Pentax DSI-200 Series User Manual page 157

CRC Check
In Modbus-CRC mode, a message includes a CRC-based error-
check field. The CRC field checks content of entire message. The
CRC field is two bytes, containing a 16-bit binary value. The CRC
field is calculated by transmitting device, and then added to
message. The receiving device recalculates a CRC value after
receiving message, and compares the calculated value with the
CRC value in the received CRC field.
The CRC is first stored to 0xFFFF. Then a procedure is invoked to
process the successive 8-bit byte in the message and the value in
the register. Only the eight bits in each character are used for the
CRC. The start bit, stop bit and the parity bit do not apply to the
CRC.
During generation of the CRC, each eight-bit character is in
exclusive-OR (XOR) with the content in the register. 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
then performs XOR with a preset value. If the LSB was a 0, no
performed. This process is repeated until eight shifts have been
performed. After the last (eighth) shift, the next eight-bit byte is in
XOR with the register's current value, and the process repeats for
eight more shifts as described above. The final value of the register,
after all the bytes of the message have been applied, is the CRC
value. The CRC is added to the message from the low-order byte
followed by the high-order byte.
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