Modbus的CRC校驗實驗

時間 2020-07-17

標籤 modbus crc 校驗實驗欄目 OS基礎简体版

原文原文鏈接

Modbus的CRC校驗實驗

表述3d

Name   : "CRC-16/MODBUS"
   Width  : 16
   Poly   : 8005
   Init   : FFFF
   RefIn  : True
   RefOut : True
   XorOut : 0000
   Check  : ?

先參考一些廠家給的MODBUS校驗程序指針

const unsigned char auchCRCHi[] = {
0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41, 0x01, 0xC0,
0x80, 0x41, 0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41,
0x00, 0xC1, 0x81, 0x40, 0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0,
0x80, 0x41, 0x01, 0xC0, 0x80, 0x41, 0x00, 0xC1, 0x81, 0x40,
0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41, 0x00, 0xC1,
0x81, 0x40, 0x01, 0xC0, 0x80, 0x41, 0x01, 0xC0, 0x80, 0x41,
0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41, 0x00, 0xC1,
0x81, 0x40, 0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41,
0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41, 0x01, 0xC0,
0x80, 0x41, 0x00, 0xC1, 0x81, 0x40, 0x00, 0xC1, 0x81, 0x40,
0x01, 0xC0, 0x80, 0x41, 0x01, 0xC0, 0x80, 0x41, 0x00, 0xC1,
0x81, 0x40, 0x01, 0xC0, 0x80, 0x41, 0x00, 0xC1, 0x81, 0x40,
0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41, 0x01, 0xC0,
0x80, 0x41, 0x00, 0xC1, 0x81, 0x40, 0x00, 0xC1, 0x81, 0x40,
0x01, 0xC0, 0x80, 0x41, 0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0,
0x80, 0x41, 0x01, 0xC0, 0x80, 0x41, 0x00, 0xC1, 0x81, 0x40,
0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41, 0x01, 0xC0,
0x80, 0x41, 0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41,
0x00, 0xC1, 0x81, 0x40, 0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0,
0x80, 0x41, 0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41,
0x01, 0xC0, 0x80, 0x41, 0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0,
0x80, 0x41, 0x00, 0xC1, 0x81, 0x40, 0x00, 0xC1, 0x81, 0x40,
0x01, 0xC0, 0x80, 0x41, 0x01, 0xC0, 0x80, 0x41, 0x00, 0xC1,
0x81, 0x40, 0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41,
0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41, 0x01, 0xC0,
0x80, 0x41, 0x00, 0xC1, 0x81, 0x40
} ;

const unsigned char auchCRCLo[] = {
0x00, 0xC0, 0xC1, 0x01, 0xC3, 0x03, 0x02, 0xC2, 0xC6, 0x06,
0x07, 0xC7, 0x05, 0xC5, 0xC4, 0x04, 0xCC, 0x0C, 0x0D, 0xCD,
0x0F, 0xCF, 0xCE, 0x0E, 0x0A, 0xCA, 0xCB, 0x0B, 0xC9, 0x09,
0x08, 0xC8, 0xD8, 0x18, 0x19, 0xD9, 0x1B, 0xDB, 0xDA, 0x1A,
0x1E, 0xDE, 0xDF, 0x1F, 0xDD, 0x1D, 0x1C, 0xDC, 0x14, 0xD4,
0xD5, 0x15, 0xD7, 0x17, 0x16, 0xD6, 0xD2, 0x12, 0x13, 0xD3,
0x11, 0xD1, 0xD0, 0x10, 0xF0, 0x30, 0x31, 0xF1, 0x33, 0xF3,
0xF2, 0x32, 0x36, 0xF6, 0xF7, 0x37, 0xF5, 0x35, 0x34, 0xF4,
0x3C, 0xFC, 0xFD, 0x3D, 0xFF, 0x3F, 0x3E, 0xFE, 0xFA, 0x3A,
0x3B, 0xFB, 0x39, 0xF9, 0xF8, 0x38, 0x28, 0xE8, 0xE9, 0x29,
0xEB, 0x2B, 0x2A, 0xEA, 0xEE, 0x2E, 0x2F, 0xEF, 0x2D, 0xED,
0xEC, 0x2C, 0xE4, 0x24, 0x25, 0xE5, 0x27, 0xE7, 0xE6, 0x26,
0x22, 0xE2, 0xE3, 0x23, 0xE1, 0x21, 0x20, 0xE0, 0xA0, 0x60,
0x61, 0xA1, 0x63, 0xA3, 0xA2, 0x62, 0x66, 0xA6, 0xA7, 0x67,
0xA5, 0x65, 0x64, 0xA4, 0x6C, 0xAC, 0xAD, 0x6D, 0xAF, 0x6F,
0x6E, 0xAE, 0xAA, 0x6A, 0x6B, 0xAB, 0x69, 0xA9, 0xA8, 0x68,
0x78, 0xB8, 0xB9, 0x79, 0xBB, 0x7B, 0x7A, 0xBA, 0xBE, 0x7E,
0x7F, 0xBF, 0x7D, 0xBD, 0xBC, 0x7C, 0xB4, 0x74, 0x75, 0xB5,
0x77, 0xB7, 0xB6, 0x76, 0x72, 0xB2, 0xB3, 0x73, 0xB1, 0x71,
0x70, 0xB0, 0x50, 0x90, 0x91, 0x51, 0x93, 0x53, 0x52, 0x92,
0x96, 0x56, 0x57, 0x97, 0x55, 0x95, 0x94, 0x54, 0x9C, 0x5C,
0x5D, 0x9D, 0x5F, 0x9F, 0x9E, 0x5E, 0x5A, 0x9A, 0x9B, 0x5B,
0x99, 0x59, 0x58, 0x98, 0x88, 0x48, 0x49, 0x89, 0x4B, 0x8B,
0x8A, 0x4A, 0x4E, 0x8E, 0x8F, 0x4F, 0x8D, 0x4D, 0x4C, 0x8C,
0x44, 0x84, 0x85, 0x45, 0x87, 0x47, 0x46, 0x86, 0x82, 0x42,
0x43, 0x83, 0x41, 0x81, 0x80, 0x40
} ;

