KROHNE ALTOSONIC V Modbus EN - Manual (Page 59)

ALTOSONIC V
ModBus Manual 0300 rev07 E 7.30855.35.00 Page 59 of 64
9.3 Appendix C: CRC generation
(As taken from the website: www.modicon.com/techpubs/crc7.html )
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 eight-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, do not
apply to the CRC.
During generation of the CRC, each eight-bit character is exclusive ORed with the register contents.
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 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.
Generating a CRC
Step 1
Load a 16-bit register with FFFF hex (all 1's). Call this the CRC register.
Step 2
Exclusive OR the first eight-bit byte of the message with the low order byte of the 16-bit CRC register,
putting the result in the CRC register.
Step 3
Shift the CRC register one bit to the right (toward the LSB), zerofilling the MSB. Extract and examine
the LSB.
Step 4
If the LSB is 0, repeat Step 3 (another shift). If the LSB is 1, Exclusive OR the CRC register with the
polynomial value A001 hex (1010 0000 0000 0001).
Step 5
Repeat Steps 3 and 4 until eight shifts have been performed. When this is done, a complete eight-bit
byte will have been processed.
Step 6
Repeat Steps 2 ... 5 for the next eight-bit byte of the message. Continue doing this until all bytes have
been processed.
Result
The final contents of the CRC register is the CRC value.
Step 7
When the CRC is placed into the message, its upper and lower bytes must be swapped as described
below.
Placing the CRC into the Message
When the 16-bit CRC (two eight-bit bytes) is transmitted in the message, the low order byte will be
transmitted first, followed by the high order byte-e.g., if the CRC value is 1241 hex (0001 0010 0100
0001):
Figure 9 CRC Byte Sequence
Example
An example of a C language function performing CRC generation is shown on the following pages. All
of the possible CRC values are preloaded into two arrays, which are simply indexed as the function
increments through the message buffer. One array contains all of the 256 possible CRC values for the
high byte of the 16-bit CRC field, and the other array contains all of the values for the low byte.