6. MODBUS COMMUNICATION PROTOCOL
IMR01J08-E1
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6.5 Calculating CRC-16
The Cyclic Redundancy Check (CRC) is a 2 byte (16-bit) error check code. After constructing the data
message, not including start, stop, or parity bit, the master calculates a CRC code and appends this to
the end of the message. The slave will calculate a CRC code from the received message, and compare
it with the CRC code from the master. If they do not coincide, a communication error has occurred and
the slave does not respond.
The CRC code is formed in the following sequence:
1.
Load a 16-bit CRC register with FFFFH.
2.
Exclusive OR
(
⊕
) the first byte (8 bits) of the message with the CRC register. Return the result to
the CRC register.
3.
Shift the CRC register 1 bit to the right.
4.
If the carry flag is
1
,
exclusive OR
the CRC register with A001 hexadecimal and return the result
to the CRC register. If the carry flag is
0
, repeat step
3
.
5.
Repeat step
3
and
4
until there have been 8 shifts.
6.
Exclusive OR
the next byte (8 bits) of the message with the CRC register.
7.
Repeat step
3
through
6
for all bytes of the message (except the CRC).
8.
The CRC register contains the 2 byte CRC error code. When they are appended to the message,
the low-order byte is appended first, followed by the high-order byte.
