Enzo Vidal - 1 year ago 337
C Question

# MODBUS RTU CRC16 calculation

I'm coding a MODBUS CRC16 calculator in C. What I have before is a python that do this, I wanted to convert it to C. I found some codes online but it's not giving me the correct answer.

For my python code, I have this as my CRC16.py

``````#!/usr/bin/env python

def calc(data):

crc_hi=0xFF
crc_lo=0xFF

for w in data:
index=crc_lo ^ ord(w)
crc_val=crc_table[index]
crc_temp=crc_val/256
crc_val_low=crc_val-(crc_temp*256)
crc_lo=crc_val_low ^ crc_hi
crc_hi=crc_temp

crc=crc_hi*256 +crc_lo
return crc
``````

Next, I'll use a script to input my variables:

``````import math
import subprocess
import serial
import time
from time import sleep
import struct
import CRC16
import sys

function_code = chr(0x04)
start_at_reg = chr(0x10) + chr(0x06)
num_of_reg = chr(0x00) + chr(0x02)

crc_hi = crc/256
crc_lo = crc & 0xFF
print "crc_lo: " +str(hex(crc_lo))
print "crc_hi: " +str(hex(crc_hi))
``````

This will give me:

``````>>
crc_lo: 0x95
crc_hi: 0xa
``````

Now, I found this C code online that to calculate CRC16:

``````WORD CRC16 (const BYTE *nData, WORD wLength)
{
static const WORD wCRCTable[] = {0x0000,0xC0C1,0xC181,0x0140,0xC301,0x03C0,0x0280,0xC241,0xC601,0x06C0,0x0780,0xC741,0x0500,0xC5C1,0xC481,0x0440,0xCC01,0x0CC0,0x0D80,0xCD41,0x0F00,0xCFC1,0xCE81,0x0E40,0x0A00,0xCAC1,0xCB81,0x0B40,0xC901,0x09C0,0x0880,0xC841,0xD801,0x18C0,0x1980,0xD941,0x1B00,0xDBC1,0xDA81,0x1A40,0x1E00,0xDEC1,0xDF81,0x1F40,0xDD01,0x1DC0,0x1C80,0xDC41,0x1400,0xD4C1,0xD581,0x1540,0xD701,0x17C0,0x1680,0xD641,0xD201,0x12C0,0x1380,0xD341,0x1100,0xD1C1,0xD081,0x1040,0xF001,0x30C0,0x3180,0xF141,0x3300,0xF3C1,0xF281,0x3240,0x3600,0xF6C1,0xF781,0x3740,0xF501,0x35C0,0x3480,0xF441,0x3C00,0xFCC1,0xFD81,0x3D40,0xFF01,0x3FC0,0x3E80,0xFE41,0xFA01,0x3AC0,0x3B80,0xFB41,0x3900,0xF9C1,0xF881,0x3840,0x2800,0xE8C1,0xE981,0x2940,0xEB01,0x2BC0,0x2A80,0xEA41,0xEE01,0x2EC0,0x2F80,0xEF41,0x2D00,0xEDC1,0xEC81,0x2C40,0xE401,0x24C0,0x2580,0xE541,0x2700,0xE7C1,0xE681,0x2640,0x2200,0xE2C1,0xE381,0x2340,0xE101,0x21C0,0x2080,0xE041,0xA001,0x60C0,0x6180,0xA141,0x6300,0xA3C1,0xA281,0x6240,0x6600,0xA6C1,0xA781,0x6740,0xA501,0x65C0,0x6480,0xA441,0x6C00,0xACC1,0xAD81,0x6D40,0xAF01,0x6FC0,0x6E80,0xAE41,0xAA01,0x6AC0,0x6B80,0xAB41,0x6900,0xA9C1,0xA881,0x6840,0x7800,0xB8C1,0xB981,0x7940,0xBB01,0x7BC0,0x7A80,0xBA41,0xBE01,0x7EC0,0x7F80,0xBF41,0x7D00,0xBDC1,0xBC81,0x7C40,0xB401,0x74C0,0x7580,0xB541,0x7700,0xB7C1,0xB681,0x7640,0x7200,0xB2C1,0xB381,0x7340,0xB101,0x71C0,0x7080,0xB041,0x5000,0x90C1,0x9181,0x5140,0x9301,0x53C0,0x5280,0x9241,0x9601,0x56C0,0x5780,0x9741,0x5500,0x95C1,0x9481,0x5440,0x9C01,0x5CC0,0x5D80,0x9D41,0x5F00,0x9FC1,0x9E81,0x5E40,0x5A00,0x9AC1,0x9B81,0x5B40,0x9901,0x59C0,0x5880,0x9841,0x8801,0x48C0,0x4980,0x8941,0x4B00,0x8BC1,0x8A81,0x4A40,0x4E00,0x8EC1,0x8F81,0x4F40,0x8D01,0x4DC0,0x4C80,0x8C41,0x4400,0x84C1,0x8581,0x4540,0x8701,0x47C0,0x4680,0x8641,0x8201,0x42C0,0x4380,0x8341,0x4100,0x81C1,0x8081,0x4040};

BYTE nTemp;
WORD wCRCWord = 0xFFFF;

while (wLength--)
{
nTemp = *nData++ ^ wCRCWord;
wCRCWord >>= 8;
wCRCWord ^= wCRCTable[nTemp];
}
return wCRCWord;

}
``````

I saved that code as header file, "CRC.h". So I can input the data that I have:

``````#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <termios.h>
#include <stdint.h>
#include <inttypes.h>
#include <stdlib.h>
#ifdef WIN32
/*windows stuff*/
#else
typedef unsigned long WORD;
typedef unsigned char BYTE;
#endif
#include "CRC.h"

int main(){
char query_m[6];
char fnc_code = 0x04;
char reg_hi = 0x10;
char reg_lo = 0x06;
char num_hi = 0x00;
char num_lo = 0x02;
query_m[1] = fnc_code;
query_m[2] = reg_hi;
query_m[3] = reg_lo;
query_m[4] = num_hi;
query_m [5] = num_lo;
int i;
for(i=0;i<7; i++){
printf("this is query_m[%d]: %d\n", i, query_m[i]);

}
int size_m = 7;
char crc_data;

crc_data = CRC16(query_m, sizeof(query_m));
printf("this is CRC: %d\n", crc_data);
return 0;
}
``````

This gives me a value of -107.
I'm relatively new to C, and I'm still doing some experiments on this

Thank you.

• `WORD` should be `unsigned short` (2 bytes), not `unsigned long` (4 or 8 bytes depended on platform)

``````typedef unsigned short WORD;
``````
• As @paddy said, loop from 0 to 6 in the for loop:

``````for (i=0; i<6; i++) {
...
}
``````
• `crc_data` should be of type `WORD`, not `char`

``````WORD crc_data;
``````
• use `%04x` in last `printf()`

``````printf("this is CRC: %04x\n", crc_data);
``````
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