GE Power Management
469 Motor Management Relay
6-3
6 COMMUNICATIONS
6.1 MODBUS COMMUNICATIONS
6
6.1.5 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.
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
6.1.6 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.
