Chapter 5 Parameters
VFD-E Series
Revision August 2006, 01EE, SW--PW V1.03/CTL V2.03 5-93
RTU mode:
Address
01H
Function 03H
21HStarting data address
02H
00HNumber of data
(count by word)
02H
CRC CHK Low 6FH
CRC CHK High F7H
CRC (Cyclical Redundancy Check) is calculated by the following steps:
Step 1: Load a 16-bit register (called CRC register) with FFFFH.
Step 2: Exclusive OR the first 8-bit byte of the command message with the low order
byte of the 16-bit CRC register, putting the result in the CRC register.
Step 3: Examine the LSB of CRC register.
Step 4: If the LSB of CRC register is 0, shift the CRC register one bit to the right with
MSB zero filling, then repeat step 3. If the LSB of CRC register is 1, shift the CRC
register one bit to the right with MSB zero filling, Exclusive OR the CRC register with the
polynomial value A001H, then repeat step 3.
Step 5: Repeat step 3 and 4 until eight shifts have been performed. When this is done, a
complete 8-bit byte will have been processed.
Step 6: Repeat step 2 to 5 for the next 8-bit byte of the command message. Continue
doing this until all bytes have been processed. The final contents of the CRC register are
the CRC value. When transmitting the CRC value in the message, the upper and lower
bytes of the CRC value must be swapped, i.e. the lower order byte will be transmitted
first.
The following is an example of CRC generation using C language. The function takes
two arguments:
Unsigned char* data Å a pointer to the message buffer
Unsigned char length Å the quantity of bytes in the message buffer
The function returns the CRC value as a type of unsigned integer.
Unsigned int crc_chk(unsigned char* data, unsigned char length){
int j;
unsigned int reg_crc=0xFFFF;
while(length--){
reg_crc ^= *data++;
for(j=0;j<8;j++){
if(reg_crc & 0x01){ /* LSB(b0)=1 */
reg_crc=(reg_crc>>1) ^ 0xA001;
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