GE Power Quality Meter Series, Инструкция по эксплуатации

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GE Power Management PQM Power Quality Meter 7-3
7 MODBUS COMMUNICATIONS 7.1 OVERVIEW
7
7.1.6 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 char-
acteristic 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 & 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
ALGORITHM:
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
7.1.7 TIMING
Data packet 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 packet, then the communication link must be reset (i.e. all slaves start listening for a new
transmission from the master). Thus at 9600 baud a delay of greater than 3.5
×
1/9600
×
10 = 3.65 ms will
cause the communication link to be reset.