SerialPort (RS-232 Serial COM Port) in C# .NET

This article is about communicating through the PC's Serial COM RS-232 port using Microsoft .NET 2.0 or later by using the System.IO.Ports.SerialPort class.  If you're looking for USB info, see here.

Way in the Past...

Back in my early (pre C# MVP) days, I had written an article on DevHood titled Serial COM Simply in C#. It became quite popular and is (at the time of writing this follow up post) the top Google search for "serial c#". Now every week I get several e-mails with people asking questions and it is high time for some serious updating.

The article was (originally) written soon after .NET was introduced to the world, and other .NET serial port controls had not yet been written. So at the time, this was an easy solution. Just use the MSComm.ocx control from VS6 which most Devs had at the time. Now however, there are many easier and preferred methods than dealing with the complexities of interoping with an old legacy (non-.NET) ActiveX OCX control.

  

A Bright Future is Here!

Today, the primary solution is to use the new SerialPort control that is part of .NET 2.0 and is freelyavailable in C# Express on MSDN. It is considerably easier to use and is a true .NET component.

Example Application & Sample Code: SerialPort Terminal

SerialPortTerminal.zip	via ClickOnce

For loyal fans of the tutorial, I've written a sample code application which you can try out to see how the SerialPort class is used.  This requires Visual Studio 2008 & .NET 3.5 or later to compile & run.  It is just a simple little application with very basic support for text or binary (hex) modes to send and receive data.  A nice feature or two is auto-detection of installed COM ports that update at runtime if you plugin or remove those USB-to-Serial adapters, also you can change the DTR and RTS levels and monitor the CTS, DSR, and CD lines.

Build Note: You will receive an error that Visual Studio isn't able to find NoahCode.pfx.  This is expected as it is the click-once certificate for publishing and is NOT NEEDED for normal code use.  Just go to Project Properties > Signing > Click on Create Test Certificate.  that’s it

  

Continued Support

Due to the high volume of e-mail I've been receiving on serial port communication in general, I've wrote this post and added an FAQ section to the bottom of the post.  Check this first if you have questions.  If you post a question as a comment that hasn't been answered in the post, I'll add it to the FAQ section. 

For those of you who I've directed to this blog post, please understand that I'm just another guy working away at a full time job (helping manage the release of Team System) who has worked with serial RS-232 ports in the past (moving on to USB now). I like to help people, hence I wrote the original article, this sample code, and the FAQ, but do not have the resources to answer additional serial port communication questions. Please check out the SerialCom FAQ for other .NET COM Port solutions and technical support options.

  

Get Connected Up

You can obtain USB to Serial adapters and have just about as many ports on your PC as you like. I carry around two adapters with a null modem (what is it?, search Froogle) between them so I can create a loopback to send & receive through to separate ports on most any computer. I'd recommend doing the same for when writing code for the serial port.

If you'd like to quickly and easily create your own external devices to communicate with the PC, I recommend starting with the Parallax BASIC Stamp modules. Parallax has lots of easy accessories(such as LCDs, RF, Sounds, AD & DA, etc) for their modules. After that you could migrate to an Atmel Microcontroller (recommended) or Microchip PIC.

  

Let's Have the Code

Here is an example of how easy it is to use the new SerialPort control.

Very simply, here is how you can send a bit of data out the port.

// This is a new namespace in .NET 2.0 // that contains the SerialPort class using System.IO.Ports; private static void SendSampleData() { // Instantiate the communications // port with some basic settings SerialPort port = new SerialPort( "COM1", 9600, Parity.None, 8 , StopBits.One); // Open the port for communications port.Open(); // Write a string port.Write("Hello World" ); // Write a set of bytes port.Write(new byte[] {0x0A, 0xE2, 0xFF}, 0, 3 ); // Close the port port.Close(); }

Now let's take a look at what it takes to read data in from the communications port. This demonstrates reading text; for an example of reading binary data, see my SerialPortTerminal.zipsample app.

