function modbus_crc($modbus_msg){
        $crctab16 = array(0X0000 .... тут таблица);

        $hexdata = pack('H*',$modbus_msg);
        $nLength = strlen($hexdata);
        $fcs = 0xFFFF;
        $pos = 0;
        while($nLength > 0)
        {
            $fcs = ($fcs >> 8) ^ $crctab16[($fcs ^ ord($hexdata[$pos])) & 0xFF];
            $nLength--;
            $pos++;
        }
        $inverted = sprintf('%04X', $fcs);
        $crc_modbus = substr($inverted,2,2)." ".substr($inverted,0,2);
        return $crc_modbus;
    }