Hitachi SJ Series, User Manual

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14-9
Chapter 14 RS485 Communication
14.3.5 Error check
• To check errors in Modbus-RTU, use CRC (Cyclic
Redundancy Check).
• To generate a CRC code, use the generating polynomial
for CRC-16 (X
16
+X
15
+X
2
+1).
Example of procedure for calculating CRC-16.
• CRC codes are16 bits data generated for a block with
arbitrary data length in 8-bit unit.
CRC-16 calculation
CRC*1=FFFFh
Hi Lo
CRC register (2 bytes)
CRC*1
Target data
Swapping high bytes and
low bytes in CRC*1.
Finished.
CRC*1= CRC*1 XOR target data
Shift by 8 bits
CRC*1= Shift CRC*1 by 1 bit to the
A bit shifted out
CRC*1= CRC*1 XOR A001h
Yes
Calculation of the
target data is
finished.
Finished
There is a remaining bit.
0
1
Shift as much as 1 byte for the target data.