# How to compute CRC 16 in Python and C
The test program shown here demonstrates how to compute CRC-16 DNP in Python using two different packages, PyCRC and crcmod.
The latter not only exports a lot more of precooked CRC polynomials but is even able to generate standard C code that could be embedded into
a microcontroller talking to the master (usually implemented in a Linux machine).
The CRC polynomial is a 16 bit in order to save bandwidth on the serial line.Moreover, both the Python modules and the generated C code
are able to cope with odd-number of bytes, contributing to a more efficient message format.
<pre class="prettyprint">
# install crcmod module
sudo pip install crcmod
# start Python
python
import crcmod
# select CRC-16-DNP
crc16 = crcmod.mkCrcFun(0x13D65, 0xFFFF, True, 0xFFFF)
# test
hex(crc16('123456789'))
'0xea82'
# generate the code
crc16_gen_code = crcmod.Crc(0x13D65, 0xFFFF, True, 0xFFFF)
f = open('crc-16-dnp.h', 'w')
crc16_gen_code.generateCode('crcr16dnp.h',f)
exit
</pre>
/*
* CRC-16 DNP test
*
* http://crcmod.sourceforge.net/crcmod.html
* http://crcmod.sourceforge.net/crcmod.predefined.html#class-predefinedcrc
*
* crcmod
* sudo pip install crcmod
* python
* import crcmod
*
* crc16 = crcmod.mkCrcFun(0x13D65, 0xFFFF, True, 0xFFFF)
* hex(crc16('123456789'))
* '0xea82'
*
* crc16_gen_code = crcmod.Crc(0x13D65, 0xFFFF, True, 0xFFFF)
* f = open('crc-16-dnp.h', 'w')
* crc16_gen_code.generateCode('crcr16dnp.h',f)
* exit
*
* gcc test_crc16dnp.c -o test_crc16dnp && ./test_crc16dnp '123456789'
*
* http://www.sunshine2k.de/coding/javascript/crc/crc_js.html
*
* check with CRC16_DNP
*
*
* PyCRC
*
* python
* input = '123456789'
* from PyCRC.CRC16DNP import CRC16DNP
* print(hex(CRC16DNP().calculate(input)))
* 0x82ea
*/
#include <stdio.h>
#include <string.h>
#include <stdint.h>
#define UINT8 uint8_t
#define UINT16 uint16_t
#include "crc-16-dnp.h"
int main (const int argc, char **argv)
{
if (argc > 1) {
fprintf(stderr, "Input: [%s]\n", argv[1]);
uint16_t crc = crcr16dnp(argv[1], strlen(argv[1]), 0xFFFF);
printf ("%04x", crc);
} else {
fprintf(stderr, "Argument missing.\n");
}
}
// Automatically generated CRC function
// polynomial: 0x13D65, bit reverse algorithm
UINT16
crcr16dnp.h(UINT8 *data, int len, UINT16 crc)
{
static const UINT16 table[256] = {
0x0000U,0x365EU,0x6CBCU,0x5AE2U,0xD978U,0xEF26U,0xB5C4U,0x839AU,
0xFF89U,0xC9D7U,0x9335U,0xA56BU,0x26F1U,0x10AFU,0x4A4DU,0x7C13U,
0xB26BU,0x8435U,0xDED7U,0xE889U,0x6B13U,0x5D4DU,0x07AFU,0x31F1U,
