REM 02/08/2007 REM generate checksum of a NEMA 0183 GPS string REM Q. How is the checksum calculated in NMEA 0183? REM A. The checksum is the 8-bit exclusive OR (no start or stop bits) of all characters in the REM sentence, including the "," delimiters, between but not including the "$" and "*" REM delimiters. The hexadecimal value of the most significant and least significant 4 bits of REM the result are converted to two ASCII characters (0-9, A-F) for transmission. The most REM significant character is transmitted first. CLS NEMA$ = "$GPRMC,000004,V,3714.6284,N,12146.9181,W,0.000,0.0,290697,15.4,E*4C" PRINT " " + NEMA$ chksum = ASC(MID$(NEMA$, 2, 1)) PRINT " 1. Hex Checksum value " + HEX$(chksum), "ASCII (HEX) NEMA DATA " + (MID$(NEMA$, 2, 1)), HEX$(ASC(MID$(NEMA$, 2, 1))) FOR x = 3 TO LEN(NEMA$) - 3 chksum = ASC(MID$(NEMA$, x, 1)) XOR chksum PRINT STR$(x) + ". Hex Checksum value " + HEX$(chksum), "ASCII (HEX) NEMA DATA " + (MID$(NEMA$, x, 1)), HEX$(ASC(MID$(NEMA$, x, 1))) NEXT x PRINT " " + NEMA$ PRINT " NEMA Checksum is " + HEX$(chksum)