I am building a simple test program for some embedded devices. They are controlled through Wi-Fi using UDP multicast messages. The test application, written using VB.NET is able to send data to the devices but I am not able to receive data back from them.
The data from the embedded devices is captured by WireShark correctly, but nothing is received by the test program.
Windows Firewall is open for the program, any port, any IP for UDP.
This is the thread code I have as a skeleton to receive the data:
Private Sub ListenForInput()
Try
Dim udpClient As New UdpClient(port)
Dim grpEndpoint As New IPEndPoint(ipAddress, port)
While keepListening
Debug.Print("Listening on IP:" + ipAddress.ToString + " port:" + port.ToString)
Dim input As Byte() = udpClient.Receive(grpEndpoint)
Debug.Print("Got data")
End While
Catch ex As SocketException
'
' What do do?
'
Debug.Print("Oops!")
End Try
End Sub
The Debug output displays the "listening" message, but the call to Receive never returns.
Related
I am writing a TCP listener (server) that needs to receive messages and send back an acknowledgement. Pretty basic stuff. There are literally dozens of examples out there, including MSDN, from which I copied much of my code. I can receive the message no problem. The problem comes when I try to send back the response. The sending client (Corepoint HL7 engine) reports the following error:
The connection was closed before a response was received
I have tested my service with my own TCP sending test app (written using code copied from MSDN) and it works. But when I receive messages from Corepoint, the response does not go back.
Below is my code. Does anyone have any idea why the NetworkStream.Write method is not actually sending the data (or why the client is not receiving it)? I've tried every idea I've found in other posts that are similar to my problem, and nothing is working. Am I doing something wrong, or is something wrong in the configuration of Corepoint?
Sub Main()
listenThread.Start()
End Sub
Private serverSocket As TcpListener
Dim listenThread As New Thread(New ThreadStart(AddressOf ListenForClients))
Private Sub ListenForClients()
Dim port As Int32 = '(pick a port #)
Dim localIP As IPAddress = 'enter your IP
serverSocket = New TcpListener(localIP, port)
serverSocket.Start()
While True 'blocks until a client has connected to the server
Dim client As TcpClient
If serverSocket.Pending Then
client = serverSocket.AcceptTcpClient
'tried these 2 settings with no effect
'client.NoDelay = True
client.Client.NoDelay = True
ProcessIncomingMessageSocketTCPClient(client) 'I was doing this in a separate thread but temporarily kept it on this thread to eliminate threading as the possible cause (but no luck)
client.Close()
Else
Threading.Thread.Sleep(1000) 'wait 1 second and poll again
End If
End While
End Sub
Private Sub ProcessIncomingMessageSocketTCPClient(ByRef objClient As TcpClient)
Dim strMessageText As String
Dim clientStream As NetworkStream
Dim msgBuffer(4096) As Byte
Dim numberOfBytesRead As Integer
Dim strChunk As String
Dim strCompleteMessage As New Text.StringBuilder
Dim sendBytes As Byte()
clientStream = objClient.GetStream()
Do
numberOfBytesRead = clientStream.Read(msgBuffer, 0, msgBuffer.Length)
strChunk = Encoding.ASCII.GetString(msgBuffer, 0, numberOfBytesRead)
strCompleteMessage.AppendFormat("{0}", strChunk)
Loop While clientStream.DataAvailable
strMessageText = strCompleteMessage.ToString
sendBytes = Encoding.ASCII.GetBytes("I received a message from you")
clientStream.Write(sendBytes, 0, sendBytes.Length)
objClient.Close() 'tried it with and without this line
End Sub
It turns out that nothing is wrong with my code. The TCP was and is working correctly. This application is an HL7 listener and I was missing the MLP wrapping around my ACK. As soon as I added that, the sending application accepted my ACK and all is good.
Is there a way to keep a TcpClient connection open constantly? I have an application which lets our users scan a carton, some database updates are performed, and a shipping label is sent to and printed from a wireless hip printer (model of printer is Zebra QLn420) which the user is using.
The application attempts to keep a connection to the wireless printer via the TcpClient connection, and multiple checks are made throughout processing to make sure the connection is good, before sending a generated ZPL to the printer for printing.
We have been having an issue with an occasional label missing, and it seems to be whenever the user stops scanning for a few minutes, then resumes. However, it is a semi-rare occurrence when a label is skipped, and as such is rather hard to reproduce (I haven't been able to replicate it myself, but I have seen it happen out in the warehouse).
