主要器件：

1、  PC机端的232/485转换接口：MC1488和MC1489实现TTL电平和RS-232通信电平的转化；PC147光电隔离器件；MAX481485驱动收发芯片。

2、  单片机端：AT89C52单片机芯片，用于数据采集和与485总线接口；MAX481485驱动收发芯片；DIP-6开关用于确定本机的设备号。

试验流程图：

主机端流程：

单片机端流程图：

试验电路图：

主机端

单片机端:

试验程序代码：

// 485Mon.h 程序

#ifndef    _485MON_H                             // 防止485Mon.h被重复引用

#define    _485MON_H

#include                    // 引用标准库的头文件

#include

#include

#define uchar unsigned char

#define uint unsigned int

#define ACTIVE  0x11

#define GETDATA       0x22

#define READY           0x33

#define SENDDATA      0x44      

#define RECFRMMAXLEN 16            // 接收帧的最大长度，超过此值认为帧超长错误

#define STATUSMAXLEN 10                     // 设备状态信息最大长度    

uchar DevNo;                             // 设备号

xdata uchar StatusBuf[STATUSMAXLEN];

//为简化起见，假设了10位固定的采集数据

#define DATA0     0x10

#define DATA1     0x20

#define DATA2     0x30

#define DATA3     0x40

#define DATA4     0x50

#define DATA5     0x60

#define DATA6     0x70

#define DATA7     0x80

#define DATA8     0x90

#define DATA9     0xA0

sbit DE = P1^6;                    //驱动器使能，1有效

sbit RE = P1^7;                    //接收器使能，0有效

void init();                                  // 系统初始化

void Get_Stat();                          // 简化的数据采集函数

bit Recv_Data(uchar *type);         // 接收数据帧函数

void Send(uchar m);                                  // 发送单字节数据

void Send_Data(uchar type,uchar len,uchar *buf);                           // 发送数据帧函数      

void Clr_StatusBuf();                   //  清除设备状态信息缓冲区函数  

#endif

// 485Mon.c程序

#include "485Mon.h"

void main(void)

{

       uchar type;

       /* 初始化 */

       init();

       while (1)

       {

              if (Recv_Data(&type)==0)                  // 接收帧错误或者地址不符合，丢弃

                     continue;

              switch (type)

              {

                     case ACTIVE:                      // 主机询问从机是否在位

                            Send_Data(READY,0,StatusBuf);  // 发送READY指令

                            break;

                     case GETDATA:                          // 主机读设备请求

                            Clr_StatusBuf();

                            Get_Stat();                          // 数据采集函数

                            Send_Data(SENDDATA,strlen(StatusBuf),StatusBuf);

                            break;

                     default:

                            break;                                 // 指令类型错误，丢弃当前帧

              }

       }

}

/* 初始化 */

void init(void)

{

       P1 = 0xff;

       DevNo = (P1&0x00111111);                // 读取本机设备号

       TMOD = 0x20;

       SCON = 0x50;

       TH1 = 0xfd;

       TL1 = 0xfd;

       TR1 = 1;

       PCON = 0x00;                                          // SMOD=0

       EA = 0;

}

/* 接收数据帧函数，实际上接收的是主机的指令 */

bit Recv_Data(uchar *type)

{

       uchar tmp,rCount,i;

       uchar r_buf[RECFRMMAXLEN];               // 保存接收到的帧

       uchar Flag_RecvOver;                        // 一帧接收结束标志  

       uchar Flag_StartRec;                          // 一帧开始接收标志

       uchar CheckSum;                                      // 校验和

       uchar DataLen;                                         // 数据字节长度变量

       /* 禁止发送，允许接收 */

       DE = 0;

       RE = 0;

       /* 接收一帧数据 */

       rCount = 0;

       Flag_StartRec = 0;

    Flag_RecvOver = 0;

       while (!Flag_RecvOver)

       {

              RI = 0;

              while (!RI);

              tmp = SBUF;

              RI=0;

              /* 判断是否收到字符'$',其数值为0x24 */         

              if ((!Flag_StartRec) && (tmp == 0x24))

              {

                     Flag_StartRec = 1;

              }

              if (Flag_StartRec)

              {

                     r_buf[rCount] = tmp;

                     rCount ++;           

                     /* 判断是否收到字符'*'，其数值为0x2A，根据接收的指令设置相应标志位 */

                     if (tmp == 0x2A)

                            Flag_RecvOver = 1;

              }

              if (rCount == RECFRMMAXLEN)              // 帧超长错误，返回0

                     return 0;

       }
 

接上篇程序:

/* 计算校验和字节 */

       CheckSum = 0;

       DataLen = r_buf[3];

       for (i=0;i++;i<3+DataLen)

       {

              CheckSum = CheckSum + r_buf[i+1];

       }

       /* 判断帧是否错误 */

       if (rCount<6)                                     // 帧过短错误，返回0，最短的指令帧为6个字节                                   

              return 0;

       if (r_buf[1]!=DevNo)                        // 地址不符合，错误，返回0

              return 0;

       if (r_buf[rCount-2]!=CheckSum)         // 校验错误，返回0

           return 0;

       *type = r_buf[2];                               // 获取指令类型

       return 1;                                            // 成功，返回1

}

/* 发送数据帧函数 */

void Send_Data(uchar type,uchar len,uchar *buf)

{

       uchar i,tmp;

       uchar CheckSum = 0;

       /* 允许发送，禁止接收 */

       DE = 1;

       RE = 1;

       /* 发送帧起始字节 */

       tmp = 0x24;

       Send(tmp);

       Send(DevNo);                                    // 发送地址字节，也即设备号

       CheckSum = CheckSum + DevNo;

       Send(type);                                               // 发送类型字节

       CheckSum = CheckSum + type;

       Send(len);                                                 // 发送数据长度字节

       CheckSum = CheckSum + len;

       /* 发送数据 */

       for (i=0;i

       {

              Send(*buf);

              CheckSum = CheckSum + *buf;

              buf++;

       }

       Send(CheckSum);                                     // 发送校验和字节

       /* 发送帧结束字节 */

       tmp = 0x2A;

       Send(tmp);

}

/* 采集数据函数经过简化处理，取固定的10个字节数据 */

void Get_Stat(void)

{

       StatusBuf[0]=DATA0;

       StatusBuf[1]=DATA1;

       StatusBuf[2]=DATA2;

       StatusBuf[3]=DATA3;

       StatusBuf[4]=DATA4;

       StatusBuf[5]=DATA5;

       StatusBuf[6]=DATA6;

       StatusBuf[7]=DATA7;

       StatusBuf[8]=DATA8;

       StatusBuf[9]=DATA9;

}

/* 发送单字节数据 */

void Send(uchar m)

{

       TI = 0;

       SBUF = m;

    while(!TI);

       TI = 0;   

}

/* 清除设备状态信息缓冲区函数*/

void Clr_StatusBuf(void)

{

       uchar i;

       for (i=0;i

              StatusBuf[i] = 0;   

}