Crc Generation - Toshiba TOSVERT VF-nC3 Instruction Manual

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5.2. CRC Generation

"CRC" is a system to check errors in communication frames during data transmission. CRC is
composed of two bytes and has hexadecimal-bit binary values. CRC values are generated by the
transmission side that adds CRC to messages. The receiving side regenerates CRC of received
messages and compares generation results of CRC regeneration with CRC values actually re-
ceived. If values do not match, data will be aborted.
Flow
CRC generation
CRC initial data: FFFF
Byte counter n = 0
Byte counter n < Length
CRC = (CRC XOR n
(0 expanded to word (higher 8
bits))
Bit counter = 0
Bit counter < 8
C = (Remainder of CRC ÷ 2)
ＣＲＣ >> １
Is remainder (C)
other than 0?
CRC＝
(CRC XOR generating polyno-
mial (A001))
Bit counter +1
Byte counter +1
End (Return CRC)
( )
Nｏ
Yes
th
send byte
Nｏ
Yes
Nｏ
Yes
A procedure for generating a CRC is:
1, Load a 16–bit register with FFFF hex (all 1's). Call this
the CRC register.
2. Exclusive OR the first 8–bit byte of the message with
the low–order byte of the 16–bit CRC register, putting
the result in the CRC register.
3. Shift the CRC register one bit to the right (toward the
LSB), zero–filling the MSB. Extract and examine the
LSB.
4. (If the LSB was 0): Repeat Step 3 (another shift).
(If the LSB was 1): Exclusive OR the CRC register with
the polynomial value A001 hex (1010 0000 0000 0001).
5. Repeat Steps 3 and 4 until 8 shifts have been per-
formed. When this is done, a complete 8–bit byte will
have been processed.
6. Repeat Steps 2 through 5 for the next 8–bit byte of the
message. Continue doing this until all bytes have been
processed.
7. The final contents of the CRC register is the CRC value.
8. When the CRC is placed into the message, its upper
and lower bytes must be swapped as described below.
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