SJ700-2 Instruction Manual NT204DX - Hitachi America, Ltd.

Chapter 4 Explanation of Functions
(iv) Error check code
The Modbus-RTU protocol uses the cyclic redundancy check (CRC) as the error check method.
The CRC code is the 16-bit data generated for a data block that has an arbitrary data length (in units of
8 bits).
A generative polynomial for CRC-16 (X 16 + X 15 + X 2 + 1) is used to generate the CRC code.
Example of CRC-16 calculation
CRC-16
operation
CRC *1
Hi
Lo
CRC *1 = FFFFh
CRC register (2 bytes)
Is the target
data found
No
Yes
Set the XOR of CRC*1 data and
target data in CRC*1
Does any
bit remain after 8-bit
shifting
No
Replace the Hi and Lo bytes
of CRC*1 with each other
Yes
Shift CRC*1 to the left by 1 bit
End
No
Is the
bit put out of
CRC*1 "1"
Yes
Set the XOR of CRC*1 data
and A001h in CRC*1
The target data is shifted by 1 byte.
(v) Header and trailer (silent interval)
The header and trailer set the total time the inverter should wait before sending a response after
having received a query from the master system.
Be sure to specify the time corresponding to the transmission of 3.5 characters (24 bits) as the waiting
time. If a shorter waiting time (corresponding to the transmission of fewer than 3.5 characters) is
specified, the inverter will not respond.
The actual waiting time is the sum of the silent interval (corresponding to the transmission of 3.5
characters) and the communication wait time (C078).
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