GE Industrial Solutions Entellisys Integrator’s Guide - Manual (Page 9)

Modbus ® protocol implementation 9
1.1.4 CRC-16 Algorithm
The CRC-16 algorithm essentially treats the entire data stream (data bits only; start, stop and
parity ignored) as one continuous binary number. This number is first shifted left 16 bits and
then divided by a characteristic polynomial (11000000000000101B). The 16-bit remainder of
the division is appended to the end of the packet, most significant byte first. The resulting
packet including CRC, when divided by the same polynomial at the receiver, will give a zero
remainder if no transmission errors have occurred. 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.
CRC-16 ALGORITHM
ALGORITHM:
1. FFFF (hex) --> A
2. 0 --> i
3. 0 --> j
4. Di (+) Alow --> Alow
5. j + 1 --> j
6. shr (A)
7. Is there a carry? No: go to 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
SYMBOLS --> data transfer
A 16-bit working register
Alow low order byte of A
Ahigh 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
Di 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 (th 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)