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Originally I made a POS application using Excel/VB. Everything worked A-OK, especially the serial communication with the POS display pole. Here's an example of the code by which I had the application communicate:

Function Send2Display(ln1$, Optional ln2$)
    On Error GoTo theEnd
    Dim ln1Space As Byte, ln2Space As Byte
    ln1 = Left(ln1, 20): ln1Space = 20 - Len(ln1)
    ln2 = Left(ln2, 20): ln2Space = 20 - Len(ln2)
    Open "COM1:" For Output As #1
        ' Write lines and then set cursor to start
        Print #1, ln1; Spc(ln1Space); ln2; Spc(ln2Space)
        Print #1, Chr$(&H1B)
    Err.Clear: Close #1
End Function

As I said, this worked ok. Now however I've moved the POS application over to RoR and I've had to make a Java applet for the purpose of communicating with the display pole. The applet works fine save for the following problem:

I'm not sure what the cause of it is, but at a given point in time Java reports that I'm using "an incorrect||missing function" when sending data to the COM port. I have to close the application (web page), then disable COM1 in the Device Manager then re-enable it. Then when I load the web page again it starts working. See this page for the Java code and JNLP.

If the COM port is unresponsive to the web app I can load the old Excel file and it will still work. I'm at a loss as to why it won't work for the Java applet.

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"POS application using Excel/VB" ← that's the very definition of horror – Hubert Kario Jan 7 '12 at 19:41
up vote 0 down vote accepted

Forget it, turns out I had installed the display pole as a printer way back and that was what was tying up the COM1 port. I removed it and so far I haven't encountered any issues.

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you can mark your own answer as the solution – Hubert Kario Jan 7 '12 at 19:42

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