MODBUS RTV PROTOCAL DESCRIPTION 3- 5
If No Parity checking is specified, no parity bit is
transmitted and no parity check can be made. An
additional stop bit is transmitted to fill out the
character frame.
3.4.2 CRC Checking
In RTU mode, messages include an error-check field
that is based on a CRC method. The CRC field checks
the contents of the entire message. It is applied
regardless of any parity check method used for the
individual characters of the message.
The CRC field is two bytes, containing a 16-bit binary
value. The CRC value is calculated by the transmitting
device, which appends the CRC to the message. The
receiving device recalculates a CRC during receipt of
the message, and compares the calculated value to
the actual value it received in the CRC field. If the two
values are not equal, an error results.
The CRC is started by first preloading a 16-bit register
to all 1’s. Then a process begins of applying
successive eight-bits bytes of the message to the
current contents of the register. Only the eight bits of
data in each character are used for generating the
CRC. Start and stop bits, and the parity bit, do not
apply to the CRC.
During generation of the CRC, each eight-bit character
is exclusive ORed with the register contents. Then the
result is shifted in the direction of the least significant
bit (LSB), with a zero filled into the most significant bit
(MSB) position. The LSB is extracted and examined.
If the LSB was a 1, the register is then exclusive ORed
with a preset, fixed value. If the LSB was a 0, no
exclusive OR takes place.
This process is repeated until eight shifts have been
performed. After the last (eighth) shift, the next
eight-bit byte is exclusive ORed with the register’s
current value, and the process repeats for eight more
shifts as described above. The final contents of the
register, after all the bytes of the message have been
applied, is the CRC value.
When the CRC is appended to the message, the low-
order byte is appended first, followed by the high-
order byte.
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