NMEA CKSUM Calculation ?

Thread Starter

myztic_man

Joined Apr 27, 2009
21
Hi there.
I have a system here which is interfacing a GPS to an AT89 Micro. It is working, in that data is coming from the GPS to the Micro via UART and some pointer arithmatic is being done to split the string into each segment.

however, I am having trouble understanding the CKSUM's and how to use them?

I am following the datasheet: http://www.usglobalsat.com/download/47/em408_ug.pdf
which on page 10 shows the structure of commands being SENT to the gps unit. I am trying to make the GPS only give me back $GPRMC every second so I am using the command:
$PSRF103,04,00,01,01*21<CR><LF>
And then i am trying to disable all the other data feeds ($GPGGA, GPRSV etc etc) via:
"$PSRF103,00,00,00,01*21<CR><LF>",
"$PSRF103,01,00,00,01*21<CR><LF>",
"$PSRF103,02,00,00,01*21<CR><LF>",
"$PSRF103,03,00,00,01*21<CR><LF>",
"$PSRF103,05,00,01,01*21<CR><LF>"

However, the device still sends me data every second for all data feeds, so I think those final 5 commands are Wrong. Can you please tell me how I go about calculating the CKSUM?

Thanks!
 
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