I am trying to create NMEA-compatible proprietary sentences, which may contain arbitrary strings.
The usual format for an NMEA sentence with checksum is:
$GPxxx,val1,val2,...,valn*ck<cr><lf>
where * marks the start of a 2-digit checksum.
My question is: Can any of the value fields contain a * character themselves?
It would seem possible for a parser to wait for the final <cr><lf>, then to look back at the previous 3 characters to find the checksum if present (rather than just waiting for the first * in the sentence). However I don’t know if the standard allows it.
Are there other characters which may cause problems?
The two ASCII characters to be careful with are $, which has to be at the start, and * which precedes the checksum. Anyone else parsing your custom NMEA wouldn’t expect to find either of those characters anywhere else. Some parsers, when they hit a $ assume that a new line has started. With serial port communication sometimes characters get lost in transit, and that’s why there’s a $ start of sentence marker.
If you’re going to make your own NMEA commands it is customary to start them with P followed by a 3 character code indicating the manufacturer or company creating the proprietary message, so you could use $PSQU. Note that although it is recommended that NMEA commands are 5 characters long, there are proprietary messages out there by various hardware and software manufacturers that are anywhere from 4 characters to 7 characters long.
Obviously if you’re writing your own parser you can do what you like.
This website is rather useful:
http://www.gpsinformation.org/dale/nmea.htm