GE 745 TRANSFORMER MANAGEMENT RELAY User Manual

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

GE 745 TRANSFORMER MANAGEMENT RELAY Specifications

General

Brand	GE
Model	745 TRANSFORMER MANAGEMENT RELAY
Category	Relays
Language	English