diff --git a/Modbus.Net/Modbus.Net.Siemens/SiemensPpiProtocol.cs b/Modbus.Net/Modbus.Net.Siemens/SiemensPpiProtocol.cs
index 52d45b3..307ac8e 100644
--- a/Modbus.Net/Modbus.Net.Siemens/SiemensPpiProtocol.cs
+++ b/Modbus.Net/Modbus.Net.Siemens/SiemensPpiProtocol.cs
@@ -18,6 +18,7 @@ namespace Modbus.Net.Siemens
         public SiemensPpiProtocol(byte slaveAddress, byte masterAddress)
             : this(ConfigurationManager.AppSettings["COM"], slaveAddress, masterAddress)
         {
+            ProtocolLinker = new SiemensPpiProtocolLinker(_com, SlaveAddress);
         }
 
         /// <summary>
@@ -71,7 +72,8 @@ namespace Modbus.Net.Siemens
         /// <returns>是否连接成功</returns>
         public override async Task<bool> ConnectAsync()
         {
-            ProtocolLinker = new SiemensPpiProtocolLinker(_com, SlaveAddress);
+            if (ProtocolLinker.IsConnected) return true;
+            if (!await ProtocolLinker.ConnectAsync()) return false;
             var inputStruct = new ComCreateReferenceSiemensInputStruct(SlaveAddress, MasterAddress);
             var outputStruct =
                 (await (await
diff --git a/Modbus.Net/Modbus.Net.Siemens/SiemensTcpProtocol.cs b/Modbus.Net/Modbus.Net.Siemens/SiemensTcpProtocol.cs
index 3e8ce00..15fd6ac 100644
--- a/Modbus.Net/Modbus.Net.Siemens/SiemensTcpProtocol.cs
+++ b/Modbus.Net/Modbus.Net.Siemens/SiemensTcpProtocol.cs
@@ -74,6 +74,7 @@ namespace Modbus.Net.Siemens
             _ip = ip;
             _port = port;
             _connectTryCount = 0;
+            ProtocolLinker = new SiemensTcpProtocolLinker(_ip, _port);
         }
 
         /// <summary>
@@ -143,7 +144,7 @@ namespace Modbus.Net.Siemens
         public override async Task<bool> ConnectAsync()
         {
             _connectTryCount++;
-            ProtocolLinker = new SiemensTcpProtocolLinker(_ip, _port);
+            if (ProtocolLinker.IsConnected) return true;
             if (!await ProtocolLinker.ConnectAsync()) return false;
             _connectTryCount = 0;
             var inputStruct = new CreateReferenceSiemensInputStruct(_tdpuSize, _taspSrc, _tsapDst);
diff --git a/Modbus.Net/Modbus.Net/BaseController.cs b/Modbus.Net/Modbus.Net/BaseController.cs
index 6e4f2a0..2358b71 100644
--- a/Modbus.Net/Modbus.Net/BaseController.cs
+++ b/Modbus.Net/Modbus.Net/BaseController.cs
@@ -55,7 +55,7 @@ namespace Modbus.Net
         public abstract void SendStop();
 
         /// <inheritdoc />
-        public void SendStart()
+        public virtual void SendStart()
         {
             if (SendingThread == null)
             {
@@ -80,7 +80,7 @@ namespace Modbus.Net
         {
             lock (WaitingMessages)
             {
-                if (WaitingMessages.FirstOrDefault(p => p.Key == def.Key) == null)
+                if (WaitingMessages.FirstOrDefault(p => p.Key == def.Key) == null || def.Key == null)
                 {
                     WaitingMessages.Add(def);
                     return true;
diff --git a/Modbus.Net/Modbus.Net/ComConnector.cs b/Modbus.Net/Modbus.Net/ComConnector.cs
index 0da1861..5492a4c 100644
--- a/Modbus.Net/Modbus.Net/ComConnector.cs
+++ b/Modbus.Net/Modbus.Net/ComConnector.cs
@@ -349,13 +349,14 @@ namespace Modbus.Net
                     {
                         lock (SerialPort)
                         {
+                            _taskCancel = false;
                             SerialPort.Open();
                             ReceiveMsgThreadStart();
+                            Controller.SendStart();
                         }
                     }
                 }
-                Controller.SendStart();
-
+                
                 Log.Information("Com client {ConnectionToken} connect success", ConnectionToken);
 
