socket 与西门子PLC仿真通讯时报错：远程主机强迫关闭了一个现有的连接。

Question

使用socket 与西门子PLC仿真通讯时，连接plc没有问题，读取数据时报错：远程主机强迫关闭了一个现有的连接。
定位到代码 tcpClient.Receive(buffer, SocketFlags.None);

相关截图

相关代码

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Net;
using System.Net.Sockets;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using System.Windows.Forms;
using thinger.DataConvertLib;
using xktComm;

namespace FrmLogin
{
    public partial class FrmMain : Form
    {
        public FrmMain()
        {
            InitializeComponent();


            SiemensS7 s7 = new SiemensS7();




            //取消跨线程访问问题
            Control.CheckForIllegalCrossThreadCalls = false;
        }

        //声明一个socket对象
        Socket tcpClient;

        //创建取消数据源
        private CancellationTokenSource cts;//= new CancellationTokenSource();

        bool isConn;
        /// 

        /// 连接PLC
        /// 
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void Btn_Conn_Click(object sender, EventArgs e)
        {
            if (this.btn_Conn.Text == "连接")
            {
                //实例化
                tcpClient = new Socket(AddressFamily.InterNetwork,SocketType.Stream,ProtocolType.Tcp);
                cts = new CancellationTokenSource();
                EndPoint ep = new IPEndPoint(IPAddress.Parse(txtb_IPAddress.Text), int.Parse(txtb_Port.Text));

                try
                {
                    tcpClient.Connect(ep);
                    this.btn_Conn.Text = "断开";
                    this.btn_Conn.BackColor = Color.Green;
                    isConn = true;
                }
                catch (Exception ex )
                {
                    ShowMessage("连接失败:" + ex.Message);
                    return;
                }

                Task.Run(new Action(() =>
                {
                    GetPLCValue();
                }));

            }
            else
            {
                tcpClient?.Close();
                cts.Cancel();
                isConn = false;
                this.btn_Conn.Text = "连接";
                this.btn_Conn.BackColor = DefaultBackColor;
            }
                
        }

        int i = 0;

        private void GetPLCValue()
        {
            while (isConn)//(!cts.IsCancellationRequested)
            {
                Thread.Sleep(10);
                byte[] buffer = new byte[200];
                int length = -1;

                try
                {
                    length = tcpClient.Receive(buffer,SocketFlags.None);
                }
                catch (Exception ex)
                {
                  //  i += 1;
                    ShowMessage("本次数据读取失败:" + ex.Message);
                }

                if (length == 20 )
                {
                    byte[] result = thinger.DataConvertLib.ByteArrayLib.GetByteArrayFromByteArray(buffer,0,length);

                    float aa = thinger.DataConvertLib.FloatLib.GetFloatFromByteArray(result, 0, DataFormat.ABCD);
                    txtsh_data.VarValue = aa.ToString();

                }
                


            }
        }



        /// 

        /// 清空显示信息
        /// 
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void Btn_Clear_Click(object sender, EventArgs e)
        {
            this.txtb_Message.Clear();
            i = 0;
        }

        private void ShowMessage(string str)
        {
            if (!txtb_Message.Text.Contains(str))
            {
                txtb_Message.AppendText(str + "\r\n");
            }
        }

        private void Button1_Click(object sender, EventArgs e)
        {
            byte[] buffer = new byte[200];
            int length = -1;

            try
            {
                length = tcpClient.Receive(buffer, SocketFlags.None);
            }
            catch (Exception ex)
            {
                i += 1;
                ShowMessage("本次数据读取失败:" + (i).ToString() + "次:" + ex.Message);
            }

            if (length == 20)
            {
                byte[] result = thinger.DataConvertLib.ByteArrayLib.GetByteArrayFromByteArray(buffer, 0, length);

                float aa = thinger.DataConvertLib.FloatLib.GetFloatFromByteArray(result, 0, DataFormat.ABCD);

                //     ShowMessage(aa.ToString());

            }




        }
    }
}

使用网口助手和PLC进行通讯正常，使用网口助手做服务器，C#程序做客户端也能正常通讯，但是使用C#程序做客户端访问plc服务器时报错。

Answer 1

建议wireshark抓取以下两个网络包，然后分析比较有何不同
1.使用网口助手和PLC进行通讯正常时
2.使用C#程序做客户端访问plc服务器报错时