1. Create a new Windows Form
2. Add the MSComm COM/OCX Control to your "Windows Forms"
   1. Right Click on the Toolbox
   2. Choose "Customize Toolbox..."
   3. Select and add the "Microsoft Communications Control"
3. Draw the new control onto your form (Telephone icon)

• com.CommPort
  Sets or gets the computer's serial port to be used.
  
  
• com.PortOpen
  Opens or closes the serial port.
  
  
• com.RThreshold
  Sets how many characters should be received before firing the OnComm event. Set to 0 to disable event calling. Set to 1 to fire OnComm every time a character is received.
  
  
• com.InputMode
  On of the MSCommLib.InputModeConstants constants to specify either sending/receiving text strings or byte arrays. Defaults to text which is easier to work with but not as reliable as byte arrays.
  
  
• com.Settings
  Used to setup the port in the format "baud,p,d,s" where baud = baud rate, p = parity, d = # data bits, and s = # stop bits. Ex: com.Settings = "9600,n,8,1"
  
  
• com.Handshaking
  On of the MSCommLib.HandshakeConstants constants to specify the type of handshaking: none, RTS/CTS hardware hs, and/or XOn/XOff software hs
  
  
• com.InBufferCount
  Returns the number of characters waiting in the receive buffer.
  
  
• com.Input
  Returns and removes a stream of data from the receive buffer. Used to check for data waiting. Returns a string if in text mode or byte array if in binary/byte mode.
  
  
• com.Output
  Writes a stream of data to the transmit buffer. Ex: com.Output = "Hello" sends "Hello" through the serial port.
  
  
• com.CommEvent
  Returns a MSCommLib.CommEventConstants, MSCommLib.ErrorConstants, or MSCommLib.OnCommConstants constant representing the most recent error or event that occurred. Check this in the OnComm event.
  
  
• com.NullDiscard
  If true, the serial control will ignore all 0x00 (null) characters come in. You will usually want to disable this so you can receive 0x00 since it may be important.
  
  
• com.InputLen
  The number of characters the Input property reads from the receive buffer. Setting InputLen to 0 reads the entire contents of the receive buffer when com.Input is used.
  

public MyForm()
{
    InitializeComponents();  // Initialize Form Components
    com.RThreshold = 1;  // Fire OnComm event after any data is received
    com.OnComm += new System.EventHandler(this.OnComm);  // Assigns the event handler }

private void OnComm(object sender, EventArgs e)  //  MSCommLib OnComm Event Handler {
    if (com.InBufferCount > 0) ProcessData((string) com.Input);
    if (com.CommEvent == MSCommLib.OnCommConstants.comEvCTS)
        Console.WriteLine("CTS Line Changed");
}

1. Beginning and Ending ("Start" & "Stop") Codes
   This is good for sending text as it lets everybody know when text starts and ends. You simply tack on a non-normal byte at the beginning and end of the text. For example, you'd use '---' to signify the start of the string and '===' to signify the end. So you would use: com.Output = "---Hello World===";
   
   
2. Fixed Length Codes
   Used for specific commands. You can create your own codes to send and specify what they mean. Say I want to control the lighting in a house, I'd setup a protocol of commands like this:
   1st byte = House Code, 2nd byte = Light Code, 3rd byte = On or Off (0 for off, 1 for on)
   So to turn on the 11th light in my house (house code #3) I'd use:
   com.Output = new byte[] {3, 11, 0};
   
   
3. Prefixed Data Packet
   This is probably the most common and flexible but requires the most coding. Just prefix your data packet with the length of the data. The prefix must be a fixed size, such as two bytes which would allow a data packet of up to 65,535 bytes. Then the receiver knows how much data is in the packet because it always takes the first two bytes and uses the rest as the data packet.
   Example: com.Output = ((char) 00) + ((char) 11) + "Hello World";
   

           DB9 Female to DB9 Female

       2 |  3 |  7 |  8 | 6&1|  5 |  4
     ---- ---- ---- ---- ---- ---- ---- 
       3 |  2 |  8 |  7 |  4 |  5 | 6&1



          DB25 Female to DB25 Female

        2 |  3 |  4 |  5 | 6&8|  7 | 20 
      ---- ---- ---- ---- ---- ---- ----
        3 |  2 |  5 |  4 | 20 |  7 | 6&8

