Appendix A. Modbus Protocol
MS300
A-7
RTU mode:
Command Message Response Message
Target register
Target register
Number of register
(Count by word)
Number of register
(Count by word)
The first data content
The second data content
6. Checksum
ASCII mode (LRC Check):
LRC (Longitudinal Redundancy Check) is calculated by summing up 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.
Example:
01H + 03H + 21H + 02H + 00H + 02H = 29H, the 2’s-complement negation of 29H is D7H.
RTU mode (CRC Check):
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, fill MSB with zero,
then repeat step 3. If the LSB of CRC register is 1, shift the CRC register one bit to the right,
fill MSB with zero, Exclusive OR the CRC register with the polynomial value A001H, then
repeat step 3.
Step 5: Repeat step 3 and 4 until you perform eight shifts. This processes a complete 8-bit byte.
Step 6: Repeat step 2 through 5 for the next 8-bit byte of the command message. Continue doing
this until all bytes are 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, that is, the lower order byte is transmitted first.
7. The following is an example of CRC generation using C language.
Unsigned char* data a pointer to the message buffer
Unsigned char length the quantity of bytes in the message buffer
unsigned int crc_chk(unsigned char* data, unsigned char length)
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