Serial communication - problems with reading

Discussion in 'Questions (Windows Mobile)' started by token, Jun 11, 2008.

  1. token

    token Member Licensed User


    I'm using BASIC4PPC 6.3 with a program based on the seriral-terminal sample.

    I'am sending data-packages of 20 Bytes with a frequency of 200 ms from a plc to the compiled B4PPC-application.

    My problem is, that not all 20 bytes will be received from the application but only the first 6-10. After a while the B4PPC-app hangs and have to be killed with the task-manager. If I only send 1 Byte it works flawlessly.

    Same problem with a connected datamatrix-camera which sends 67 byte-packages with a frequence of about 180 ms.

    I tried changing the timeout of the serial-object but it didn't helped.

    Thanks a lot for any suggestions in advance!


  2. Erel

    Erel Administrator Staff Member Licensed User

    Can you post the code you use for reading the data?
  3. token

    token Member Licensed User

    Hello Erel,

    I've attached the code. I've used the serialterminal exmaple as base. I'am using it for desktop only. Com-Parameters are: 115200b,8databits, 1 stopbit, no parity.

    Btw.: If I connect a datamatrix-cam and I'am using the original serialterminal-example the problem I encounter is somewhat different: For each scan the camera sends 67 Bytes. There are about 5-12 scans per second. Data seems to come in correctly, but after a while no more data is receive, the transmission stops without an error-message, and/or the application hangs.

    Attached Files:

  4. Erel

    Erel Administrator Staff Member Licensed User

    Are you using the OnCom event to read the data?
  5. token

    token Member Licensed User

    Yes, I'am using the OnCom event. Should I mange the buffer directly?
  6. Erel

    Erel Administrator Staff Member Licensed User

    Try to use a Timer instead.
    Something like:
    Sub Timer1_Tick
    If Serial1.InBufferCount > 0 Then
    'Read the data.
        End If
    End Sub
  7. token

    token Member Licensed User

    Thanks Erel! I'll try this.
  8. konisek

    konisek Member Licensed User

    I have similar problem. I need to send 22 bytes and receive them back. For a test, I made a loopback on the COM1 port (connected pins 2+3) so that I can see what data are coming back. First I tried Serial1_OnCom but it reads the data only in 8 byte packets, then I tried the Timer1_Tick but received only 14 bytes (not those 22 sent!), last 8 bytes are lost. I don't know what is wrong with OnCom or Tick but neither of them reads all 22 bytes. Any help, please?

    Sub Timer1_Tick
    If Serial1.InBufferCount > 0 Then
    Buffer()= serial1.InputArray
    For i = 0 To ArrayLen(Buffer())-1
    ReceivedString = ReceivedString & DecToHex(Buffer(i))
    textBox1.Text = ReceivedString
    End If
    End Sub

    Sub Button6_Click
    Timer1.Enabled = True
    textBox2.Text = "AAAA0100000000010B30363036423030303236010368"
    End Sub

  9. agraham

    agraham Expert Licensed User

    Which serial library are you using, SerialDevice, SerialDesktop, Serial2 or SerialEx? Are you on the desktop or a device?

    Although not your (current) problem, as you appear to want to receive binary data, if you are using Serial2 you should use my (.NET 2.0 only) library instead due to this problem in Serial2.

    Try my library anyway although it should behave identically to Serial2 in all situations that don't need the enhancements. However it looks like the default setting on your system may be to only interrupt or transfer data every 8 bytes. Try sending an extra byte at the end and see if it pushes the missing 8 bytes out. If you use SerialEx you can try setting the RxBytesThreshold to 1 and see if that alters the behaviour.
  10. konisek

    konisek Member Licensed User

    I am on the desktop and use a Serial2. I tested the comunication rather with strings but behaves the same. In my example I send two strings "AAAAAAAAAAAAAAAAAAAAAA" and "BBBBBBBBBBBBBBBBBBBBBB" with a 1000 ms time period. But the receiving textbox always concatenates both strings together but I expect to see in the receiving TextBox1 the first string only, and then in 1 second later the second string only. The TextBox2 is just for information what I am sending.

    Sub App_Start
    serial1.PortOpen = True
    End Sub

    Sub casovac
    End Sub

    Sub ButtonTe_Click
    serial1.EnableOnComm = True
    TextBox1.Text = ""
    TextBox1.Text = ""
    End Sub

    Sub Serial1_OnCom
    char = serial1.InputString
    buffer = buffer & char
    TextBox1.Text = TextBox1.Text & buffer
    buffer = ""
    End Sub

    When I enable the Msgbox("break"), I get the correct result

    however when I disable it, it concatenates it together, and that I don't want.
    The same code written in VB6 works fine!!!
    Finaly, the application should send 8 different strings (bytes) with 1 second interval to a connected device on COM1 and should receive single replies from the device based on the single sent string. The firtst 6 strings inicialise the device, the last 2 strings will continuously monitor the device in the loop. For testing the Tx/Rx communication I use a loopback on COM1.
  11. TWELVE

    TWELVE Active Member Licensed User


    from my perspective of view you should avoid to manipulate a textbox's content ( or a variable's content) in different places in your code "at the same time", if you don't have implemented some mechanisms to synchronize them.Look at the red arrows i inserted below:

    You send out the string, and shortly after this you clear the TextBox1 content.Virtually in parallel your OnCom event is also manipulating the Textbox1 content.Since you don't know, which of both "Textbox1.Text = " will occur first, this is kinda race condition.At the moment it looks like, the statement in the main part is winning this everything.So you end up in this order:

    Textbox = ""
    serial.output first string
    Textbox ="" <--you clear the empty box, because the OnCom didn't trigger yet
    OnCom <--- Textbox gets filled with first string
    wait a second
    serial.output second string
    OnCom <--- Textbox adds the second string

    In other words: you clear the textbox that was already empty and then the OnCom Event puts in the first string.

    If you put in a wait loop in here:

    Although a wait loop is not safe ( some factors like send string length may vary), it would be a workaround for your particular piece of code.In real world you would need to ensure that all bytes of your string have been received by the buffer, so you know the textbox can be cleared.You could also place something like an end-of-line character at the very end of your string, if your OnCom detects this character, it clears out the textbox.


    Last edited: Feb 7, 2009
  12. konisek

    konisek Member Licensed User

    Finally, I solved it in this way:
    TextBox19.Text = TextBox4.Text
    and replaced the OnCom event because it did not work:
    Sub frombuffer 'Serial1_OnCom
       Buffers()= serial1.InputArray
       For i = 0 To ArrayLen(Buffers())-1
          buffer = buffer & DecToHex(Buffers(i))
       TextBox4.Text = buffer
       buffer = 
    End Sub
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