C# COM port communication problem

Question

I have a C# code which communicates with three different COM ports. The COM ports are actually three serial port to USB converters.

The code each time switches 'off' and 'on' the devices to which it is communicating, then initializes the three com ports, tries to send and read data and then closes the com port. This keeps continuing for a pre-defined number of loops. My problem is that after about 8 or 9 iterations, the COM port communication stops working. Sometime it throws an error saying the port is closed, sometime it does not throw any exception but it is actually not reading or writing anything from the com port. Some point it was only writing but not reading back the data.

What might be the reason and any tips to debug this problem?

EDIT:

The port abruptly closes or stops working even in the middle of the program as shown below:

SerialPort.Write("ss");
SerialPort.Read("ss"); // FAILS!!

Some part of the code I am using

public string Read(string readCommand)
        {
            string str = "";
            _port.WriteLine("\r");
            _port.WriteLine(readCommand + "\r");
            Thread.Sleep(0x3e8);
            str = _port.ReadExisting();
            return str;
        }

public void Write(string command)
        {
            _port.WriteLine(command + "\r");
            Thread.Sleep(100);
            if (_port.ReadExisting() == string.Empty)
            {
                throw new IOException("Error writing to COM");
            }
        }

    public void Initialize()
    {
        if (_port == null)
        {
            _port = new SerialPort(this.PortName.ToString(), this.BaudRate, this.Parity, this.DataBits, this.StopBits);
            _port.Handshake = this.Handshake;
        }
        try
        {
            if (!_port.IsOpen)
            {
                _port.Open();
                if (Read("") == string.Empty)
                {
                    throw new IOException("Device not connected or powered on");
                }

            }
        }
        catch (Exception)
        {
            this.Close();
        }
    }

Thanks...

Answer 1

        _port.WriteLine(command + "\r");
        Thread.Sleep(100);
        if (_port.ReadExisting() == string.Empty)
        {
            throw new IOException("Error writing to COM");
        }

That's evil code and bound to throw sooner or later. Windows cannot provide a service guarantee like that. Or for that matter the device itself, especially when you power it on and off. Use SerialPort.ReadTimeout, set it to at least 2 seconds. And make a blocking call, like ReadLine().

    catch (Exception)
    {
        this.Close();
    }

That's tops the previous snippet. You have no idea what's going wrong when that runs. And your code will try to use a closed port. Just delete the statements, it does nothing but harm.

Do not close the ports until your program ends. SerialPort uses a background thread to watch for events on the port, that thread needs to shutdown after the Close() call before you can open the port again. How long it takes to shutdown is unpredictable, it could be seconds worst case. There's no point in closing the port, it isn't going to be useful to anything else.

Answer 2

You need to use SetCommTimeouts (not sure what the .NET wrapper is, I gave up on the .NET serial classes long ago and call the Win32 API directly) to force the USB/serial converter to send the data back to your program.

By default it may try to collect a block equal in size to a USB transfer block, for efficiency.

Answer 3

Its tough to tell exactly what the problem might be without see some of the code. My guess would be that you are not waiting long enough for the COM port to close after reopening it. Note from the SerialPort.Close page, that:

The best practice for any application is to wait for some amount of time after calling the Close method before attempting to call the Open method, as the port may not be closed instantly.

Can you just open the COM ports and leave them open until you are done? For example from this post:

using (SerialPort serialPort = new SerialPort("COM1", 9600))
{
    serialPort.Open();
    while (true)
    {
        Thread.Sleep(1000);
        // serialPort.Write();
        Thread.Sleep(1000);
        // serialPort.Read();
        // break at some point to end
    }
    serialPort.Close();
}