GE 239, Instruction Manual

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GE Multilin 239 Motor Protection Relay 7-3
7 COMMUNICATIONS 7 COMMUNICATIONS
7
If a 239 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 that one
or more bytes of the transmission were received incorrectly and thus the entire transmission should
be ignored in order to avoid the 239 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 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 characteristic polynomial is dropped since it does not affect the value of the remain-
der. The following symbols are used in the algorithm:
--> data transfer
A16 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 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:
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