A sentence is a line of text with a dollar sign.
The whole of NMEA 0183 fits in a paragraph. A message starts with $, carries a five-character identifier, then comma-separated fields, then * and two hexadecimal digits, then a carriage return and a line feed. Fields may be empty — two commas in a row mean the sender has nothing to say about that quantity, which is different from saying zero. The line is limited to 82 characters. That is the standard.
The identifier splits into a two-letter talker and a three-letter sentence type. GP is a GPS receiver, so $GPRMC is the recommended minimum position fix — time, a validity flag, latitude and longitude in the degrees-and-decimal-minutes format the sea gave us, speed in knots, track in degrees true, and the date.
Its companion, $GPGGA, adds fix quality, satellite count, dilution of precision, altitude and the geoid separation that l'articolo sulle quote spends its time on. Neither of them knows anything about lift.
The soaring lives in the proprietary sentences.
Standard NMEA has nothing to say about lift, so every manufacturer added its own. A proprietary sentence starts with P and a three-letter maker code — $PFLAU and $PFLAA are FLARM's, carrying traffic status and one line per target. LX's own sentences ignore that convention — the older $LXWP family and the newer $LXBC and $LXDT — a well-known exception every soaring app has a special case for. They carry the soaring: airspeed, barometric altitude, a burst of vario samples, heading and the computed wind. $LXDT is the request-and-answer half, so a tablet can ask for the task or set the MacCready rather than only listening.
The reason a panel assembled from four manufacturers works at all is a single design decision made in 1983: a receiver silently ignores any sentence it does not recognise. Nothing negotiates, nothing announces itself, and nothing breaks when a stranger starts talking on the same wire. It is not elegant. It is why your glider works.
One sentence, and the wire it has to fit on.
Drag the baud rate down to 4800 and the timeline overflows: at 480 characters a second, one busy second of traffic does not fit, and something gets dropped. That is most of the reason a modern panel runs at 38400. Then drag the vario and watch the checksum change with it — the two digits are recomputed from every character in the line, which is how a receiver knows a noisy wire mangled something.
The checksum is a parity, not a signature.
Take every character between the dollar sign and the asterisk and exclusive-or them together. That is it. Two hexadecimal digits catch a flipped bit, a truncated line and most of what a bad crimp does to a serial wire — and catch nothing at all about whether the data is right. A sentence claiming you are at 30,000 metres over the Atlantic will pass its checksum happily. The two digits protect the wire, not the truth.
They also do not protect against a dropped line. If a whole sentence never arrives, nothing anywhere notices; the receiver simply has slightly older data than it thinks. This is the failure mode of an overloaded serial link, and it is silent by design.
Baud is a budget, and traffic is expensive.
Serial speed is quoted in bits per second, and with a start bit and a stop bit each byte costs ten. So 4800 baud is 480 characters a second, and 38400 is 3840. A position fix and a vario sentence at one hertz cost almost nothing. Traffic is what fills the wire: one line per target per second, and a busy gaggle produces a dozen. This is why the same panel that works perfectly on a quiet Tuesday drops data on a competition grid.
Bluetooth changes nothing about any of this. It is a longer wire: the same characters, the same checksums, the same sentences, carried over a radio link that pretends to be a serial cable. A tablet paired over Bluetooth is reading the identical stream it would have read through a plug.
Where the panel really outgrows serial is between the instruments themselves, which is what the CAN bus article is about — but even a CAN panel keeps one NMEA port, because that is what the tablet in the pilot's bag understands.
Nel tuo cockpit.
You will meet this protocol exactly twice: on the afternoon you connect a tablet for the first time, and on the afternoon it stops working. Both go faster if you know that the stream is human-readable text, that any serial terminal will show it to you, and that a screen full of valid sentences means your problem is entirely in software. The step-by-step wiring and pairing for our instruments is in the knowledge-base guide, and the sentence-by-sentence reference for developers is in the knowledge base too.