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PPC-2000 User’s Guide Appendix A: Modbus Protocol
Doc.# 30002-00 Rev 2.3 Watlow Anafaze 275
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.
CRC Checking
All messages include an error-checking field that is based on a
Cyclical Redundancy Check (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 pre-loading a 16-bit register to all
1’s. Then a process begins of applying successive 8-bit 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 if one is used, do
not apply to the CRC.
During generation of the CRC, each 8-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 0 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 (A001 hex). If the
LSB was a 0, no exclusive OR takes place.
This process is repeated until eight shifts have been performed.
After the last shift, the next 8-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.