Timewave DSP-D300, Manual

Page: 99 / 112

99
Note the presets on bits 6 and 7 on byte 0. This is the EIF key combined with the CRC.
The CRC is symmetric so that the CRC for the four bytes equals 0 when the CRC is in
the packet.
// Get the CRC polynomial for the entry.
// See: http://docs.afws.net/supportsite/iflows/enhanced_iflows_format.htm
BYTE jp_getcrc( BYTE * eifmsg)
{
BYTE rem;
BYTE curbyte;
int bytecnt, bitcnt;
rem = 0;
for ( bytecnt = 0; bytecnt < 4; ++bytecnt )
{
curbyte
= eifmsg[bytecnt];
for (bitcnt = 0; bitcnt < 8; ++bitcnt)
{
rem >>= 1;
if ( ( curbyte & 0x01 ) == 0x01 )
rem |= 0x80;
curbyte
>>= 1;
if ( ( rem & 0x02 ) == 0x02 )
rem ^= 0x9a; // XOR 10011010
}
}
return rem;
}
Note: This is from the NWS web site. Please visit the website for a more complete
explanation.