Hi,
I am doing some modbus application but I have problem with calculating CRC16 for my modbus commands. can any one help me how to calculate CRC16 or is any component or dll for crc16 calculation?

Thanks
Saeid Yousefpour

Saied_Yousefpour [at] Yahoo.Com

 

This spreadsheet explains and calculates the CRC. Download from:
www.simplymodbus.ca/crc.xls

 

unsigned short crc16( unsigned char *data, short N)
{

unsigned short reg, bit, yy, flag;

yy = 0;
reg = 0xffff;

while( N-- > 0 )
{
reg = reg ^ data[yy++];

for ( bit = 0; bit <= 7; bit++ )
{
flag = reg & 0x0001;
reg = reg >> 1;
if ( flag == 1 )
reg = reg ^ 0xa001;
}
}

return ( reg );
}



Example of calling above crc16()
{

crc = crc16( polldata, bytes);
polldata[bytes] = crc & 0xff;
polldata[++bytes] = (crc >> 8) & 0xff;
txd( polldata, bytes);
}

 

http://www.modbus.org/docs/Modbus_over_serial_line_V1_02.pdf