I would like to know either if there is a way to make sure that the connection is always open (by "pinging" the device every so often), or if there is a way to get feedback that the data has been received and printed.
This is the code I'm calling to ensure a connection:
Public Function Connect(strIP As String, intPort As Integer) As Boolean
Try
'connect to printer via TcpClient, need ip address and port number
'connects without thread, hangs program for 10-20 seconds if printer is not turned on, replaced with code below to thread the connection and set timeout
For i As Integer = 1 To 2
If Not (client IsNot Nothing AndAlso client.Connected) Then
'uses ClientSocketParameters structure to pass to recursive function ConnectionReturned()
clntSockParams = New ClientSocketParameters
clntSockParams.addrs = strIP
clntSockParams.prt = intPort
'create client and call BeginConnect (attempts to connect on separate thread until TimeoutTime has elapsed)
client = New System.Net.Sockets.TcpClient
client.SendTimeout = 5000
client.ReceiveTimeout = 5000
'setup timer with timeout length and start, if timer goes past intTimeoutLength, the Timeout() function is called which closes everything and leaves client = Nothing
AddHandler TimeoutTime.Elapsed, AddressOf Timeout
TimeoutTime.Interval = intTimeoutLength
TimeoutTime.Start()
client.BeginConnect(strIP, intPort, New AsyncCallback(AddressOf ConnectionReturned), clntSockParams)
'keeps the program from doing anything else until BeginConnect either succeeds or fails (due to connect on separate thread)
Do While TimeoutTime.Enabled
System.Threading.Thread.Sleep(500)
Loop
End If
'if TimeoutTime is elapsed and client is Nothing, connection didn't happen, throw an error
If client Is Nothing Then
blnConnected = False
Else
blnConnected = True
Exit For
End If
Next
Catch ex As Exception
blnConnected = False
End Try
Return blnConnected
End Function
Private Sub ConnectionReturned(ByVal ar As System.IAsyncResult)
'this method is called from the client.BeginConnect line in Connect(), make sure timer is running
If TimeoutTime.Enabled Then
'ensure client is initialized
If client Is Nothing Then client = New System.Net.Sockets.TcpClient
'keep calling ConnectionReturned until client.Connected is true
If client.Connected Then
TimeoutTime.Stop()
Else
Dim actualParameters As ClientSocketParameters = DirectCast(ar.AsyncState, ClientSocketParameters)
client.BeginConnect(actualParameters.addrs, actualParameters.prt, New AsyncCallback(AddressOf ConnectionReturned), clntSockParams)
End If
End If
End Sub
Private Sub Timeout(ByVal sender As Object, ByVal e As EventArgs)
'this method is only called if TimeoutTime elapsed, which means no connection was made. close the client object if needed, set to Nothing, and stop TimeoutTime
If TimeoutTime.Enabled Then
Try
client.Close()
Catch ex As Exception
End Try
client = Nothing
TimeoutTime.Stop()
End If
End Sub
According to this question:
tcp client in vb.net not receiving the entire data response data from server
TcpClient is not always guaranteed to deliver all data to the other end of a connection, so if a more reliable connection method is available, that would be worth a try as well.
Please let me know if more information is needed. Thank you!
Originally I got the code for connecting via this link. I've modified it since because it would hang the application for 10-20 seconds if it took longer to connect. Code here is in C# and I translated to VB:
Send ZPL Commands via TCP/IP in C#
This is the link to the docs for the class:
TcpClient Class
Zebra printers have a timeout setting on TCP that I think has a 3 or 5 minute default. The first thing to do is to turn that timeout off. There will still be other reasons that the printer would disconnect so you will need to handle that as well.
embed this into your program:
! U1 setvar "wlan.ip.timeout.enable" "off"
Make sure you send a CR/LF before and after that line.
if you send a query after your format you can know that the whole format made it to the printer. Something like the following would work:
! U1 getvar "device.uptime"
That's not a sane solution. A sane solution is this: If a label isn't completely sent to the printer because the connection failed, make a new connection and send the label.
I am using people count device to read the InCount, Out Count record and it is connected with my PC COM3 USB port.. I have written the code to fetch the data, I am continuously receiving the below message while reading the data..... can I have some code or idea to fetch the record?
message is.... The operation has timed out.
mycode is below:
Function ReceiveSerialData() As String
' Receive strings from a serial port.