                 return true;
diff --git a/Modbus.Net/Modbus.Net/ComProtocalLinker.cs b/Modbus.Net/Modbus.Net/ComProtocalLinker.cs
index 2c266c3..a845e37 100644
--- a/Modbus.Net/Modbus.Net/ComProtocalLinker.cs
+++ b/Modbus.Net/Modbus.Net/ComProtocalLinker.cs
@@ -34,7 +34,7 @@ namespace Modbus.Net
             int slaveAddress)
             : this(
                 com, baudRate, parity, stopBits, dataBits,
-                int.Parse(ConfigurationManager.AppSettings["ComConnectionTimeout"] ?? "3000"), slaveAddress)
+                int.Parse(ConfigurationManager.AppSettings["ComConnectionTimeout"] ?? "-1"), slaveAddress)
         {
         }
 
@@ -51,8 +51,16 @@ namespace Modbus.Net
         protected ComProtocolLinker(string com, int baudRate, Parity parity, StopBits stopBits, int dataBits,
             int connectionTimeout, int slaveAddress)
         {
-            BaseConnector = new ComConnector(com + ":" + slaveAddress, baudRate, parity, stopBits, dataBits,
-                connectionTimeout);
+            if (connectionTimeout == -1)
+            {
+                BaseConnector = new ComConnector(com + ":" + slaveAddress, baudRate, parity, stopBits, dataBits);
+            }
+            else
+            {
+                BaseConnector = new ComConnector(com + ":" + slaveAddress, baudRate, parity, stopBits, dataBits,
+                    connectionTimeout);
+            }
+            
         }
     }
 }
\ No newline at end of file
diff --git a/Modbus.Net/Modbus.Net/FifoController.cs b/Modbus.Net/Modbus.Net/FifoController.cs
index 0b8267b..58134fb 100644
--- a/Modbus.Net/Modbus.Net/FifoController.cs
+++ b/Modbus.Net/Modbus.Net/FifoController.cs
@@ -44,6 +44,14 @@ namespace Modbus.Net
                     }
                     lock (WaitingMessages)
                     {
+                        if (_currentSendingPos == null)
+                        {
+                            if (WaitingMessages.Count > 0)
+                            {
+                                _currentSendingPos = WaitingMessages.First();
+                                _currentSendingPos.SendMutex.Set();
+                            }
+                        }
                         if (_currentSendingPos != null)
                         {
                             if (WaitingMessages.Count <= 0)
@@ -70,6 +78,13 @@ namespace Modbus.Net
 
         }
 
+        /// <inheritdoc />
+        public override void SendStart()
+        {
+            _taskCancel = false;
+            base.SendStart();
+        }
+
         /// <inheritdoc />
         public override void SendStop()
         {
diff --git a/Modbus.Net/Modbus.Net/MatchController.cs b/Modbus.Net/Modbus.Net/MatchController.cs
index a4f00f5..252f811 100644
--- a/Modbus.Net/Modbus.Net/MatchController.cs
+++ b/Modbus.Net/Modbus.Net/MatchController.cs
@@ -85,6 +85,13 @@ namespace Modbus.Net
 
