使用COM口的2、3针的通断作为中端源（有一个读图像的摄像头，当把卡插到位时触发中端，防止在插卡的过程中出现不稳定的图像）

利用串口2读，串口3发数据的特点。建立不断的发送流，再从接收端接收。如果收到，则数据畅通，否则断开。相当于产生一个中断。这样电脑对外部事件可作出反应。

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;

namespace WindowsApplication1
{
public partial class Form1 : Form
{
int i = 0;
bool myflag = true;

public Form1()
{
InitializeComponent();

}
//开始
private void button2_Click(object sender, EventArgs e)
{
myflag = true;
this.serialPort1.Open();

System.Threading.Thread t = new System.Threading.Thread(send_data);
t.Start();

this.button2.Enabled = false;
this.button3.Enabled = true;

}

//不断的发送数据
private void send_data(object sender)
{
while (myflag)
{
this.serialPort1.WriteLine(i.ToString());
System.Threading.Thread.Sleep(300);
i++;
}
}

private void serialPort1_DataReceived(object sender, System.IO.Ports.SerialDataReceivedEventArgs e)
{
this.listBox1.Items.Insert(0,this.serialPort1.ReadLine()+" @ "+System.DateTime.Now.ToString());
}

//停止
private void button3_Click(object sender, EventArgs e)
{
myflag = false;
System.Threading.Thread.Sleep(800);

this.serialPort1.Close();

this.button2.Enabled = true;
this.button3.Enabled = false;
}
}
}