//crc low byte in high 8 bit
unsigned short Modbus_CRC16(unsigned char *Buff_addr,unsigned short len)
{
	unsigned char uchCRCHi = 0xFF;             // CRC高字節的初始化
	unsigned char uchCRCLo = 0xFF;             // CRC低字節的初始化
	unsigned short uIndex;                           // CRC查找表的指針
	while (len--)								
	{
		uIndex = uchCRCHi ^ *Buff_addr++;      // 計算CRC
		uchCRCHi = uchCRCLo ^ auchCRCHi[uIndex];
		uchCRCLo = auchCRCLo[uIndex];
	}
	return(uchCRCHi <<8 | uchCRCLo);
}
//crc high byte in high 8 bit
unsigned short Modbus_CRC16_2(unsigned char *Buff_addr,unsigned short len)
{
	unsigned char uchCRCHi = 0xFF ; 
    unsigned char uchCRCLo = 0xFF ; 
    unsigned uIndex ; 
    while (len--) 
    {
        uIndex = uchCRCLo ^ *Buff_addr++ ; /* calculate the CRC*/
        uchCRCLo = uchCRCHi ^ auchCRCHi[uIndex] ;
        uchCRCHi = auchCRCLo[uIndex] ;
    }
    return (uchCRCHi << 8 | uchCRCLo) ;
}

由上以程序，可看到使用的方法是直驅表法,而這個生成的表卻與正向poly直接計算生成的表是不一致的。
緣由在於modbus使用的輸入字節倒轉的選項，所以生成的表是不同的
那麼這個表是怎麼生成的呢？code

poly        18005
            1 1 0 0 0 0 0 0 0 0 0 0 0 0 1 0 1 //0x8005
reserved    1 0 1 0 0 0 0 0 0 0 0 0 0 0 0 1 1 //0xa001 || 0x4003 兩種表述方法

由於MODBUS中CRC選擇了REFIN這個選項，那咱們看這個表格是如何生成it

unsigned short crcTable[256] = {0};
unsigned short poly = 0xa001;
void crcTableCreate(void)
{
    int i = 0;
    for(i = 0; i < 256; i++)
    {
        unsigned short crc = i;
        for(int j = 0; j < 8; j++)
        {
            if(crc & 0x0001)
            {
                crc = (crc >> 1) ^ poly;
            }
            else
            {
                crc = crc >> 1;
            }
        }
        crcTable[i] = crc;
    }
}

0000,c0c1,c181,0140,c301,03c0,0280,c241,c601,06c0,0780,c741,0500,c5c1,c481,0440,
cc01,0cc0,0d80,cd41,0f00,cfc1,ce81,0e40,0a00,cac1,cb81,0b40,c901,09c0,0880,c841,
d801,18c0,1980,d941,1b00,dbc1,da81,1a40,1e00,dec1,df81,1f40,dd01,1dc0,1c80,dc41,
1400,d4c1,d581,1540,d701,17c0,1680,d641,d201,12c0,1380,d341,1100,d1c1,d081,1040,
f001,30c0,3180,f141,3300,f3c1,f281,3240,3600,f6c1,f781,3740,f501,35c0,3480,f441,
3c00,fcc1,fd81,3d40,ff01,3fc0,3e80,fe41,fa01,3ac0,3b80,fb41,3900,f9c1,f881,3840,
2800,e8c1,e981,2940,eb01,2bc0,2a80,ea41,ee01,2ec0,2f80,ef41,2d00,edc1,ec81,2c40,
e401,24c0,2580,e541,2700,e7c1,e681,2640,2200,e2c1,e381,2340,e101,21c0,2080,e041,
a001,60c0,6180,a141,6300,a3c1,a281,6240,6600,a6c1,a781,6740,a501,65c0,6480,a441,
6c00,acc1,ad81,6d40,af01,6fc0,6e80,ae41,aa01,6ac0,6b80,ab41,6900,a9c1,a881,6840,
7800,b8c1,b981,7940,bb01,7bc0,7a80,ba41,be01,7ec0,7f80,bf41,7d00,bdc1,bc81,7c40,
b401,74c0,7580,b541,7700,b7c1,b681,7640,7200,b2c1,b381,7340,b101,71c0,7080,b041,
5000,90c1,9181,5140,9301,53c0,5280,9241,9601,56c0,5780,9741,5500,95c1,9481,5440,
9c01,5cc0,5d80,9d41,5f00,9fc1,9e81,5e40,5a00,9ac1,9b81,5b40,9901,59c0,5880,9841,
8801,48c0,4980,8941,4b00,8bc1,8a81,4a40,4e00,8ec1,8f81,4f40,8d01,4dc0,4c80,8c41,
4400,84c1,8581,4540,8701,47c0,4680,8641,8201,42c0,4380,8341,4100,81c1,8081,4040,

