ok, then look for modbus, and 16 bit CRC on www.google.com

I got this in under 10 seconds....

unsigned int add2CRC16(unsigned char newData,
unsigned int oldCRC,
unsigned int genPoly)
{
int i = 8;

oldCRC ^= newData;

for(i = 8; i; i--)
{
if(oldCRC & 1)
{
oldCRC = (oldCRC >>= 1) ^ genPoly;
}
else
{
oldCRC >>= 1;
}
}
return oldCRC;
}


Chris Brooks