Dim returnStr As String = ""
Dim com1 As IO.Ports.SerialPort
'SerialPort sp = new SerialPort("COM3", 115200, Parity.None, 8, StopBits.One);
Try
com1 = My.Computer.Ports.OpenSerialPort("COM3")
com1.BaudRate = 115200
com1.ReadTimeout = 10000
Do
Dim Incoming As String = com1.ReadLine()
If Incoming Is Nothing Then
Exit Do
Else
returnStr &= Incoming & vbCrLf
End If
Loop
Catch ex As TimeoutException
returnStr = "Error: Serial Port read timed out."
Finally
If com1 IsNot Nothing Then com1.Close()
End Try
Return returnStr
End Function
You must know at least the following 7 parameter settings for the device you are trying to communicate with and set your serial port properties to match.
PortName
BaudRate
Parity
DataBits
StopBits
NewLine
Handshake
Some of these you might guess (Parity is usually none, Databits is usually 8 stop bits is usually 1, handshake is often none). But Hans is correct unless you get all these set properly you will never communicate with your device. Also it is better to open your serial port once during initialization of your program and then leave it open until the program closes.
I am not a software programmer but I have a task to create a TCP Server (a program that is listening on its network card interfaces for incoming data streams).
I have searched on the internet and I found that I can use two methods: Socket or TCPListener class.
I have created an example for the Socket class, but I was wondering how I can test it?
If another computer in the network sends some string data to the listener computer, then the message should be displayed.
Here is the example from Microsoft that I am using for the TCP server using a Socket:
Public Shared Sub Main()
' Data buffer for incoming data.
Dim data = nothing
Dim bytes() As Byte = New [Byte](1024) {}
Dim ipAddress As IPAddress = ipAddress.Any
Dim localEndPoint As New IPEndPoint(ipAddress, 0)
Dim intI As Integer = 0
'Display the NIC interfaces from the listener
For Each ipAddress In ipHostInfo.AddressList
Console.WriteLine("The NIC are {0}", ipHostInfo.AddressList(intI))
intI += 1
Next
Console.WriteLine("You are listening on {0}",localEndPoint)
' Create a TCP/IP socket.
Dim listener As New Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp)
' Bind the socket to the local endpoint and
' listen for incoming connections.
Try
listener.Bind(localEndPoint)
listener.Listen(200)
Catch e As SocketException
Console.WriteLine("An application is alreading using that combination of ip adress/port", e.ErrorCode.ToString)
End Try
' Start listening for connections.
While True
Console.WriteLine("Waiting for a connection...")
' Program is suspended while waiting for an incoming connection.
Dim handler As Socket = listener.Accept()
data = Nothing
' An incoming connection needs to be processed.
While True
bytes = New Byte(1024) {}
Dim bytesRec As Integer = handler.Receive(bytes)
data += Encoding.ASCII.GetString(bytes, 0, bytesRec)
Console.WriteLine("The string captured is {0}", data)
If data.IndexOf("something") > -1 Then
Exit While
End If
End While
' Show the data on the console.
Console.WriteLine("Text received : {0}", data)
' Echo the data back to the client.
Dim msg As Byte() = Encoding.ASCII.GetBytes(data)
handler.Shutdown(SocketShutdown.Both)
handler.Close()
End While
End Sub
End Class
Am I on the right lead?
Thanks
Later Edit:
I have used that code in a Console Application created with Visual Studio and I want to check the scenario when a device is sending some string message through the network.
E.g:
I have two devices :Computer A, computer B connected through LAN
I have tried this command : telnet computerA port ( from computer B) but nothing is displayed in the TCP server running from computer A.
telnet 192.168.0.150 3232
I also made a TCP client for testing (derived from the Microsoft example):
Public Class SynchronousSocketClient
Public Shared Sub Main()
' Data buffer for incoming data.
Dim bytes(1024) As Byte
Dim ipHostInfo As IPHostEntry = Dns.GetHostEntry(Dns.GetHostName())
Dim ipAddress As IPAddress = ipHostInfo.AddressList(0)
Dim remoteEP As New IPEndPoint(ipAddress, 11000)
' Create a TCP/IP socket.
Dim sender As New Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp)
' Connect the socket to the remote endpoint.
sender.Connect(remoteEP)
Console.WriteLine("Socket connected to {0}", _
sender.RemoteEndPoint.ToString())
' Encode the data string into a byte array.
