Are there ciritical differences betweem how the old VB6 MSComm control communicates with a serial port and how the .Net SerialPort does this?

Question

We have a device that is connected over a USB cable and emulates a COM port. Using our old VB6 application and the MSComm control after opening the connection the device sends a byte back whenever the VB6 application sends a byte to the device.

Now, we are porting the VB6 application to .Net and use the SerialPortclass for communication. port.Write succeeds, but port.ReadByte runs into a TimeOutException for every thinkable ReadTimeOut we use.

On the face of it all settings like Baudrate, Parity, etc are the same. The strange thing is, that if we first launch the old VB6 application and close it (which connects to the device) the .Net code starts working as well until we unplug and replug the device.

So, in some way the MSComm control kicks the device to life in a way that the SerialPort can not. The code in the VB6 application that opens the connection and starts communication is straightforward and has been converted to C#.

I used a port monitor to see what data was actually sent and indeed the MSComm control sends a lot more control bytes to the device before starting the actual data transfer. The only meaningful difference I was able to discern is that the MSComm control uses other control chars for Xon Xof, EvtChar, etc. But after adjusting these for the SerialPort as well (see this link nothing changed.

What else can be going on? Should investigating the data sent over the port even more closely give the answer in the end, or can it be that something else entirely is going on?

Answer 1

Make sure the MSComm port is open (when your program initializes and communicate with the device)

In C# .NET , it is something like:

  serialPort = new SerialPort();

            if (serialPort is SerialPort)
            {
                serialPort.PortName = "COM1";
                serialPort.DataBits = 8;
                serialPort.Parity = Parity.None;
                serialPort.StopBits = StopBits.One;
                serialPort.BaudRate = 9600;

                try
                {
                    serialPort.Open();

/// and other commands

}}