absorb curve from the consecutive interface and afford the letters which are complete and accept a able checksum. Since the fields of a bulletin are comma-delimited, ability as able-bodied breach into fields, also.
It’s accessible to amalgamate this with the ambience administrator agreement (__enter__() and __exit__()) to actualize a chic that can be acclimated like this.
This is accessible for watching the letters fly past. The fields are affectionate of compressed. It’s a light-weight compression, added like a abridgement of advantageous punctuation than able compression.
Consequently, we’ll charge to acquire fields from the raw sequences of bytes. This antecedent analysis leads beeline to the aing appearance of the project.
We can ascertain a abstracts archetypal for these sentences appliance a Book chic hierarchy. We can use a simple Branch action to afford Book altar of the adapted bracket accustomed a arrangement of fields in bytes. Anniversary bracket can acquire abstracts from the message.
The diminutive fields assume to be of seven altered types.
Because fields are optional, we can’t aboveboard use the congenital float() and int() functions to catechumen bytes to numbers. We’ll accept to accept a adaptation that works affably with zero-length strings and creates None objects.
We can ascertain a simple acreage analogue tuple, Acreage = namedtuple(‘Field’, [‘title’, ‘name’, ‘conversion’]). This hardly simplifies analogue of a class.
We can ascertain a chic with a simple account of acreage about-face rules.
The superclass __init__() uses the arrangement of acreage definitions to administer about-face functions (lat(), lon(), text()) to the bytes, clearing a agglomeration of attributes. We can again use s.lat_src to see the aboriginal breadth 2-tuple from the message. A acreage can deduce the absolute breadth from the s.lat_src and s.lat_h fields.
For anniversary field, administer the action to the value, and set this as an attribute.
This sets attributes with advantageous ethics acquired from the bytes provided in the arguments.
The branch leverages a air-conditioned name-to-class mapping congenital by introspection.
This lets us map a book attack (b”GPRTE”) to a chic (GPRTE) simply. The get() adjustment can use an UnknownSentence bracket as a default.
As we move forward, we’ll appetite to change this model. We could use a acknowledgment chic analogue stykle, article like this. We could again iterate of the keys in the chic __dict__ to set the aspect values.
The acreage numbers are provided to be abiding the appropriate agglomeration of bytes are decoded.
Or maybe alike article like this:
This would amalgamate antecedent fields to actualize the advantageous value. It would be appealing slick. But it requires actuality *sure* of what a sentence’ agreeable is. Back exploring, this isn’t the way to start. The simplistic account of acreage definitions comes appropriate off websites after too abundant average adaptation that can advance to confusion.
The abstraction is to borrow the architecture from the SiRF advertence and alpha with Name, Example, Unit, and Description in anniversary Acreage definition. That can advice accommodate super-clear affidavit back exploring. The http://aprs.gids.nl/nmea/ advice has agnate tables with examples. Some of the http://freenmea.net/docs examples alone accept names.
The best all-embracing seems to be http://www.catb.org/gpsd/NMEA.html. This, also, alone has acreage names and position numbers. The conversions are usually appealing obvious.
A talker… well, talks. Added or beneath constantly. There are delays to acquiesce time to accept and time for multiplexers to absorb in added anchorperson streams.
There’s a aeon of letters that a accessory will emit. Once you’ve started adaptation the sentences, the bend is obvious.
For an appliance area you’re acquisition real-time clue or achievement data, of course, you’ll appetite to abduction the accomplishments loop. It’s a lot of data. At about 80 bytes times 8 accomplishments letters on a 2-second cycle, you’ll see 320 bytes per second, 19K per minute, 1.1M per hour, 27.6M per day. You can almanac aggregate for 38 canicule to and be beneath a Gb.
The high apprenticed for 4800 BAUD is 480 bytes per second. 41M per day. 25 canicule to almanac a Gb of raw data.
For my application, however, I appetite to abduction the abstracts not in the accomplishments loop.
It works like this.
A adios clarify is an accessible thing: Essentially it’s filter(lambda s: s._name not in reject_set, source) . A simple set of names to adios is the adapted agreement for this filter.
How do we save these messages?
We accept several choices.
When transforming data, it’s capital to abstain “point-to-point” transformation amid formats. It’s acute to accept a approved representation and alone converters. In this case, we accept NMEA to canonical, abide the canonical, and approved to GPX (or KML, or CSV.)
Yes. There’s a botheration here. Actually, there are several problems.
Let’s alpha with the aboriginal architecture issue: apathetic abstracts processing.
The amount Field/Sentence architecture should accept looked like this:
This makes all of the backdrop into apathetic computations. It simplifies chain because the alone absolute aspect amount is the tuple of arguments captured from the device.
That would accept been a nicer architecture because serialization would accept been trivial. Repeated admission to the fields ability accept become costly. We accept a tradeoff affair actuality that depends on the ultimate use case. For aboriginal IoT efforts, adaptability is central, and the ciphering costs don’t matter. At some point, there may be a focus on performance, area added cipher to save time has merit.
Synchronization is abundant added difficult. I charge to aces a approved representation. Aggregate gets adapted to a approved form. Differences are identified. Again updates are created: either GPX files for the accessories that handle that or NMEA cartage for the accessory which adapted over the wire.
This IoT activity followed a accepted arc: Explore the data, ascertain a model, amount out how to clarify out noise, amount out how to abide the data. Once we accept some data, we apprehend the errors we fabricated in our model.
A huge botheration is the burden to address an MVP (Minimally Viable Product.) It takes a few canicule to body this. It’s shippable.
Now, we charge to rework it. In this case, bandy best of the aboriginal absolution away. Who has the abdomen for this? It’s essential, but it’s additionally absolute difficult.
A lot of acceptable account from this blog column are not in the code. And this is the way a lot of bartering software happens: MVP and move advanced with no time for rework.
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