Crc-16 Algorithm; Timing - GE MM3 Instruction Manual

6 COMMUNICATIONS
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.
a) 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. 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: 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 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? No: go to 8.
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
Data packet synchronization is maintained by timing constraints. The receiving device must measure the time
between the reception of characters. If 3.5 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 commu-
nication link to be reset.
GE Power Management
Yes: G (+) A —> A
MM3 Motor Manager 3
6.1 MM3 MODBUS PROTOCOL

6.1.6 TIMING

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6-3