Dim msg As Byte() = _
Encoding.ASCII.GetBytes("This is a test<EOF>")
' Send the data through the socket.
Dim bytesSent As Integer = sender.Send(msg)
' Receive the response from the remote device.
Dim bytesRec As Integer = sender.Receive(bytes)
Console.WriteLine("Echoed test = {0}", _
Encoding.ASCII.GetString(bytes, 0, bytesRec))
' Release the socket.
sender.Shutdown(SocketShutdown.Both)
sender.Close()
Console.ReadLine()
End Sub
End Class 'SynchronousSocketClient
But it does not work because of the PORT setting.
If in the TCP Server I have "Dim localEndPoint As New IPEndPoint(ipAddress, 0)" then the client crashes, but if I change the port from any (0) to 11000 for example, the client works fine.
Do you know why?
Later edit 2:
Maybe I should have started with this question: Which method is recommended for my scope - asynchronous or synchronous method?
Yes, you are on the right path.
The next thing to do is to introduce message detection since TCP is stream based and not message based like UDP. This means that TCP might decide to send two of your messages in the same packet (so that one socket.Recieve will get two messages) or that it will split up your message into two packets (thus requiring you to use two socket.Recieve to get it).
The two most common ways to create message detection is:
Create a fixed size header which includes message size
Create a delimiter which is appended to all messages.
Your "server" isn't listening on a set port, so you'll need to pay attention to the "You are listening on" message that appears. Then, from another machine on the network, telnet the.ip.add.ress port. (This may require installing "telnet client", or enabling it in the Programs and Features stuff, or whatever.)
Side note...if you actually intend for this to be a server of some sort, you'll want to decide what port you want to use, so that other computers can find your service. Most people won't be able to read your screen to figure out where to connect. :)
As for your "client"...when you connect to another computer, you don't just "pick a port" (which is what a port number of 0 means in an endpoint). You need to know what port the server uses. (Reread what i said in the previous paragraph. A program running on another computer has no idea what port to use to connect to the server -- any server could be running on any port.) You need to pick a port number for the server (say, 11000...good as any, really) rather than letting it use port 0.
I'm using TCP socket connetion between a server program and a client program. Multiple client programs shall connect to the same port. The problem is that if I close a client program I get the following error on the server-program:
System.IO.IOException: Unable to read data from the transport connection: An existing connection was forcibly closed by the remote host. ---> System.Net.Sockets.SocketException: An existing connection was forcibly closed by the remote host
at System.Net.Sockets.Socket.EndReceive(IAsyncResult asyncResult)
at System.Net.Sockets.NetworkStream.EndRead(IAsyncResult asyncResult)
http://i55.tinypic.com/nh135w.png
If I handle this by a try/catch, I am then not able to re-connect with the client-program, as it gives the following error (in the client program):
No connection could be made because the target machine actively refused it 127.0.0.1: 1234
Below is the listener code in the server program. I hope to get some help to understand how I can handle client program shutdown/restart&reconnect without the server program failing..
' This is the callback function for TcpClient.GetStream.Begin, It begins an asynchronous read from a stream.
Private Sub DoRead(ByVal aread As IAsyncResult)
Dim BytesRead As Integer
Dim strMessage As String
' Try
' Ensure that no other threads try to use the stream at the same time.
SyncLock _client.GetStream
' Finish asynchronous read into readBuffer and get number of bytes read.
If Client.Connected Then
BytesRead = _client.GetStream.EndRead(aread)
End If
End SyncLock
' Convert the byte array the message was saved into, minus one for the Chr(13) (carriage return).
strMessage = Encoding.ASCII.GetString(readBuffer, 0, BytesRead - 1)
ProcessIncoming(strMessage)
' Ensure that no other threads try to use the stream at the same time.
SyncLock Client.GetStream
' Start a new asynchronous read into readBuffer.
Client.GetStream.BeginRead(readBuffer, 0, READ_BUFFER_SIZE, AddressOf DoRead, Nothing)
End SyncLock
'Catch e As Exception
' ' This triggers if userlist is found empty
' ' Then gives problem, as it cant close the connection or something.. ??
' Debug.Print("UserConnection.DoRead Exception: " & e.ToString)
' CloseConnetion("Error: Stream Reciever Exception")
'End Try
End Sub
You don't. You're the server. You close the socket and forget about it. If the client wants more service it is up to him to reconnect.