Appendix B – Modbus Protocol
NMBA-01 Installation and Start-up Guide
B-19
CRC Generation
The Cyclical Redundancy Check (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 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 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 8-bit character 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
characters of the message have been applied, is the CRC value.
A procedure for generating a CRC is:
1. Load a 1 6-bit register with FFFF hex (all 1’s). Call this the CRC
register.
2. Exclusive OR the first 8-bit byte of the message with the low-order
byte of the 16-bit CRC register, putting the result in the CRC
register.
3. Shift the CRC register one bit to the right (toward the LSB), zero-
filling the MSB. Extract and examine the LSB.
4. (If the LSB was 0): Repeat Step 3 (another shift).
(if the LSB was 1): Exclusive OR the CRC register with the
polynomial value A001 hex (101 0 0000 0000 0001).
5. Repeat Steps 3 and 4 until 8 shifts have been performed. When this
is done, a complete 8-bit byte will have been processed.
6. Repeat Steps 2 through 5 for the next 8-bit byte of the message.
Continue doing this until all bytes have been processed.
7. The final contents of the CRC register is the CRC value.
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