计算Modbus RTU CRC 16 [英] Calculating Modbus RTU CRC 16

问题描述

我正在实现一个软件，其中我通过串行在Modbus RTU protocolo中读取和写入数据。为此，我需要计算字节字符串末尾的两个CRC字节，但是我无法这样做。

I'm implementing a software where I read and write data in Modbus RTU protocolo via serial. For that, I need to calculate the two CRC byte at the end of the string of bytes, but I'm being incapable of doing this.

在整个网络中搜索，我发现两个函数似乎正确计算CRC：

Searching throughout the web, I found two functions that seems to calculate the CRC correctly:

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;
} // End: CRC16

和

uint CRC16_2(QByteArray buf, int len)
{
  uint crc = 0xFFFF;

  for (int pos = 0; pos < len; pos++)
  {
    crc ^= (uint)buf[pos];          // XOR byte into least sig. byte of crc

    for (int i = 8; i != 0; i--) {    // Loop over each bit
      if ((crc & 0x0001) != 0) {      // If the LSB is set
        crc >>= 1;                    // Shift right and XOR 0xA001
        crc ^= 0xA001;
      }
      else                            // Else LSB is not set
        crc >>= 1;                    // Just shift right
    }
  }
  // Note, this number has low and high bytes swapped, so use it accordingly (or swap bytes)
  return crc;
}

问题是我应该得到两个十六进制字节作为CRC数而此函数返回一个整数值。例如，对于01（1字节），我应该得到一个7E80，而我得到21695，我无法做某种转换从这到十六进制数据。

The problem is that I'm supposed to get two hex bytes as CRC numbers while this functions returns a integer value. For example, for "01" (1 byte), I was supposed to get a "7E80" while I get "21695", and I'm being unable to do some sort of conversion from this to that hex data.

因此，我的问题是：如何从整数结果转到需要的双精度结果？我尝试了几个选项，没有成功。

My question, therefore, is: how do I go from the integer result to the double hex result needed? I tried a couple of options, with no success.

我很高兴任何帮助，

Momergil 。

Momergil.

注意：我使用Qt，所以如果一个人能找到一个解决方案实现QByteArray或另一个Qt友好的代码，我会很高兴。任何一种方法，不使用Qt，C或C ++的解决方案是没有用的：P

Note: I'm using Qt, so if one could find a solution implementing QByteArray or another Qt friendly code, I'll be glad. Either way a solution not using Qt, C or C++ is useless :P

推荐答案

根据 MODBUS通过串行线规范和实现指南V1.02 ，CRC以小端字节（低字节优先）发送。

According to MODBUS over serial line specification and implementation guide V1.02, the CRC is sent little-endian (low byte first).

但我不知道，你是怎么想出需要任何十六进制字节的CRC。 MODBUS RTU是一个二进制协议，CRC是作为两个字节，而不是作为四个十六进制数字发送！

I have no idea, though, how you came up with needing any hexadecimal bytes for the CRC. MODBUS RTU is a binary protocol, and the CRC is sent as two bytes, not as four hexadecimal digits!

这是你怎么做，使用CRC16函数

Here's how you'd do it, using the CRC16 function you provided.

QByteArray makeRTUFrame(int slave, int function, const QByteArray & data) {
    Q_ASSERT(data.size() <= 252);
    QByteArray frame;
    QDataStream ds(&frame, QIODevice::WriteOnly);
    ds.setByteOrder(QDataStream::LittleEndian);
    ds << quint8(slave) << quint8(function);
    ds.writeRawData(data.constData(), data.size());
    int const crc = CRC16((BYTE*)frame.constData(), frame.size());
    ds << quint16(crc);
    return frame;
}

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