Algorithm; Timing - GE Power Quality Meter Series Instruction Manual

7 MODBUS COMMUNICATIONS
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
-->
16 bit working register
A
low order byte of A
AL
high order byte of A
AH
16 bit CRC-16 value
CRC
loop counters
i,j
logical exclusive-or operator
(+)
i-th data byte (i = 0 to N-1)
Di
16-bit characteristic polynomial = 1010000000000001 with MSbit dropped & bit order reversed
G
shift right (the LSbit of the low order byte of x shifts into a carry flag, a '0' is shifted into the
shr(x)
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?
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
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.
GE Power Management
No: go to 8.
Yes: G (+) A --> A
PQM Power Quality Meter
7.1 OVERVIEW
7.1.6 CRC-16 ALGORITHM

7.1.7 TIMING

7-3
7