![]() XOR the checksum with this character's valueĬhecksum = Checksum Xor Convert.ToByte(Character) ' Is this the first value for the checksum? ' Loop through all chars to get a checksum Public Function CalculateChecksum(ByVal sentence As String) As String Below is a VB.Net code sample used to calculate the checksum: At that point, if you’re reading the data, you can compare your computed checksum with the one at the end of the sentence. That data needs to have the checksum calculated for it. If you are reading NMEA data, you will need to separate the line to get everything between the dollar sign ($) and the asterisk (*), highlighted above in green. The checksum is actually computed as a single byte of data (8 bits), but is formatted as a two-character hexadecimal so that it is valid in ASCII. The checksum allows you to confirm that the message is the same as when it was created, and no data corruption along the way. Those two characters are the checksum for the message. Sentences also end with an asterisk (*) followed by two characters. The Checksum: The next thing you'll notice is that all NMEA sentences start with a dollar sign ($) indicating the start of a message. NMEA messages should never be anywhere near this long, so hitting this limit means that you aren’t receiving NMEA data. The buffer is also checked to see if it is over 4000 characters long, and if so, it is cleared. If any data remains after the last CrLf, it is placed back into the buffer so that future incoming data can be appended to it. One by one, those lines are checked to see if they are at least 5 characters long, and then sent to the ParseLines() subroutine for further processing. ![]() If any are found, the buffer is split into individual lines. That buffer is then checked to see if it contains any carriage return and line feed character sequences (vbCrLf). Below is a VB.Net code sample used to read data coming in from a serial port:ĭim lines() As String = Split(ReceiveBuffer, vbCrLf)ĭata coming in from the serial port is appended to a buffer. For this reason, you can usually separate lines by looking for these two characters. Here is an example of multiple messages from a GPS receiver: If you're looking at the data stream in a terminal program such a PuTTy, you'll see this as a new line. NEMA Sentence Structure: The first thing you need to know is that each NMEA sentence is on its own line, separated by two characters, a carriage return (ASCII 13) and a line feed (ASCII 10). This page is intended to help you understand the various parts of the message so that you can quickly write software to read or create NMEA sentences, and hopefully overcome problems such as corrupted or invalid data. However, the above characters contain a lot of useful info, and are actually somewhat human-readable if you know what you're looking at. If you've never seen this before, it will look a little cryptic. Here is an example of a NMEA GGA sentence: The world standard for this data is the NMEA sentence. Introduction: I've written a handful of applications that deal with GPS position data.
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