Chapter 4 Parameters|
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4-78 Revision 2016/03, 6ELE, V1.14
3.4 Check sum
ASCII mode:
LRC (Longitudinal Redundancy Check) is calculated by summing up, module 256, the values of the bytes
from ADR1 to last data character then calculating the hexadecimal representation of the 2’s-complement
negation of the sum.
For example, reading 1 word from address 0401H of the AC drive with address 01H.
STX ‘:’
Address 1
Address 0
‘0’
‘1’
Function 1
Function 0
‘0’
‘3’
Starting data address
‘0’
‘4’
‘0’
‘1’
Number of data
‘0’
‘0’
‘0’
‘1’
LRC Check 1
LRC Check 0
‘F’
‘6’
END 1
END 0
CR
LF
01H+03H+04H+01H+00H+01H=0AH, the 2’s-complement negation of 0AH is F6H.
RTU mode:
Address 01H
Function 03H
Starting data address 21H
02H
Number of data
(count by word)
00H
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 a pointer to the message buffer
Unsigned char length 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;
}else{
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