CG40 369 MOTOR MANAGEMENT RELAY – COMMUNICATIONS GUIDE
MODBUS RTU PROTOCOL CGCOMMUNICATIONS GUIDE
If an 369 Modbus slave device receives a transmission in which an error is indicated by the
CRC-16 calculation, the slave device will not respond to the transmission. A CRC-16 error
indicates than one or more bytes of the transmission were received incorrectly and thus
the entire transmission should be ignored in order to avoid the 369 performing any
incorrect operation.
The CRC-16 calculation is an industry standard method used for error detection. An
algorithm is included here to assist programmers in situations where no standard CRC-16
calculation routines are available.
5.4 CRC-16 Algorithm
Once the following algorithm is complete, the working register "A" will contain the CRC
value to be transmitted. Note that this algorithm requires the characteristic polynomial to
be reverse bit ordered. The MSbit of the characteristic polynomial is dropped since it does
not affect the value of the remainder. The following symbols are used in the algorithm:
--> data transfer
A 16 bit working register
AL low order byte of A
AH high order byte of A
CRC 16 bit CRC-16 value
i, j loop counters
(+) logical exclusive or operator
Di i-th data byte (i = 0 to N-1)
G 16 bit characteristic polynomial = 1010000000000001 with MSbit
dropped and bit order
reversed
shr(x) shift right (the LSbit of the low order byte of x shifts into a carry flag, a '0'
is shifted into the
MSbit of the high order byte of x, all other bits shift right one location
The algorithm is as follows:
1. FFFF hex --> A
2. 0 --> i
3. 0 --> j
4. Di (+) AL --> AL
5. j+1 --> j
6. shr(A)
7. is there a carry? No: go to 8.
Yes: G (+) A --> A
8. is j = 8? No: go to 5.
Yes: go to 9.
9. i+1 --> i
10. is i = N? No: go to 3.
Yes: go to 11.
11. A --> CRC
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