一、485硬件原理

差分对传输数据的原理
- IO数据的传输→差分对
rs232传输的距离在15米以下，RS485传输距离是几十米到1000米以上
为什么485可以传输这么远
- 差分对的机制可以降低电磁场的干扰
- 衰减
485传输距离和传输线有关系
- 注意：双绞线和屏蔽线

二、485原理图

嵌入式上一般使用串口转485

分析芯片datasheet

串口的信号转化为485则：

D→（A，B），DE高电平，RE高电平

485信号转化为串口信号则：

（A，B），DE低电平，RD低电平

三、驱动

485驱动=串口驱动+GPIO的字符驱动

BUF_XURTS1高电平发送，低电平接收

串口驱动是drivers/char/max485_ctl.c

485驱动=串口驱动+GPIO的字符驱动
BUF_XURTS1高电平发送，低电平接收
（GPIO是GPA0_7，串口设备节点是ttySAC1）

驱动中只需要操作GPIO

设备节点/dev/max485_ctl

应用中，控制GPIO和串口=类似led+串口的操作

ioctl是参数是1，则输出高电平，发送
ioctl是参数是0，则输出低电平，接收
串口的节点/dev/ttySAC1

运行程序

发送./test_485 /dev/ttySAC1 1
接收./test_485 /dev/ttySAC1 0

测试程序发送的信息：iTOP-4412: max485 test app(times:%d)

测试程序：

//#include <stdio.h>

#include <unistd.h>

//#include "uart.h"

#include <stdio.h>

#include <stdlib.h>

#include <string.h>

#include <sys/types.h>

#include <sys/stat.h>

#include <fcntl.h>

#include <unistd.h>

#include <termios.h>

#include <string.h>

#define MAX485_CONTROL

//#include "uart.c"

int set_opt(int fd,int nSpeed, int nBits, char nEvent, int nStop)

{

    struct termios newtio,oldtio;

    if  ( tcgetattr( fd,&oldtio)  !=  ) {

        perror("SetupSerial 1");

        return -;

    }

    bzero( &newtio, sizeof( newtio ) );

    newtio.c_cflag  |=  CLOCAL | CREAD;

    newtio.c_cflag &= ~CSIZE;

    switch( nBits )

    {

    case :

        newtio.c_cflag |= CS7;

        break;

    case :

        newtio.c_cflag |= CS8;

        break;

    }

    switch( nEvent )

    {

    case 'O':

        newtio.c_cflag |= PARENB;

        newtio.c_cflag |= PARODD;

        newtio.c_iflag |= (INPCK | ISTRIP);

        break;

    case 'E':

        newtio.c_iflag |= (INPCK | ISTRIP);

        newtio.c_cflag |= PARENB;

        newtio.c_cflag &= ~PARODD;

        break;

    case 'N':

        newtio.c_cflag &= ~PARENB;

        break;

    }

    printf("Baund Rate: %d\n", nSpeed);

    switch( nSpeed )

    {

    case :

        cfsetispeed(&newtio, B2400);

        cfsetospeed(&newtio, B2400);

        break;

    case :

        cfsetispeed(&newtio, B4800);

        cfsetospeed(&newtio, B4800);

        break;

    case :

        cfsetispeed(&newtio, B9600);

        cfsetospeed(&newtio, B9600);

        break;

    case :

        cfsetispeed(&newtio, B115200);

        cfsetospeed(&newtio, B115200);

        break;

    case :

        cfsetispeed(&newtio, B460800);

        cfsetospeed(&newtio, B460800);

        break;

    case :

        printf("Rate:921600\n");

        cfsetispeed(&newtio, B921600);

                cfsetospeed(&newtio, B921600);

                break;

    default:

        cfsetispeed(&newtio, B9600);

        cfsetospeed(&newtio, B9600);

        break;

    }

    if( nStop ==  )

        newtio.c_cflag &=  ~CSTOPB;

    else if ( nStop ==  )

    newtio.c_cflag |=  CSTOPB;

    newtio.c_cc[VTIME]  = ;

    newtio.c_cc[VMIN] = ;

    tcflush(fd,TCIFLUSH);

    if((tcsetattr(fd,TCSANOW,&newtio))!=)

    {

        perror("com set error");

        return -;

    }

//    printf("set done!\n\r");

    return ;

}

int prepare_to_send(int fd)

{

    int ret;

    ret = ioctl(fd, , );

    if(ret<)

    {

        printf("max485 set ctl to high failed!\r\n");

        return -;

    }

    else

    {

        return ;

    }

}

int prepare_to_recv(int fd)

{

    int ret;

    ret = ioctl(fd, , );

    if(ret<)

    {

        printf("max485 set ctl to low failed!\r\n");

        return -;

    }

    else

    {

        return ;

    }

}

void main(int argc, char* argv[])

{

    unsigned char ucTmp;

    int fd1,fd2,nset1,nset2,nread;

    char buf[];

    //char buf1[1];

    //char *buff = "Hello\n\r";

    int i = ;

    char *max485_ctl = "/dev/max485_ctl_pin";

    if( != argc)

        {

        printf("Usage:    test_485 [uart port] [type]\r\n");

        printf("        type: 0--recv, 1--send\r\n");

        return;

    }

    fd1 = open(argv[], O_RDWR);

    if (fd1 == -)

    {

        printf("Open %s faild\n", argv[]);

        exit();

    }

    nset1 = set_opt(fd1, , , 'N', );

    if (nset2 == -)

    {

        printf("Set uart faild\n");

        exit();

    }

#ifdef MAX485_CONTROL

    if((fd2=open(max485_ctl, O_RDWR|O_NOCTTY|O_NDELAY))<)

    {

        printf("Open %s faild\n", max485_ctl);

        close(fd1);

        exit();

    }

#endif

    if( == atoi(argv[]))    //recv

    {

#ifdef MAX485_CONTROL

        prepare_to_recv(fd2);

#endif

        while()

        {    

            nread = read(fd1, buf, );

            if (nread > )

            {

                for(i=; i<nread; i++)

                {

                    printf("%c", buf[i]);

                    if(buf[i] == 'q')

                        //break;

                        goto exit;

                }

            }

            //if(nread)

            //{

            //    printf("\r\n");

            //}

            sleep();

        }

    }

    else    //send

    {

#ifdef MAX485_CONTROL

        prepare_to_send(fd2);

#endif

        while()

        {

            printf("Send data, time:%d\r\n", i);

            sprintf(buf, "iTOP-4412: max485 test app(times:%d)\r\n", i++);

            //nread = write(fd1, "iTOP-4412: max485 test app\r\n", strlen("iTOP-4412: max485 test app\r\n"));

            nread = write(fd1, buf, strlen(buf));

            sleep();

#if 0

            nread = read(fd1, buf, );

                        if (nread > )

                        {

                                for(i=; i<nread; i++)

                                {

                                        printf("%c", buf[i]);

                                        if(buf[i] == 'q')

                                                //break;

                                                goto exit;

                                }

                        }

                        if(nread)

                        {

                                printf("\r\n");

                        }

#endif

        }

    }

exit:

    close(fd1);

        return;

}

测试程序

巴特西

4412 RS485

一、485硬件原理

二、485原理图

三、驱动

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