          DB9 Male (Pin Side)                   DB9 Female (Pin Side)
          DB9 Female (Solder Side)              DB9 Male (Solder Side)
              -------------                          -------------
              \ 1 2 3 4 5 /                          \ 5 4 3 2 1 /
               \ 6 7 8 9 /                            \ 9 8 7 6 /
                ---------                              ---------


   DB25 Male (Pin Side)                    DB25 Female (Pin Side)
   DB25 Female (Solder Side)               DB25 Male (Solder Side)
    ---------------------------------       ---------------------------------
    \ 1  2  3  4  5  6  7  8 ... 13 /       \ 13 ... 8  7  6  5  4  3  2  1 /
     \ 14 15 16 17 18 19 20 ... 25 /         \ 25 ... 20 19 18 17 16 15 14 /
      -----------------------------           -----------------------------

1. Create a new form
2. Add the AxMSCommLib control and name it "com"
3. Add a Rich Text Box named "rtfTerminal"
4. Put the constructor code below in the form's constructor
5. Add the rest of the code. Make sure the settings are right for your system.
6. Have another computer with a null modem connecting the two and a terminal programming (such as Tera Term Pro) running.

// Constructor for Form with an AxMSCommLib control on it named "com"
public SerialTerm()
{
    // Initialize Form Components
    InitializeComponent();

    // Initialize the COM Port control
    InitComPort();

    // Send data out through the COM port
    com.Output = "Serial Terminal Initialized";
}

private void InitComPort()
{
    // Set the com port to be 1
    com.CommPort = 1;
    
    // This port is already open, close it to reset it.
    if (com.PortOpen) com.PortOpen = false;
    
    // Trigger the OnComm event whenever data is received
    com.RThreshold = 1;  
    
    // Set the port to 9600 baud, no parity bit, 8 data bits, 1 stop bit (all standard)
    com.Settings = "9600,n,8,1";

    // Force the DTR line high, used sometimes to hang up modems
    com.DTREnable = true;
    
    // No handshaking is used
    com.Handshaking = MSCommLib.HandshakeConstants.comNone;

    // Don't mess with byte arrays, only works with simple data (characters A-Z and numbers)
    com.InputMode = MSCommLib.InputModeConstants.comInputModeText;
    
    // Use this line instead for byte array input, best for most communications
    //com.InputMode = MSCommLib.InputModeConstants.comInputModeText;
    
    // Read the entire waiting data when com.Input is used
    com.InputLen = 0;

    // Don't discard nulls, 0x00 is a useful byte
    com.NullDiscard = false;
    
    // Attach the event handler
    com.OnComm += new System.EventHandler(this.OnComm);
    
    // Open the com port
    com.PortOpen = true;  
}

private void OnComm(object sender, EventArgs e)  //  MSCommLib OnComm Event Handler {
    // If data is waiting in the buffer, process it.
    // Note: This is using the string method for simple data, be sure
    //       to use byte arrays (described below) for more generic data.
    if (com.InBufferCount > 0) ProcessComData((string) com.Input);
}

private void ProcessComData(string input)
{
    // Send incoming data to a Rich Text Box
    rtfTerminal.AppendText(input + "\n");
}

public short FindDevicePort()
{
    bool PortOkay = true; short TestPort = 0;
    bool found = false;  // Init variables for checking.

    // Stop the OnComm event and normal processing.
    com.RThreshold = 0;
    
    // Close the current port if one is already open.
    if (com.PortOpen) com.PortOpen = false;

    do
    {
        TestPort++;  // Try the next port
        PortOkay = true;
        
        // Attempt to access this port number (mayby it doesn't exist on this computer)
        try {com.CommPort = TestPort;}
        catch (System.Runtime.InteropServices.COMException)
        {PortOkay = false;}
        
        if (PortOkay)  // Continue if the port exists
        {
            // Attempt to open the port (maybe it's already in use by another program)
            try {com.PortOpen = true;}
            catch (System.Runtime.InteropServices.COMException)
            {PortOkay = false;}

            if (PortOkay) // Continue if port was available and opened
            {
                com.Output = "Hello?";  // Send string to request a response.

                long TimeStamp = DateTime.Now.Ticks;  // Time Stamp for Time-out
                string buffer = "";
                bool ElapsedTime;
                do
                {
                    // Don't lock up the entire application,
                    // release control to process other messages.
                    System.Windows.Forms.Application.DoEvents();
                    
                    // If there is data waiting, buffer it in our own string buffer.
                    if (com.InBufferCount > 0) buffer += com.Input;