1. Create a new "Console Application" and replace all the default class code with this code
2. Add a reference to "System.Windows.Forms" to the project
3. Run w/ F5, to exit the app, press Ctrl-Break.
4. Get Connected Up with two USB to Serial adapters and a null modem
5. Use another app, the code above, or the SerialPortTerminal.zip example to send data and watch it come in with this code

#region Namespace Inclusions using System; using System.IO.Ports; using System.Windows.Forms; #endregion namespace SerialPortExample { class SerialPortProgram { // Create the serial port with basic settings private SerialPort port = new SerialPort("COM1" , 9600, Parity.None, 8 , StopBits.One); [STAThread] static void Main(string [] args) { // Instatiate this class new SerialPortProgram(); } private SerialPortProgram() { Console.WriteLine("Incoming Data:" ); // Attach a method to be called when there // is data waiting in the port's buffer port.DataReceived += new SerialDataReceivedEventHandler(port_DataReceived); // Begin communications port.Open(); // Enter an application loop to keep this thread alive Application.Run(); } private void port_DataReceived(object sender, SerialDataReceivedEventArgs e) { // Show all the incoming data in the port's buffer Console.WriteLine(port.ReadExisting()); } } }

One of the (several) new methods that is supported, and one I'm very glad is finally here, is the ability to obtain a list of the COM ports installed on the computer (ex: COM1, COM2, COM4). This is definately helpful when you want to present the list of ports avalible for the user to select from (as in the SerialPortTerminal.zip Win App example).

foreach (string s in SerialPort.GetPortNames()) System.Diagnostics.Trace.WriteLine(s);

Here are two helpful little methods for sending files through the serial port. Of course, these are the bare essentials and as always, you should check to make sure the port is open first (port.IsOpen) and use try/catch around trying to open a file, but you get the gist with this code. The binary sending routine is limited to about 2GB (the size of an int), but this should be okay for most uses. 

using System.IO; private static void SendTextFile( SerialPort port, string FileName) { port.Write(File.OpenText(FileName).ReadToEnd()); } private static void SendBinaryFile( SerialPort port, string FileName) { using (FileStream fs = File.OpenRead(FileName)) port.Write((new BinaryReader(fs)).ReadBytes( (int)fs.Length), 0, (int )fs.Length); }

  

RS-232 Project Photos

Each of these involve RS-232 serial port communications.

Just what's needed to get started with microcontrollers,  a Basic Stamp, mini LCD display, power, and RS-232 port.

Two USB to Serial adapters with a null modem  to loopback and test your serial software.

The brains to a mini automated radio station that let me  control my PC & home using my HAM radio from around town.

Port Wiring Notes

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 /

      ---------                              ---------



DB9 Female to DB9 Female Null-Modem Wiring

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

---- ---- ---- ---- ---- ---- ---- 

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



9-pin   25-pin  Assignment                 From PC

------  ------  -------------------------  ------------

Shield  1       Case Ground                Gnd

1       8       DCD (Data Carrier Detect)  Input

2       3       RX  (Receive Data)         Input

3       2       TX  (Transmit Data)        Output

4       20      DTR (Data Terminal Ready)  Output

5       7       GND (Signal Ground)        Gnd

6       6       DSR (Data Set Ready)       Input

7       4       RTS (Request To Send)      Output

8       5       CTS (Clear To Send)        Input

9       22      RI  (Ring Indicator)       Input



- RTS & DTR are binary outputs that can be manually set and held

- DCD, DSR, CTS, and RI are binary inputs that can be read

- RX & TX can not be set manually and are controlled by the UART

- maximum voltages are between -15 volts and +15 volts

- binary outputs are between +5 to +15 volts and -5 to -15 volts

- binary inputs are between +3 to +15 volts and -3 to -15 volts

- input voltages between -3 to +3 are undefined while output voltages

  between -5 and +5 are undefined

- positive voltages indicate ON or SPACE, negative voltages indicate

  OFF or MARK

Other Resources

Here are some additional sites, libraries, tutorials, etc. These are links that I just found around the net and am providing for convenience (they are not endorsed).

• RS-232 on Wikipedia
• SerialPort on MSDN
• Search on Google #1, #2, #3
• CFSerialIO
• OpenNETCF Port

Where CF = Compact Framework

  

The Final Say (aka Conclusion)

The new SerialPort class in .NET 2.0 rocks! It is much easier to use than getting the old MSComm.ocx control going in a .NET app, contains new functionality, is a 'native' .NET control, has docs built into the MSDN Library, and is easy to use.

Frequently Asked Questions (FAQ)

I'm adding this section (as of 8/10/06) to address the common questions I get on this post and through e-mail.  Chances are, if you ask a good question in the comments here, I'll post it here for others to see easily. Some of the most popular topics are usb, device, cable, lines, power, packets,bridge, permissions, help.

