Question

I am trying to use a Serial Port to interface with a camera. Now, the camera has its own proprietary protocol, which is fairly simple and uses 6-byte commands and acknowledges.

Now, I am using C# to write the program which interfaces with the camera, since that allows me to use WPF, and I am comfortable with C# anyway.

My problem arises when I try to set up the camera to retreive an image. The Serial Port object works absolutely fine in doing this, when I step through the program. However, as soon as I run it at full speed, for some reason I end up with the messages at the port being very badly read and even missed. I get to the end of the setup process, just before I begin to retreive the actual data, and there are still commands and things which are still in the port’s in buffer.

Now, this solution is, admittedly, just a very hacky attempt to learn the protocol and get the thing working at the moment, and I fully intend to improve it later, but I just cannot figure out why it does this. The camera isn’t supposed to sent the next packet until I have acknowledged the last one, so I can only assume that the serial port object is problematic, not the camera or my handling of the protocol.

Could it be because I have (lazily, yes) got this function on the UI thread?

Here’s the code:

private void retreivePhoto()
    {
        progressBar1.Visibility = Visibility.Visible;
        photo = new FileStream(Environment.CurrentDirectory + @"/photos/" + (photoNum++) + ".jpg", FileMode.OpenOrCreate);
        port.Open();
        progressBar1.Value = 0.5;
        setup.theMessage = setup.JPEG();
        port.Write(setup.theMessage, 0, setup.theMessage.Length);
        while (port.BytesToRead < 0) ;
        port.Read(received.theMessage, 0, received.theMessage.Length);
        setup.theMessage = setup.Package();
        port.Write(setup.theMessage, 0, setup.theMessage.Length);
        while (port.BytesToRead < 0) ;
        port.Read(received.theMessage, 0, received.theMessage.Length);
        setup.theMessage = (byte[])Message.SNAPSHOT.Clone();
        port.Write(setup.theMessage, 0, setup.theMessage.Length);
        while (port.BytesToRead < 0) ;
        port.Read(received.theMessage, 0, received.theMessage.Length);
        setup.theMessage = (byte[])Message.BLANK.Clone();
        setup.theMessage[1] = (byte)Message.MessageTypes.GETPICTURE;
        setup.theMessage[2] = (byte)Message.PictureType.SNAPSHOT;
        port.Write(setup.theMessage, 0, setup.theMessage.Length);
        while (port.BytesToRead < 0) ;
        port.Read(received.theMessage, 0, received.theMessage.Length);
        while (port.BytesToRead < 0) ;
        port.Read(received.theMessage, 0, received.theMessage.Length);
        int length = (received.theMessage[5] << 16) + (received.theMessage[4] << 8) + received.theMessage[3];
        length /= data.theMessage.Length-6;
        byte[] temp = {0xaa, 0x0e, 0x00, 0x00, 0x00, 0x00};
        port.DiscardInBuffer();
        port.Write(temp, 0, temp.Length);

        int next = 0;
        int bytes;

        for (int i = 0; i < length; i++)
        {
            if ((bytes = port.BytesToRead) > 0)
            {
                port.Read(data.theMessage, 0, data.theMessage.Length);
                if (bytes < data.theMessage.Length)
                {
                    photo.Write(data.theMessage, 4, bytes - 6);
                }
                else
                {
                    photo.Write(data.theMessage, 4, data.theMessage.Length - 6);
                }
                photo.Flush();
            }


            next = ((data.theMessage[1] << 8) + data.theMessage[0]) + 1;

            byte[] nextBytes = System.BitConverter.GetBytes(next);

            if (length - i == 1)
            {
                temp[4] = temp[5] = 0xf0;
            }
            else
            {
                temp[4] = nextBytes[0];
                temp[5] = nextBytes[1];
            }

            port.Write(temp, 0, temp.Length);
        }

        progressBar1.Value = 1.0;
        photo.Flush();
        photo.Close();
        port.Close();
        progressBar1.Visibility = Visibility.Hidden;

    }

Here is the camera’s datasheet which describes the protocol. http://www.4dsystems.com.au/downloads/micro-CAM/Docs/uCAM-DS-rev7.pdf

Answer 1

while (port.BytesToRead < 0) ;
port.Read(received.theMessage, 0, received.theMessage.Length);

Doesn’t mean that you will read received.theMessage.Length bytes. it can be anything between 1 & received.theMessage.Length. You should check the returned value