Xylem Expert 7060_7070_Modbus - Manual (Page 27)

Communication protocol Serie30
Page 33/37
6.4 Calculation of the CRC16 checksum
The checksum can either be calculated or derived from a table.
Here is an example of CRC16 calculation in C:
//////////////////////////////////////////////////////////////////////////
// CRC-16 calculation in C
//
// Calculation of CRC-16 checksum over an amount of bytes in the serial buffer.
// The calculation is done without the 2byte from crc16 (receive-mode).
// SC_Buffer[]: Byte-Buffer for the serial interface. Type: unsigned char (8bit)
// SC_Amount : Amount of Bytes which should be transmitted or are received (without CRC16)
//
//////////////////////////////////////////////////////////////////////////
void CalcCRC16(unsigned char* CRC_H, unsigned char* CRC_L)
{
CRC := $FFFF
N := 0
M := 0
CRC mod 2
= 1
CRC := CRC div 2
CRC := CRC xor $A001
CRC := CRC div 2
M < 8
M := M + 1
N := N + 1
N < Message
l th
START
STOP
CRC := CRC xor DATA[N]
yes
yes
yes
// locals
unsigned int Crc;
unsigned char n, m, x;
// initialisation
Crc= 0xFFFF;
m= SC_Amount;
x= 0;
// loop over all bits
while(m>0)
{
Crc^= SC_Buffer[x];
for(n=0; n<8; n++)
{
if(Crc&1)
{
Crc>>= 1;
Crc^= 0xA001;
}
else
Crc>>= 1;
}
m--;
x++;
}
// result
*CRC_H= (Crc>>8)&0xFF;
*CRC_L= Crc&0xFF;
}// end CalcCRC16
This results in the following calculation for function 48 with device address 250: CRC16_H= 4, CRC16_L= 67.
Examples showing use based on a table are to be found in the MODBUS documentation at:
http://www.modbus.org