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4 MODBUS COMMUNICATIONS
The RTU version of Modbus includes a two byte CRC-16 (16 bit cyclic redundancy check) with every transmission. 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 transmission, most significant
byte first. The resulting message including CRC, when divided by the same polynomial at the receiver will give a zero
remainder if no transmission errors have occurred.
If a 735/737 Modbus slave device receives a transmission in which an error is indicated by the CRC-16 calculation, the
slave device will not respond to the transmission. A CRC-16 error indicates than one or more bytes of the transmission
were received incorrectly and thus the entire transmission should be ignored in order to avoid the slave device performing
any incorrect operation. The CRC-16 calculation is an industry standard method used for error detection. An algorithm is
included here to assist programmers in situations where no standard CRC-16 calculation routines are available.
CRC-16 Algorithm:
Once the algorithm is complete, the working register "A" will contain the CRC value to be transmitted. Note that this algo-
rithm requires the characteristic polynomial to be reverse bit ordered. The most significant bit of the characteristic poly-
nomial 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
ith data byte (i = 0 to N-1)
G
16 bit characteristic polynomial = 1010000000000001 with the MSbit dropped and 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)
The algorithm is:
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
GE Power Management
No: go to 8.
Yes: G (+) A --> A
735/737 Feeder Protection Relay
4.1 OVERVIEW
4.1.6 ERROR CHECKING
4-3
4
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