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| 4 | File : lib_crc.txt
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| 5 | Date : 2005-02-05
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| 6 | Author : Lammert Bies
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| 7 | E-mail : info@lammertbies.nl
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| 8 | Version : 1.11
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| 9 |
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| 10 | Contents : Documentation for the CRC functions in lib_crc.c
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| 11 |
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| 12 |
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| 13 |
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| 14 | The file lib_crc.c contains source code for functions to calculate
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| 15 | four commonly used CRC values: CRC-16, CRC-32, CRC-DNP and CRC-CCITT.
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| 16 | The functions can be freely used.
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| 17 |
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| 18 | To calculate a CRC, the following three steps must be followed:
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| 19 |
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| 20 | 1. Initialize the CRC value. For CRC-16 and CRC-DNP the initial value
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| 21 | of the CRC is 0. For CRC-CCITT the value 0xffff is used. CRC-32
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| 22 | starts with an initial value of 0xffffffffL.
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| 23 |
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| 24 | 2. For each byte of the data starting with the first byte, call the
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| 25 | function update_crc_16(), update_crc_32(), update_crc_dnp() or
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| 26 | update_crc_ccitt() to recalculate the value of the CRC.
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| 27 |
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| 28 | 3. Only for CRC-32: When all bytes have been processed, take the
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| 29 | one's complement of the obtained CRC value.
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| 30 |
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| 31 | 4. Only for CRC-DNP: After all input processing, the one's complement
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| 32 | of the CRC is calcluated and the two bytes of the CRC are swapped.
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| 33 |
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| 34 |
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| 35 |
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| 36 | An example of this calculation process can be found in the tst_crc.c
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| 37 | sample program. The program and other CRC implementations can be
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| 38 | tested with the test string "123456789" without the quotes. The
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| 39 | results should be:
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| 40 |
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| 41 | CRC16 : BB3D
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| 42 | CRC-CCITT : 29B1
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| 43 | CRC-DNP : 82EA
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| 44 | CRC32 : CBF43926
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| 45 |
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| 46 |
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| 47 |
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| 48 | The example program tst_crc.exe can be invoked in three ways:
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| 49 |
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| 50 | tst_crc -a
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| 51 |
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| 52 | The program will prompt for an input string. All characters in the
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| 53 | input string are used for the CRC calculation, based on their ASCII
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| 54 | value.
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| 55 |
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| 56 | Example input string: ABC
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| 57 | CRC16 = 0x4521
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| 58 | CRC-CCITT = 0xF508
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| 59 | CRC-DNP = 0x5AD3
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| 60 | CRC32 = 0xA3830348
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| 61 |
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| 62 | tst_crc -x
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| 63 |
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| 64 | The program will prompt for an input string. All characters will
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| 65 | be filtered out, except for 0..9, a..f and A..F. The remaining characters
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| 66 | will be paired, and every pair of two characters represent the hexadecimal
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| 67 | value to be used for one byte in the CRC calculation. The result if an
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| 68 | od number of valud characters is provided is undefined.
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| 69 |
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| 70 | Example input string: 41 42 43
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| 71 | CRC16 = 0x4521
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| 72 | CRC-CCITT = 0xF508
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| 73 | CRC-DNP = 0x5AD3
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| 74 | CRC32 = 0xA3830348
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| 75 |
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| 76 | You see, that the result is the same as for the ASCII input "ABC". This
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| 77 | is, because A, B and C are represented in ASCII by the hexadecimal
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| 78 | values 41, 42 and 43. So it is obvious that the result should be
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| 79 | the same in both cases.
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| 80 |
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| 81 | tst_crc file1 file2 ...
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| 82 |
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| 83 | If neither the -a, nor the -x parameter is used, the test program
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| 84 | assumes that the parameters are file names. Each file is opened and
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| 85 | the CRC values are calculated.
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| 86 |
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| 87 |
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| 88 |
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| 89 | The newest version of these files can be found at:
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| 90 |
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| 91 | http://www.lammertbies.nl/download/lib_crc.zip
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| 92 |
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| 93 | On-line CRC calculations of strings can be performed at:
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| 94 |
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| 95 | http://www.lammertbies.nl/comm/info/crc-calculation.html
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