com0com virtual port not found

StackOverflow https://stackoverflow.com/questions/8772858

  •  14-04-2021
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Question

I have installed, apparently successfully, com0com v2.2.2.0 on a 64bit Windows 7 machine.

I run the command utility, like this:

command> install PortName=COM9 PortName=COM8
       CNCA0 PortName=COM9
       CNCB0 PortName=COM8
ComDB: COM8 - logged as "in use"
ComDB: COM9 - logged as "in use"

command> busynames COM?*
COM3
COM4
COM5
COM6
COM8
COM9
COMPOSITEBATTERY

Note: logged as "in use" is NOT an error message?

Then I try to open COM8 like this

m_hIDComDev = CreateFileA( szCodedPort, GENERIC_READ | GENERIC_WRITE, 0, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL | FILE_FLAG_OVERLAPPED, NULL );
if( m_hIDComDev == NULL  || m_hIDComDev == INVALID_HANDLE_VALUE ) {

    wchar_t * lpMsgBuf;
    DWORD dw = ::GetLastError(); 
    FormatMessage(
        FORMAT_MESSAGE_ALLOCATE_BUFFER | 
        FORMAT_MESSAGE_FROM_SYSTEM |
        FORMAT_MESSAGE_IGNORE_INSERTS,
        NULL,
        dw,
        MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
        (LPTSTR) &lpMsgBuf,
        0, NULL );

    myErrorMsg = (char*)malloc( 200 );
    sprintf(myErrorMsg,"Error creating connection %s ( %s ) %S",
        szPort,szCodedPort,lpMsgBuf);

This fails with

Error creating connection COM8 ( COM8 ) The system cannot find the file specified.

I have tried both "COM8" and "\\.\COM8".

Note that the exact same call to CreateFileA works perfectly on a 'real' COM port.

Q: Can someone tell me what the problem is?

A: On windows 7 64 bit you have to enable testing of unsigned drivers.

  • Run a command prompt as administrator. ( It is not enough just to run from an account with admin privileges. See here for details of how to do this. )

  • Type in the command bcdedit -set TESTSIGNING ON

  • reboot. ( Windows will place a "test Mode" note on the bottom right of the desktop )

For Windows 8 this method does not work satisfactorily or easily. Here is some discussion of the issues and workarounds.

Alternatively you might want to try a signed version - I have NOT tried this myself.

Was it helpful?

Solution

Here are my points:

  • Microsoft requires 64-bit drivers to be signed.
  • com0com.sys (64bit) is a mere unsigned kernel-mode driver.

SoftWx has written an interesting tutorial on installing it on a 64 bit Windows 7 machine. You can fetch it here.

OTHER TIPS

In case anyone is referencing this, and interested in this topic, the 2.2.2.0 version of com0com is signed for 64-bit Windows. This removes the need to jump through the hoops of putting Windows in "test" mode.

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