看到这个标题，我的反应也是，Modbus 是什么，RTU 是什么，CRC16 又是什么？好像只认识一个 php，有点丢脸。但认识新的事物就是这样的一个过程，短时间内一堆东西抛到你脸上，之后再慢慢的了解相关的概念。

首先了解一下 Modbus、RTU、CRC16 等概念：【Modbus】 RTU CRC校验码计算方法。简单来讲，Modbus 是一种通讯协议，或者标准，Modbus RTU 是 Modbus 的三种实现方式之一。

Modbus RTU 采用 16 位的循环冗余校验码(CRC)。通过一个对数据进行“或”运算以及移位运算的复杂程序，由主设备产生 CRC，并且由接收设备进行检查。如果双方计算出的 CRC 值不符，从设备就会要求重新传送信息。

【Modbus】 RTU CRC校验码计算方法 内还带了一个 Java 版的 Modbus RTU CRC16 校验码实现方法。

CRC16 校验码计算方法是对接一个新的 tcp 协议报文里提到的，里面有一个 c++ 的实现：

WORD ModbusCRC(BYTE * pData, BYTE len)
{
BYTE byCRCHi = 0xff;
BYTE byCRCLo = 0xff;
BYTE byIdx;
WORD crc;
while(len--)
{
byIdx = byCRCHi ^* pData++;
byCRCHi = byCRCLo ^ gabyCRCHi[byIdx];
byCRCLo = gabyCRCLo[byIdx];
}
crc = byCRCHi;
crc <<= 8;
crc += byCRCLo;
return crc;
}

# CRC 码表高字节
BYTE gabyCRCHi[] =
{
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
};

# CRC 码表高字节
BYTE gabyCRCLo[] =
{
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
};

根据这个 C 语言版本，改出了 PHP 的版本：

function modBusCRC($pData, $len = 0)
{
    $len = ($len <= 0 ? strlen($pData) : $len);
    $byCRCHi = 0xff;
    $byCRCLo = 0xff;
    $byIdx = 0;
    for ($i = 0; $i < $len; $i++) {
        $byIdx = $byCRCHi ^ ord(substr($pData, $i, 1));
        $byCRCHi = $byCRCLo ^ self::$gabyCRCHi[$byIdx];
        $byCRCLo = self::$gabyCRCLo[$byIdx];
    }

    $crc = $byCRCHi;
    $crc <<= 8;
    $crc += $byCRCLo;
    return $crc;
}

输入的 $pData 是字符，而非 16 进制表示字符串，需要调整；输出是一个整数，而我想要的结果的是 4 位的校验码（16 进制表示字符串）。

网上搜索 php modbus crc16 检验码，找到一个 PHP实现 Modbus RTU CRC16 校验，主体的算法、高字节、地字节数组都基本一致，除了输出和调用时的 pack 与 unpack。需要注意的是，他的算法输出时是低字节在前，高字节在后，而我根据 c++ 原函数拿到的是高字节在前，低字节在后。

修改后的版本：

function modBusCRC($pData, $len = 0)
{
    $len = ($len <= 0 ? strlen($pData) : $len);
    $byCRCHi = 0xff;
    $byCRCLo = 0xff;
    $byIdx = 0;
    for ($i = 0; $i < $len; $i++) {
        $byIdx = $byCRCHi ^ ord(substr($pData, $i, 1));
        $byCRCHi = $byCRCLo ^ self::$gabyCRCHi[$byIdx];
        $byCRCLo = self::$gabyCRCLo[$byIdx];
    }
    
    // echo $byCRCHi . '|' . $byCRCLo . PHP_EOL;
    return (chr($byCRCHi) . chr($byCRCLo));
}

尝试直接输出返回值，结果是一个字符串，但并不是我想要的 16 进制表示字符串，需要进行转化：

$s = pack('H*', $data);
$t = Utils::modBusCRC($s);
$crc = strtoupper(unpack("H*",$t)[1]);
// echo $crc . PHP_EOL; // C40B
return $crc;

pack 把数据装入一个二进制字符串，H* 表示数据格式，多个 16 进制（表示）字符串，高位在前；unpack 是逆过程，将二进制字符串解包成格式化数据，unpack 会返回数组，且下标从 1 开始。

自此，PHP 版本的 Modbus CRC16 检验码算法就完成了。