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8.2 MODBUS PROTOCOL
Once the following algorithm is completed, the working register "A" will contain the CRC value to be transmit-
ted. Note that this algorithm requires the characteristic polynomial to be reverse bit ordered. The most signifi-
cant bit of the characteristic polynomial is dropped, since it does not affect the value of the remainder. The
following symbols are used in the algorithm:
Symbols:
Algorithm:
8
GE Power Management will provide a C programming language implementation of this algorithm
upon request.)
NOTE
Communication message synchronization is maintained by timing constraints. The receiving device must mea-
sure the time between the reception of characters. If three and one half character times elapse without a new
character or completion of the message, then the communication link must be reset (i.e. all slaves start listen-
ing for a new query message from the master). Thus at 1200 baud a delay of greater than 3.5 x 1/1200 x 10 =
29.2 ms will cause the communication link to be reset. At 9600 baud a delay of greater than 3.5 x 1/9600 x 10
= 3.6 ms will cause the communication link to be reset. Most master query messages will be responded to in
less than 50 ms.
8-4
-->
data transfer
A
16 bit working register
A
low order byte of A
low
A
high order byte of A
high
CRC
16 bit CRC-16 result
i, j
loop counters
(+)
logical EXCLUSIVE-OR operator
N
total number of data bytes
D
i-th data byte (i = 0 to N-1)
i
G
16 bit characteristic polynomial = 1010000000000001 (binary) with MSbit dropped
and bit order reversed
shr (x) right shift operator (the LSbit of x is shifted into a carry flag, a '0' is shifted into the
MSbit of x, all other bits are shifted right one location)
1.
FFFF (hex) --> A
2.
0 --> i
3.
0 --> j
4.
D
(+) A
--> A
i
low
5.
j + 1 --> j
6.
shr (A)
7.
Is there a carry? No: go to step 8.
8.
Is j = 8?
No: go to 5.
Yes: continue.
9.
i + 1 --> i
10.
Is i = N?
No: go to 3.
Yes: continue.
11.
A --> CRC
745 Transformer Management Relay
low
Yes: G (+) A --> A and continue.
8 COMMUNICATIONS
8.2.6 CRC-16 ALGORITHM
8.2.7 MESSAGE TIMING
GE Power Management
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