1. Q: When updating a control (like a text box) while in the DataRecieved event, I get an error. 
   A: The SerialPort class raises events on a separate thread than the main form was create on.  Windows Forms controls must be modified only on their original thread.  Thankfully there is an easy way to do this.  Each Windows control has a "Invoke" method which will run code on the control's original thread.  So to put the recently received data into a text box (txtLog), this would do it:   txtLog.Invoke(new EventHandler(delegate { txtLog.Text += comport.ReadExisting(); });   You can see this more in action in the "Log" event of "Terminal.cs" my sample code project, SerialPortTerminal.zip. 
2. Q: I can't find the System.IO.Ports namespace. 
   A: Using Visual Studio 2003?  The new namespace, and SerialPort class, is part of .NET 2.0 and Visual Studio 2005.  It is not included in .NET 1.x (and Visual Studio 2003).  Even if you have .NET 2.0 or Visual Studio 2005 installed, you can not access the class from within Visual Studio 2003. 
3. Q: I only have .NET 1.1, what can I do? 
   A: Upgrade to .NET 2.0.  Seriously, it's free.  In fact, you can get the great C# and VB Visual Studio Interactive Development Environment (IDE) editors for FREE now with C# Express andVB Express.  The .NET 2.0 Software Development Kit (SDK) for command-line development is also free.  If you really must stay in .NET 1.1, you can use the technique I talk about in Serial COM Simply in C# or a 3rd party library. 
4. Q: I'm sending data to my device, but it is not responding. 
   A: First make sure the device will respond using a standard app like Hyperterminal.  Check the settings (baud rate, data bits, stop bits, etc) and make sure they match with your code.  Try sending binary data via binary arrays.  Many devices expect a carriage return at the end of a command, so be sure to send 0x0D or \n.  String data can be easily converted to a binary array using: 
   byte[] data = System.Text.ASCIIEncoding.Default.GetBytes("Hello World\n"); 
   com.Write(data, 0, data.Length); 
   Many devices require several carriage returns first to sync baud rates, so send several, like: com.Output("".PadLeft(9, '\n'));  It you're communicating with a modem, make sure Echo Back is turned on.  Send "ATE1\n".  Other than this, just keep trying and playing around with it.  It can be hard because you don't see the response from the device as easily as you would with a terminal app. 
5. Q: When I put received data to a text box or rich text box, I get a strange symbols. 
   A: The default font of text boxes is designed only to show standard characters.  Try using "CharMap" (a free tool in WinXP, click "Start", "Run", type "CharMap", enter).  "Terminal" is a font designed to show classic ASCII characters and is what most terminal apps (like my sample code and Hyperterminal) use.  There are also many ASCII codes that won't display correctly.  This is why I choose to show the hex data instead of an ASCII string a lot of the time.  System.Convert.ToString(mybyte, 16) will convert a byte to a string hex code, for example: byte b = 13; string s = Convert.ToStrong(b, 16).PadLeft(2, '0'), then s will contain "0D".  See the "ByteArrayToHexString" and "HexStringToByteArray" methods in my sample app,SerialPortTerminal.zip. 
6. Q: What about USB communications?  How can I do USB? 
   This blog post isn't about USB.  Believe me, I wish the .NET framework supported USB natively, and I'm doing what I can here at Microsoft to see USB get into the framework in the future.  For now, you can use a USB to Serial adapter.  I use a lot of these.  They plug into the USB port, then appear just as a SerialPort to the PC.  
   
   For native USB devices, microcontroller vendors such as Microchip, Atmel, and TI make chips that do this for projects as well as have chips that support USB natively (I'm using thePIC18F2455).  There is a lot of info and USB C# libraries readily available online with a quick search, try USB C#, which turns up resources like this great article or this USB HID C# open source project.  
   
   For simple USB-to-Serial bridges that allow your device to communicate via RS-232 but connect via USB to the PC, check out the section below on USB-to-Serial Bridge Chipsets 
7. Q: Can I use the sample code here in my own projects (commercial or not)? 
   Yes!  All sample code on my blog is free public domain material.  I'm not a legal guy so I don't know the exact words to use, but I'll try...  I'm not responsible for any problems!  Use at your own rick etc.  However, have at it, if it helps you out, fantastic, that's what it's here for. 
8. Q: When using SerialPort.Open() (or SerialPort.PortOpen = true) I get the exception "UnauthorizedAccessException" or the error "An unhandled exception of type 'System.Runtime.InteropServices.COMException' occurred in axinterop.mscommlib.dll" 
   It may be one of a few factors: 
   
   • It may require Administrative privileges to use the port.
   • The port may already be open by another program, only one app can use a port at a time.
   • The port may not exist on the computer (this happens a lot).  Verify the port you're trying to open is listed in the Device Manager (FAQ #9).
   • The name being provided is not exactly correct. 
     Use the full name of the port when opening, like "COM1" (not "1") 
9. Q: How do I know what COM ports are on my PC? 
   Use the Device Manager ("Start", "Run", "devmgmt.msc") and look for the "Ports" node (see below).  If you don't see a Ports node, it's because there are no Serial or Parallel ports installed in the PC.  You can also use System.IO.Ports.SerialPort.GetPortNames() to return the list of available ports.  Many laptops these day's don't have a serial port.  You can get more serial ports very easily today with USB to Serial adapters. 
   
