GE Feeder Management Relay 750 User Manual

GE Multilin 750/760 Feeder Management Relay 7-3
7 COMMUNICATIONS 7.2 MODBUS PROTOCOL
7
7.2.3 CRC-16 ALGORITHM
The CRC-16 algorithm essentially treats the entire data stream (data bits only; start, stop and parity ignored) as one contin-
uous 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, MSByte first. The
resulting packet including CRC, when divided by the same polynomial at the receiver will give a zero remainder if no trans-
mission errors have occurred. This algorithm requires the characteristic polynomial to be reverse bit ordered. The most sig-
nificant bit of the characteristic 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
GE Multilin will provide a C programming language implementation of this algorithm upon request.
7.2.4 MESSAGE TIMING
Communication message 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 message, then the communication link must be reset (i.e. all slaves start listening for a new query message from the
master). Thus at 1200 baud a delay of greater than 3.5 × 1 / 1200 × 10 = 29.2 ms will cause the communication link to be
reset. At 9600 baud a delay of greater than 3.5 × 1 / 9600 × 10 = 3.6 ms will cause the communication link to be reset. Most
master query messages will be responded to in less than 50 ms.
NOTE
Courtesy of NationalSwitchgear.com

GE Feeder Management Relay 750 Specifications

General

Brand	GE
Model	Feeder Management Relay 750
Category	Protection Device
Language	English