ymodem与modbus crc校验

最近接触到CRC校验，发现不同的地方Modbus的CRC校验码高低位说法不同。为了把相关问题弄清楚。本文按以下顺序来讲述：

1.CRC校验码是什么？

2.ModBus协议是怎样计算CRC校验码的？

3.是什么原因造成了CRC校验码高低位说法不同？

首先来第一个问题，CRC的全名是循环冗余校验（Cyclic Redundancy Check），基本原理是将K位信息码写成多项式的形式

，左移R位变成

，然后除以生成多项式

（Modbus协议使用的标准16位生成多项式），得到的余式是R-1位多项式，写成的R位数据即为CRC校验码。

第二个问题，ModBus协议中，CRC校验码计算方法为：

1．预置1个16位的寄存器为十六进制FFFF（即全为1）；称此寄存器为CRC寄存器；
2．把第一个8位二进制数据（既通讯信息帧的第一个字节）与16位的CRC寄存器的低8位相异
或，把结果放于CRC寄存器；
3．把CRC寄存器的内容右移一位（朝低位）用0填补最高位，并检查右移后的移出位；
4．如果移出位为0：重复第3步（再次右移一位）； 如果移出位为1：CRC寄存器与多项式A001
（1010 0000 0000 0001）进行异或；
5．重复步骤3和4，直到右移8次，这样整个8位数据全部进行了处理；
6．重复步骤2到步骤5，进行通讯信息帧下一个字节的处理；
7．将该通讯信息帧所有字节按上述步骤计算完成后，得到的16位CRC寄存器的高、低字节进
行交换；
8．最后得到的CRC寄存器内容即为：CRC码。

第三个问题，

网上有段程序

U16 crc16(U8 *ptr, U8 len)
 {
   U16 crc=0xFFFF;
   U8 i;
   while(len--)
   {
     crc ^=*ptr++;
     for(i=0;i<8;i++)
     {
       if(crc & 0x01)
       {
         crc>>=1;
         crc^=0xA001;
       }
       else
       {
         crc>>=1;
       }
     }
   }
   return crc;
 }这段程序完成了第二个问题中的1-6步，但是没有做第7步的交换工作，这就是造成高低位说法不同的原因所在。

再把CRC查表程序拿来作下比较
// -----------------------------------------------------------------------------
 // DESCRIPTION: RTU CRC 校验的高位字节表
 // -----------------------------------------------------------------------------
 static const U8 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};
 // -----------------------------------------------------------------------------
 // DESCRIPTION: RTU CRC 校验的低位字节表
 // -----------------------------------------------------------------------------
 static const U8 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};


 //校验函数：
 U16 RTU_CRC( U8 * puchMsg,U16 usDataLen )
 {
   U8 uchCRCHi; // high byte of CRC initialized
   U8 uchCRCLo; // low byte of CRC initialized
   U32 uIndex; // will index into CRC lookup table
   uchCRCHi = 0xFF;
   uchCRCLo = 0xFF;
   while ( usDataLen-- )
   {
 // calculate the CRC
     uIndex = uchCRCHi ^ (U8)( *puchMsg++ );
     uchCRCHi = uchCRCLo ^ auchCRCHi[uIndex];
     uchCRCLo = auchCRCLo[uIndex];
   }
   return ( uchCRCHi << 8 | uchCRCLo );
 }


 U16 usMBCRC16( U8 * pucFrame, U16 usLen )
 {
     U8 ucCRCHi = 0xFF;
     U8 ucCRCLo = 0xFF;
     int iIndex;
     while( usLen-- )
     {
         iIndex = ucCRCLo ^ *( pucFrame++ );
         ucCRCLo = ( U8 )( ucCRCHi ^ auchCRCHi[iIndex] );
         ucCRCHi = auchCRCLo[iIndex];
     }
     return ( U16 )( ucCRCHi << 8 | ucCRCLo );
 }

RTU_CRC函数结果是CRC校验码，usMBCRC16函数算的是CRC校验码的高低位交换后的值

usMBCRC16函数与crc16函数的结果是一致的