485引脚悬空，却接收到一串乱码。

2018-08-04 03:23:25 1k 阅读 AI 写同款 · GEO 优化AI 选题数据监测一键分发 ›

51单片机RS-485通信全解析：从硬件电路到软件协议与抗干扰设计串行通信是嵌入式系统与外部设备进行数据交换的基础技术，其中RS-485标准因其差分传输原理，在抗共模干扰和远距离传输方面展现出显著优势，成为工业自动化、楼宇控制等场景中多点通信的主流选择。其技术价值在于通过一对双绞线实现长距离、多节点的可靠数据链路，核心在于硬件上的电平转换与软件上的半双工协调机制。在实际工程应用中，围绕单片机（如经典的51系列）构建RS-485网络，需要综合考量收发器电路设计、总线终端匹配、软件状态机、通信协议（如自定义帧结构或Modbus RTU）以及环境抗干扰措施，以确保在复杂工业现场阅读详情

cpu芯片是ar9344，485芯片是sp485re，测试485模块读写，A发送一串数据指令，B接收到指令后，给A反馈一串数据指令，但是在485 引脚悬空的情况下却能收到一串乱码数据。485读写程序如下：
ucHostAddress=(unsigned char)atol(argv[1]);
ucSlaveAddress=(unsigned char)atol(argv[2]);
ucSensorAddress=(unsigned char)atol(argv[3]);
ucRegAddress=(unsigned char)atol(argv[4]);
uiRegNum=(unsigned int)atol(argv[5]);
uiTTLtime=(unsigned int)atol(argv[6]);
if((ucSensorAddress >= 0)&&(ucSensorAddress <= 180))
{

/*generate command package*/
cpCommandPackage[0] = (char)0xA5;
cpCommandPackage[1] = (char)ucHostAddress; //HostAddress
cpCommandPackage[2] = (char)uilength; //data length
cpCommandPackage[3] = (char)ucSlaveAddress; //SlaveAddress
cpCommandPackage[4] = (char)((uiTTLtime >> 24) & 0x00FF); //TTLtime
cpCommandPackage[5] = (char)((uiTTLtime >> 16) & 0x00FF);
cpCommandPackage[6] = (char)((uiTTLtime >> 8) & 0x00FF);
cpCommandPackage[7] = (char)(uiTTLtime & 0x00FF);
cpCommandPackage[8] = (char)ucSensorAddress; //SensorAddress
cpCommandPackage[9] = (char)0x03; //function code
cpCommandPackage[10] = (char)((ucRegAddress >> 8) & 0x00FF); //reg address high byte
cpCommandPackage[11] = (char)(ucRegAddress & 0x00FF); //reg address low byte
cpCommandPackage[12] = (char)((uiRegNum >> 8) & 0x00FF); //read reg number high byte
cpCommandPackage[13] = (char)(uiRegNum & 0x00FF); //read reg number low byte

/*get CRC16*/
wCRC16 = CRC16_2(&cpCommandPackage[8], 6);
cpCommandPackage[14] = (char)(wCRC16 & 0x00FF); //crc16 code low byte
cpCommandPackage[15] = (char)((wCRC16 >> 8) & 0x00FF); //crc16 code high byte
xor=xorCheck((char*)cpCommandPackage, 16);
cpCommandPackage[16] = (char) xor; //xor code

iFile = open("/dev/ttyATH0", O_RDWR | O_NOCTTY | O_NONBLOCK, 0);
if(-1 == iFile )
{
printf("open 485 port err...\n");
return -1;
}
else
{
if((ret=setport(iFile,4800,8,1,'N')) < 0)
{
printf("set port faild!\n");
return -1;
}
len = write(iFile, cpCommandPackage, uilength);
printf("\n** Getsensor -->Tx_commandpackage -- length:%d , sensoraddr:%d , Regaddr:%d.\n", len,(int)ucSensorAddress,(int)ucRegAddress);
printf("[");
for(i = 0;i< len ;i++)
printf("%02x,",cpCommandPackage[i]);
printf("]\n");
printf("\r\n");
if((cpCommandPackage[4] == 0)&&(cpCommandPackage[5] == 0)&&(cpCommandPackage[6] == 0)&&(cpCommandPackage[7] == 0))
{
sleep(2);
}
else
{
usleep(100000);
}
ret = read(iFile, cpAnswerPackage,15+uiRegNum*2);
// cpAnswerPackage[ret] = '\0';
close(iFile);
printf("\n** Getsensor -->Rx_ackpackage --length:%d , Sensoraddr:%d , Regaddr:%d.\n",ret,(int)ucSensorAddress,(int)ucRegAddress);
printf("[");
for(i = 0;i < ret;i++)
printf("%02x,",cpAnswerPackage[i]);
printf("]");
printf("\r\n");