7. MODBUS
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7.1.5 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-bit) 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-bit) 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.