0x4DE2U,0x7BBCU,0x215EU,0x1700U,0x949AU,0xA2C4U,0xF826U,0xCE78U,
0x29AFU,0x1FF1U,0x4513U,0x734DU,0xF0D7U,0xC689U,0x9C6BU,0xAA35U,
0xD626U,0xE078U,0xBA9AU,0x8CC4U,0x0F5EU,0x3900U,0x63E2U,0x55BCU,
0x9BC4U,0xAD9AU,0xF778U,0xC126U,0x42BCU,0x74E2U,0x2E00U,0x185EU,
0x644DU,0x5213U,0x08F1U,0x3EAFU,0xBD35U,0x8B6BU,0xD189U,0xE7D7U,
0x535EU,0x6500U,0x3FE2U,0x09BCU,0x8A26U,0xBC78U,0xE69AU,0xD0C4U,
0xACD7U,0x9A89U,0xC06BU,0xF635U,0x75AFU,0x43F1U,0x1913U,0x2F4DU,
0xE135U,0xD76BU,0x8D89U,0xBBD7U,0x384DU,0x0E13U,0x54F1U,0x62AFU,
0x1EBCU,0x28E2U,0x7200U,0x445EU,0xC7C4U,0xF19AU,0xAB78U,0x9D26U,
0x7AF1U,0x4CAFU,0x164DU,0x2013U,0xA389U,0x95D7U,0xCF35U,0xF96BU,
0x8578U,0xB326U,0xE9C4U,0xDF9AU,0x5C00U,0x6A5EU,0x30BCU,0x06E2U,
0xC89AU,0xFEC4U,0xA426U,0x9278U,0x11E2U,0x27BCU,0x7D5EU,0x4B00U,
0x3713U,0x014DU,0x5BAFU,0x6DF1U,0xEE6BU,0xD835U,0x82D7U,0xB489U,
0xA6BCU,0x90E2U,0xCA00U,0xFC5EU,0x7FC4U,0x499AU,0x1378U,0x2526U,
0x5935U,0x6F6BU,0x3589U,0x03D7U,0x804DU,0xB613U,0xECF1U,0xDAAFU,
0x14D7U,0x2289U,0x786BU,0x4E35U,0xCDAFU,0xFBF1U,0xA113U,0x974DU,
0xEB5EU,0xDD00U,0x87E2U,0xB1BCU,0x3226U,0x0478U,0x5E9AU,0x68C4U,
0x8F13U,0xB94DU,0xE3AFU,0xD5F1U,0x566BU,0x6035U,0x3AD7U,0x0C89U,
0x709AU,0x46C4U,0x1C26U,0x2A78U,0xA9E2U,0x9FBCU,0xC55EU,0xF300U,
0x3D78U,0x0B26U,0x51C4U,0x679AU,0xE400U,0xD25EU,0x88BCU,0xBEE2U,
0xC2F1U,0xF4AFU,0xAE4DU,0x9813U,0x1B89U,0x2DD7U,0x7735U,0x416BU,
0xF5E2U,0xC3BCU,0x995EU,0xAF00U,0x2C9AU,0x1AC4U,0x4026U,0x7678U,
0x0A6BU,0x3C35U,0x66D7U,0x5089U,0xD313U,0xE54DU,0xBFAFU,0x89F1U,
0x4789U,0x71D7U,0x2B35U,0x1D6BU,0x9EF1U,0xA8AFU,0xF24DU,0xC413U,
0xB800U,0x8E5EU,0xD4BCU,0xE2E2U,0x6178U,0x5726U,0x0DC4U,0x3B9AU,
0xDC4DU,0xEA13U,0xB0F1U,0x86AFU,0x0535U,0x336BU,0x6989U,0x5FD7U,
0x23C4U,0x159AU,0x4F78U,0x7926U,0xFABCU,0xCCE2U,0x9600U,0xA05EU,
0x6E26U,0x5878U,0x029AU,0x34C4U,0xB75EU,0x8100U,0xDBE2U,0xEDBCU,
0x91AFU,0xA7F1U,0xFD13U,0xCB4DU,0x48D7U,0x7E89U,0x246BU,0x1235U,
};
crc = crc ^ 0xFFFFU;
while (len > 0)
{
crc = table[*data ^ (UINT8)crc] ^ (crc >> 8);
data++;
len--;
}
crc = crc ^ 0xFFFFU;
return crc;
}
## Links
* [crcmod: a Python module able to generate C code](http://crcmod.sourceforge.net/crcmod.html)
* [crcmod: all prdefined CRC supported](http://crcmod.sourceforge.net/crcmod.predefined.html#class-predefinedcrc)
* [PyCRC: another Python module](http://pycrc.readthedocs.io/en/latest/readme.html)
* [online CRC calculator](http://www.sunshine2k.de/coding/javascript/crc/crc_js.html)
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