C++ USB communication

Question

I have a problem regarding communication with a USB device on Windows. I can't use libusb or WinUSB as I have a specific driver for that (Silabs USB to UART, which is a USB-to-serial bridge). This is how I initialize a device file, send&read data and close the handle.

HANDLE hDevFile = CreateFile(L"\\??\\USB#VID_10C4&PID_EA60#0001#{a5dcbf10-6530-11d2-901f-00c04fb951ed}",
    GENERIC_READ | GENERIC_WRITE, 0, 0, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL | FILE_FLAG_OVERLAPPED, NULL);
PurgeComm(hDevFile, PURGE_TXABORT | PURGE_RXABORT | PURGE_TXCLEAR | PURGE_RXCLEAR);

DCB dcbInitState;
GetCommState(hDevFile, &dcbInitState);

DCB dcbNewState = dcbInitState;
dcbNewState.BaudRate = 57600;
dcbNewState.Parity = NOPARITY;
dcbNewState.ByteSize = 8;
dcbNewState.StopBits = ONESTOPBIT;

if (SetCommState(hDevFile, &dcbNewState) == 0)
{
    printf("Could not set COM state. Error: %i", GetLastError());
    return -1;
}

Sleep(60);

BYTE outData[8];
outData[0] = 0x53;
outData[1] = 0x10;
outData[2] = 0x04;
outData[3] = 0x10;
outData[4] = 0x40;
outData[5] = outData[3] ^ outData[4];
outData[6] = 0xAA;
outData[7] = 0x00;
DWORD dwWritten;

if (!WriteData(hDevFile, outData, 8, &dwWritten))
{
    printf("Could not write data. Error: %i", GetLastError());
    return -1;
}

BYTE inData[8];
DWORD dwRead;

if (!ReadData(hDevFile, inData, 8, &dwRead, 2000))
{
    printf("Could not read data. Error: %i", GetLastError());
    return -1;
}

SetCommState(hDevFile, &dcbInitState);
Sleep(60);
CloseHandle(hDevFile);
hDevFile = INVALID_HANDLE_VALUE;

(I get the device symbolic name from the registry but I've skipped that part to make my question concise. WriteData() and ReadData() are custom functions that write and read accordingly.)

~~The problem is that SetCommState() returns a zero-value. GetLastError() returns 122, which is ERROR_INSUFFICIENT_BUFFER.~~

The problem now is that PurgeComm() generates ERROR_INSUFFICIENT_BUFFER, too. CreateFile() gives ERROR_SUCCESS, so it must be opened properly.

What's wrong? Did I miss something?

Edit: I tried enumerating COM ports and found an interesting thing - there are no COM ports on my computer. Even though the device is connected and enabled, with the driver present and all that stuff. I also tried forcefully putting \\.\COM1, \\.\COM2, and so on as the file name for CreateFile, but with no luck. Everytime got an ERROR_FILE_NOT_FOUND.

Please, help. This is very important to me.

Have you checked the return value from GetCommState() to ensure that it's filling in the DCB structure that you go on to modify? — Andy Brown
Well, apparently, GetCommState() returns ERROR_INSUFFICIENT_BUFFER, too. — wassup
The DCB parameter to GetCommState() is documented as [in,out]. Zero it and set the DCBlength parameter to sizeof(DCB) before making the call. — Andy Brown
PurgeComm() generates ERROR_INSUFFICIENT_BUFFER too, so there isn't any problem with DCB. — wassup

Preston Preston · Accepted Answer · 2014-09-02T15:58:36

Because this is a CP210x device, it's a virtual COM port, so you should be opening it as such in CreateFile. You had it right when you said that you tried using \.\COMx, you just need to find out which COM port your CP210x device has been assigned and you will not get the ERROR_FILE_NOT_FOUND error. You can find this by looking in device manager:

enter image description here

Take a look a the Serial Communications Guide for the CP210x, this explains how to make these types of calls to your device, there's even a COM port discovery function that will help you find the COMxx name dynamically. It also has accompanying software, AN197SW.zip.

C++ USB communication

2 Answers