Algorithm; Message Timing - GE TRANSFORMER MANAGEMENT RELAY 745 Instruction Manual

7.2 MODBUS PROTOCOL
Once the following algorithm is completed, 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 most significant 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 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 (XOR) 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)

Algorithm: 1.

FFFF (hex) --> A
2. 0 --> i
3. 0 --> j
4. D
i
5. j + 1 --> j
6. shr (A)
7. Is there a carry?
8. Is j = 8?
9. i + 1 --> i
10. Is i = N?
11. A --> CRC
7
GE Multilin 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 measure 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 listening 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.
7-4
Courtesy of NationalSwitchgear.com
(+) A --> A
low low
No: go to step 8.
Yes: G (+) A --> A and continue.
No: go to 5.
Yes: continue.
No: go to 3.
Yes: continue.
745 Transformer Management Relay
7 COMMUNICATIONS
7.2.6 CRC-16 ALGORITHM

7.2.7 MESSAGE TIMING

GE Multilin