使用這種方法生成的表格與官方給的是相一致的，只是高低位是顛倒的，那這個又如何理解呢，那是由於採用低位先XOR的方式，那麼字節填充的方式就是跟正向是不一致的，反向方式至關於數據是填充在數據串的左側，因而生成CRC的後8位反而與後進來的數據高位進行XOR。
這種方式來進行直驅表法的公式與POLY是正向的也有所區別sed

unsigned short crcUpdate3(unsigned short crcIn, unsigned char data)
{
    unsigned short result = 0;
    //result = (crcIn << 8) ^ crcTable[(crcIn >> 8) ^ data]; // poly
    result = (crcIn >> 8) ^ crcTable[(crcIn & 0xff) ^ data]; // reversed poly
    return result;
}

unsigned short crcCheck3(unsigned char *pData, unsigned char size)
{
    unsigned short crcResult = 0xffff;//Initial Value
    for(int i = 0; i < size; i++)
    {
        crcResult = crcUpdate3(crcResult, *(pData+i));
    }
    return crcResult;
}

若是想要獲得三種方法獲得同樣的結果那邊直驅表法的寄存器初始值必須都是0才能夠達到三者的輸出結果一致，否則直驅表法獲得與其餘兩種方法同樣的結果，由於直驅表法原本就是直接計數方法的變種。
下面是針對MODBUS的三種結果一致的具體實現，寄存器初始值設置爲0date

unsigned short crcUpdate(unsigned short crcIn, unsigned char data)
{
    unsigned short result = 0;
    unsigned int poly = (0xa001 << 1) + 1;
    unsigned int tmp =  crcIn;
    for(int i = 0; i < 8; i++)
    {
        if(data & 0x01)
        {
            tmp += 0x10000;
        }
        data >>= 1;
        if(tmp & 0x0001)
        {
            tmp ^= poly;
        }
        tmp >>= 1;
    }
    result = tmp & 0xffff;
    return result;
}

unsigned short crcCheck(unsigned char *pData, unsigned char size)
{
    unsigned short crcResult = 0x0000;//Initial Value
    for(int i = 0; i < size; i++)
    {
        crcResult = crcUpdate(crcResult, *(pData+i));
    }
    for(int i = 0; i < 2; i++)
    {
        crcResult = crcUpdate(crcResult, 0x00);
    }
    return crcResult;
}

unsigned short crcUpdate2(unsigned short crcIn, unsigned char data)
{
    unsigned short result = 0;
    result = (crcIn >> 8 | data << 8) ^ crcTable[(crcIn & 0xff)];
    return result;
}

unsigned short crcCheck2(unsigned char *pData, unsigned char size)
{
    unsigned short crcResult = 0x0000;//Initial Value
    for(int i = 0; i < size; i++)
    {
        crcResult = crcUpdate2(crcResult, *(pData+i));
    }
    for(int i = 0; i < 2; i++)
    {
        crcResult = crcUpdate2(crcResult, 0x00);
    }
    return crcResult;
}

unsigned short crcUpdate3(unsigned short crcIn, unsigned char data)
{
    unsigned short result = 0;
    result = (crcIn >> 8) ^ crcTable[(crcIn & 0xff) ^ data];
    return result;
}

unsigned short crcCheck3(unsigned char *pData, unsigned char size)
{
    unsigned short crcResult = 0x0000;//Initial Value
    for(int i = 0; i < size; i++)
    {
        crcResult = crcUpdate3(crcResult, *(pData+i));
    }
    return crcResult;
}

將字節倒過來，先對小位進行計算，是由於一些硬件電路他是LSB先發送的，若是對小位開始計算能夠直接對硬件進入的數據直接開始CRC計算，方便硬件電路的實現。硬件

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