8-4
745 Transformer Management Relay
GE Power Management
8.2 MODBUS PROTOCOL
8 COMMUNICATIONS
8
8.2.6 CRC-16 ALGORITHM
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:
-->
data transfer
A
16 bit working register
A
low
low order byte of A
A
high
high order byte of A
CRC
16 bit CRC-16 result
i, j
loop counters
(+)
logical EXCLUSIVE-OR operator
N
total number of data bytes
D
i
i-th data byte (i = 0 to N-1)
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)
Algorithm:
1.
FFFF (hex) --> A
2.
0 --> i
3.
0 --> j
4.
D
i
(+) A
low
--> A
low
5.
j + 1 --> j
6.
shr (A)
7.
Is there a carry? No: go to step 8.
Yes: G (+) A --> A and continue.
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
GE Power Management will provide a C programming language implementation of this algorithm
upon request.)
8.2.7 MESSAGE TIMING
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.
NOTE
Summary of Contents for 745 TRANSFORMER MANAGEMENT RELAY
Page 30: ...2 8 745 Transformer Management Relay GE Power Management 2 3 SECURITY 2 GETTING STARTED 2 ...
Page 210: ...7 36 745 Transformer Management Relay GE Power Management 7 2 BLOCK DIAGRAMS 7 SCHEME LOGIC 7 ...
Page 322: ...9 12 745 Transformer Management Relay GE Power Management 9 3 USING 745PC 9 745 PC SOFTWARE 9 ...
Page 396: ...A 4 745 Transformer Management Relay GE Power Management A 1 FIGURES AND TABLES APPENDIXA A ...
Page 400: ...C 2 745 Transformer Management Relay GE Power Management C 1 WARRANTY INFORMATION APPENDIXC C ...
Page 406: ...vi 745 Transformer Management Relay GE Power Management ...
Page 407: ...GE Power Management 745 Transformer Management Relay NOTES ...