                    // Look for response from device.
                    found = (buffer.IndexOf("Hi There") > -1);  

                    // True if time is up, change the 0.2 for human interaction
                    ElapsedTime = DateTime.Now.Ticks - TimeStamp >
                                  TimeSpan.TicksPerSecond * 0.2;

                // Keep waiting until found or 0.2 seconds are up.
              } while (!ElapsedTime & !found);
            }
        }
    } while ((TestPort < 4) & !found);

    // If the device was found, return the port it was found on
    if (found) return TestPort;
    
    // Device not found, return 0.
    return 0;
}

1. Setup com.Input for receiving byte arrays. Change your "com.InputMode" to:
   com.InputMode = MSCommLib.InputModeConstants.comInputModeBinary;
   
   
2. When receiving data, store into a byte array.
   Here is an example OnComm event:
   

   private void OnComm(object sender, EventArgs e)  //  MSCommLib OnComm Event Handler {
     // Receive data into a byte array, good for any type of data
       byte[] indata = (byte[]) com.Input;
       
       // Show each byte received in the "Output" window
       foreach (byte b in indata)
         Console.WriteLine("Byte Data: " + b);
   }
   
       
   
   
   
   
   
   

   
   

1. Use "Start" & "Stop" Tokens
   This is by far the preferred method of the three. If you can design your protocol, use one of the methods described above, such as the "Start" and "Stop" codes. Prefix and suffix the data with known codes to signify the beginning and ends of a data packet. Then when data comes in, buffer it and just check the buffer. The example below is with strings since they are generally easier to work with.
   

   // We'll use Regular Expressions to match the data 'start'/'stop' tokens using System.Text.RegularExpressions;
   
   // Used to buffer the incoming serial data
   private string ComBuffer = "";
   
   private void OnComm(object sender, EventArgs e)  //  MSCommLib OnComm Event Handler {
       // Add to the buffer the incoming data 
       ComBuffer += (string) com.Input;
   
       // Example regular expression test string
       // string ComBuffer = "trash---Hello World===trash---How Are You?===trash";
   
       // Build a regular expression to match data that
       // starts with '---' and ends with '==='
       Regex r = new Regex("---.*?===");
       
       // Cycle through the matches
       for (Match m = r.Match(ComBuffer); m.Success; m = m.NextMatch())
       {
           // Display the result to the 'Output' debug window
           Console.WriteLine(m.Value);
           
           // Remove the find from the string buffer
           ComBuffer = ComBuffer.Replace(m.Value, "");
       }
   }
   
       
   
   
   
   
   
   

   
   
2. Time Interval Packets, Using a Timer
   If you know that data will be coming in at defined time intervals, say there is always at least a second before the next set of data, then use a timer method. If you used a loop you could easily tie up system resources. Again you will need to buffer the data.
   

   // Use the system timers library using System.Timers;
   
   // Used to buffer the incoming serial data
   private string ComBuffer = "";
   
   // Create a new timer for serial data watching
   private Timer tmrWaitData = new Timer();
   
   // You will need to initialize the timer
   private void ExtraInitCode()
   {
       // Set the timer interval to one second
       tmrWaitData.Interval = 1000;
     
       // Attach the timer tick event handler
       tmrWaitData.Elapsed += new ElapsedEventHandler(tmrWaitData_Elapsed);
   }
   
   private void OnComm(object sender, EventArgs e)  //  MSCommLib OnComm Event Handler {
       // Check to see if data is waiting
       if (com.InBufferCount > 0) 
       {
           // Add to the buffer the incoming data 
           ComBuffer += (string) com.Input;
           
           // Reset the timer
           tmrWaitData.Stop();
           tmrWaitData.Start();
       }
   }
   
   private void tmrWaitData_Elapsed(object sender, ElapsedEventArgs e)
   {
       // Show to the 'Output' window the data that was received
       Console.WriteLine("Data Packet: " + ComBuffer);    
       
       // Stop the timer now that this data packet
       // has been successfully received
       tmrWaitData.Stop();
   }
   
   
   
   
   
   
   
   

   
   
3. Limit the Incoming Buffer Trigger
   If you know that data will always come in at a fixed amount, for example 10 characters, you can change the com.RThreshold value to trigger the OnComm event only when that amount has been received. Normally you would want OnComm to be triggered whenever any data is received, but you can change this if you are careful that all your data comes in the same amount of bytes/characters.