         }
 
+        /// <inheritdoc />
+        public override void SendStart()
+        {
+            _taskCancel = false;
+            base.SendStart();
+        }
+
         /// <inheritdoc />
         public override void SendStop()
         {
diff --git a/Modbus.Net/Modbus.Net/TcpConnector.cs b/Modbus.Net/Modbus.Net/TcpConnector.cs
index bdcbe9b..174968a 100644
--- a/Modbus.Net/Modbus.Net/TcpConnector.cs
+++ b/Modbus.Net/Modbus.Net/TcpConnector.cs
@@ -139,40 +139,46 @@ namespace Modbus.Net
         /// <inheritdoc />
         public override async Task<bool> ConnectAsync()
         {
-            if (_socketClient != null)
-                Disconnect();
-            try
+            using (await Lock.LockAsync())
             {
-                _socketClient = new TcpClient
+                if (_socketClient != null)
                 {
-                    SendTimeout = TimeoutTime,
-                    ReceiveTimeout = TimeoutTime
-                };
-
-                try
-                {
-                    var cts = new CancellationTokenSource();
-                    cts.CancelAfter(TimeoutTime);
-                    await _socketClient.ConnectAsync(_host, _port).WithCancellation(cts.Token);
-                }
-                catch (Exception e)
-                {
-                    Log.Error(e, "Tcp client {ConnectionToken} connect error", ConnectionToken);
-                }
-                if (_socketClient.Connected)
-                {
-                    Controller.SendStart();
-                    ReceiveMsgThreadStart();
-                    Log.Information("Tcp client {ConnectionToken} connected", ConnectionToken);                   
                     return true;
                 }
-                Log.Error("Tcp client {ConnectionToken} connect failed.", ConnectionToken);
-                return false;
-            }
-            catch (Exception err)
-            {
-                Log.Error(err, "Tcp client {ConnectionToken} connect exception", ConnectionToken);
-                return false;
+                try
+                {
+                    _socketClient = new TcpClient
+                    {
+                        SendTimeout = TimeoutTime,
+                        ReceiveTimeout = TimeoutTime
+                    };
+
+                    try
+                    {
+                        var cts = new CancellationTokenSource();
+                        cts.CancelAfter(TimeoutTime);
+                        await _socketClient.ConnectAsync(_host, _port).WithCancellation(cts.Token);
+                    }
+                    catch (Exception e)
+                    {
+                        Log.Error(e, "Tcp client {ConnectionToken} connect error", ConnectionToken);
+                    }
+                    if (_socketClient.Connected)
+                    {
+                        _taskCancel = false;
+                        Controller.SendStart();
+                        ReceiveMsgThreadStart();
+                        Log.Information("Tcp client {ConnectionToken} connected", ConnectionToken);
+                        return true;
+                    }
+                    Log.Error("Tcp client {ConnectionToken} connect failed.", ConnectionToken);
+                    return false;
+                }
+                catch (Exception err)
+                {
+                    Log.Error(err, "Tcp client {ConnectionToken} connect exception", ConnectionToken);
+                    return false;
+                }
             }
         }
 
@@ -185,7 +191,6 @@ namespace Modbus.Net
             try
             {
                 Dispose();
-                Controller.SendStop();
                 Log.Information("Tcp client {ConnectionToken} disconnected successfully", ConnectionToken);
                 return true;
             }
@@ -276,7 +281,7 @@ namespace Modbus.Net
             catch (Exception err)
             {
                 Log.Error(err, "Tcp client {ConnectionToken} receive exception", ConnectionToken);
-                CloseClientSocket();
+                //CloseClientSocket();
             }
         }
 
@@ -320,7 +325,10 @@ namespace Modbus.Net
                 Controller.SendStop();
                 Controller.Clear();
                 ReceiveMsgThreadStop();
-                _socketClient?.GetStream().Dispose();
+                if (_socketClient.Connected)
+                {
+                    _socketClient?.GetStream().Dispose();
+                }
                 _socketClient?.Close();           
             }
             catch (Exception ex)
diff --git a/Modbus.Net/Modbus.Net/TcpProtocalLinker.cs b/Modbus.Net/Modbus.Net/TcpProtocalLinker.cs
index 34eb76c..d3f155f 100644
--- a/Modbus.Net/Modbus.Net/TcpProtocalLinker.cs
+++ b/Modbus.Net/Modbus.Net/TcpProtocalLinker.cs
@@ -21,7 +21,7 @@ namespace Modbus.Net
         /// <param name="ip">Ip地址</param>
         /// <param name="port">端口</param>
         protected TcpProtocolLinker(string ip, int port)
-            : this(ip, port, int.Parse(ConfigurationManager.AppSettings["IPConnectionTimeout"] ?? "5000"))
+            : this(ip, port, int.Parse(ConfigurationManager.AppSettings["IPConnectionTimeout"] ?? "-1"))
         {
         }
 
@@ -33,8 +33,16 @@ namespace Modbus.Net
         /// <param name="connectionTimeout">超时时间</param>
         protected TcpProtocolLinker(string ip, int port, int connectionTimeout)
         {
-            //初始化连接对象
-            BaseConnector = new TcpConnector(ip, port, connectionTimeout);
+            if (connectionTimeout == -1)
+            {
+                //初始化连接对象
+                BaseConnector = new TcpConnector(ip, port);
+            }
+            else
+            {
+                //初始化连接对象
+                BaseConnector = new TcpConnector(ip, port, connectionTimeout);
+            }            
         }
     }
 }
\ No newline at end of file