Virtual comport communication with windows 10 in C

Question

im trying to communicate with a device via a virtual comport (comport to usb adapter,PL2303) on Win10. The device is an Eltek RC250 datalogger.

I have already installed an older PL2303 driver. The devicemanager recognized the device without any errors. Sending and receiving data between the device and the official software is working properly.

My problem is that after ReadFile is executed the program is doing nothing. I think ReadFile is waiting for more input from the device and therefore stucked in this function. Trying it on a win7 System gets to the same issue.

The message which i write to the device is a valid message.

The following code shows the communication.

hComm = CreateFile("COM3",                       //port name
    GENERIC_READ | GENERIC_WRITE, //Read/Write
    0,                            // No Sharing
    NULL,                         // No Security
    OPEN_EXISTING,// Open existing port only
    0,            // Non Overlapped I/O
    NULL);        // Null for Comm Devices                                                  /* establish connection to serial port */

if (hComm == INVALID_HANDLE_VALUE)
    printf("Error in opening serial port");
else
    printf("opening serial port successfully");


nNumberOfBytesToWrite = sizeof(message);

resW = WriteFile(
    hComm,
    message,
    nNumberOfBytesToWrite,
    &lpNumberOfBytesWritten,
    NULL);

do
    {
    printf("\nread");
    resR = ReadFile(
        hComm,
        &answer,
        sizeof(lpNumberOfBytesRead),
        &lpNumberOfBytesRead,
        NULL);
        SerialBuffer[i] = answer;
        i++;
    }
while (lpNumberOfBytesRead > 0);

return 0;

Please help me, i have no clue what the problem might be. Thomas

Answer 1

In the ReadFile() call, third parameter should be sizeof(answer) (or possibly just 1 since it appears to be a single byte), but certainly not sizeof(lpNumberOfBytesRead). It is blocking waiting for 4 bytes (size of a DWORD) when presumably answer is a single byte?

Also if you have not explicitly set a Comm timeout, you have no idea how long ReadFile() will wait before returning 0 to exit the loop. If the timeout is indefinite then it will never exist the loop.

There are other potential issues in this call, but without seeing how the parameters are declared, it is not possible to say.