Chapter 5 Parameters_VFD-B Series
Revision 10/2005, BE13, SW V4.08 5-99
RTU mode:
Address
01H
Function 03H
21H Starting data address
02H
00H Number 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++){
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