7/13/2023 0 Comments Nmea checksum![]() For example messages from a GPS receiver will begin with GP, from a GLONASS receiver – GL, from Galileo – GA and from any combination of GNSS receiver – GN. The first two characters define the ‘talker identifier’ or the source of the message. This address field is divided into two fields. It is followed by an ‘address’ field consisting only of digits and uppercase letters. NMEA 0183 takes the form of a series of ASCII comma delimited string, for example: $GPVTG,333.93,T,M,0.343,N,0.635,K,A30 The u-blox 8 receiver description & protocol specification, section 31: NMEA Protocol describes all you need to know about NMEA messages. NMEA-0183 is a National Marine Electronics Association standard describing the electrical specification and data protocol for communication with common marine electronics such as echo sounders, sonars, anemometers and of course, GPS. I also believed the PI Zero was an overkill for the project, and was considering a lower power microcontroller that didn’t have all the overhead of Linux. However, as I was targeting my own PCB containing a ublox MAX-M8W, a lack of compatibility didn’t rate high in my list of project risks. While NMEA 0183 is a standard, there are subtle differences, interpretations or even bugs between vendors that can throw the spanner in the works. One advantage of using gpsd would have been compatibility with various GPS. gpsd can also be used used in conjunction with NTPd to set the time. Robust, feature rich software packages currently exist such as gpsd – a GPS service daemon. With the logger fitted to a vehicle, a network connection wasn’t always available for NTP. Upon boot, it doesn’t know what the actual time it is, unless it has access to a network and can retrieve the time via Network Time Protocol (NTP). The Raspberry PI doesn’t include a Real Time Clock (RTC). In C# you don't need the Asc function to get the character code, you can just cast the char to an int.For a recent OBD2 (vehicle/fleet data-logging) project running on a Raspberry PI Zero, I needed access to GPS data – both position/speed and time. The loop starts at the fourth character, to skip the first "", which should logically not be part of the data that the checksum should be calculated for. Int transCheckSum = Convert.ToByte(value.Substring(lastStar + 1, 2), 16) The C# code would look like this: string value = strValues More advanced algorithms like CRC and MD5 are often used to calculate checksums for more demanding applications. The idea is the same as when calculating a parity bit, but there is eight bits calculated across the bytes. Using Xor is a simple algorithm to calculate a checksum. PS: As a bonus, if anyone can provide a c# translation I would be most grateful. I just need an explanation of what the Xor is doing and how that is used for the checksum. TransCheckSum = Val("&h" + strValues(CheckLoop).Substring(LastStar + 1, (StrLen - (LastStar + 1))))ĬheckSum = CheckSum Xor Asc(strValues(CheckLoop)(CheckString)) Here is 1 line of the body of a soap request that gets parsed (some values have been obscured so the checksum may not work in this case): CHANGEDTHIS01,W-A,0,7753.2018E,1122.6674N, 0.00,1,CID_V_01*3BĪnd this is the snippet of vb code that checks it LastStar = strValues(CheckLoop).IndexOf("*") ![]() I am trying to reverse engineer someone else's vb.NET code and I am stuck with what a Xor is doing here. ![]() This question may already have been asked but nothing on SO actually gave me the answer I need.
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