    
10. Q: How do I communicate with my specific device?  Modem, Mobile Phone, LED/LCD Display, Scanner, GPS, GSM/GPRS, CDMA, test equipment, etc 
    This post is specific to device independent serial port communications.  RS-232 is a hardware standard and there are hundreds (if not thousands) of software protocols that are created for communicating over RS-232.  An analogy would be that physical telephone hardware is used around the globe (like the RS-232 hardware standard), but there are hundreds of languages people can speak through the phone to each other to communicate (the software protocol).  You will need to find information about the protocol used for your specific device elsewhere.  I'd recommend looking on the manufacture's website, writing/calling the manufacture, or searching online for your specific device.  Sometimes you can find fans or hackers of a device that congregate on a forum.  Some protocols can be found through wikipedia, like the GPSNMEA 0183 serial protocol that GPS units use to send their data via a serial connection.  Honestly the RS-232 communications is the easy part, I personally find the more interesting challenge is writing software for the particular protocol in use.  Best of luck in finding specifications on the protocol you’re device uses! 
11. Q: What control lines can I manually manipulate? 
    Control lines on an RS-232 serial port were introduced to provide flow control of serial data.  Even though a modem or PC may be rated for a particular baud rate, it doesn’t always mean that the device can process the data that fast.  For example, bits may flow in through a modem at 2400 bps (bits per second) but in the early days a PC couldn’t process the data fast enough, so it would toggle control lines to tell the modem that it was read for more data or to ‘pause’ send it data.  Now since PCs are quite fast these lines are mostly ignored.  They can be used as a simple way to toggle an LED or send or receive high or low states. RTS/CTS control flow is one such technique which is supported natively by the control via the Handshake property. 
    
    Lines marked ‘Output’ in the table can be set by the serial port to a high or low state (True or False) and the ‘Input’ lines can be read as high or low.  You can use the PinChanged event to be notified when one of the input pins changes.  The Handshake property can be set to use RTS/CTS (uses hardware control lines) or XON/XOFF (uses software) or both. 
    
    

    Control Line	Name	DB9 Pin	SerialPort Property	I/O
    DTR	Data Terminal Ready	4	DtrEnable	Output
    RTS	Request to Send	7	RtsEnable	Output
    CTS	Clear to Send	8	CtsHolding	Input
    DSR	Data Set Ready	6	DsrHolding	Input
    CD (or DCD)	Data Carrier Detect	1	CDHolding	Input

    
    For detecting a Ring Indication, see the next question. 
12. Q: How do you detect the Ring Indicator line? 
    There isn’t a property off of the SerialPort class to test the current state of the Ring Indicator (DB9 pin 9), but there is an event that gets triggered, PinChanged.   You can attach an event listener to PinChanged and when the event is called, SerialPinChangedEventArgs is passed with a EventType property of type SerialPinChange which will indicate if a ring indication was detected.  Here is an example: 
    
    

    comport.PinChanged += newSerialPinChangedEventHandler(comport_PinChanged);

    void comport_PinChanged(object sender, SerialPinChangedEventArgs e)

    {

      if (e.EventType == SerialPinChange.Ring) RingDetected();

    }

13. Q: What pins can I use for powering devices, a high signal, or for boolean input & output? 
    The TX & RX pins carry the standard serial signal, but the other pins can be used as high/low input/output pins.  The output pins (4 DTR or 8 CTS), supply 5 to 15 volts (15v is proper RS-232 standard) when high and 0 to -15 volts when low.  They only supply flea current so they're not meant to be used for powering any devices (like USB is designed for).  However, they can be used as a reference voltage or for switching to one of the input pins for a high or low signal.  The input pins (1 DCD, 6 DSR, 8 CTS, and 9 RI) can be used to detect a high or low signal.  Proper RS-232 signal levels are -15v for a low and +15v for a high (compared to ground, pin 5).  A MAX232 or similar chip takes a TTL 0-5v input and produces the -15v to +15v levels.  However, most PC RS-232 COM ports will accept a 0v for low and 5v for high, but it is not guaranteed and alters from PC to PC.  If you want a simple "toggle high", just hold pin 4 DTR high, and switch it to pin 1 DCD.  The .NET SerialPort class has easy to use properties for switching the output pins high or low and for detecting the current level of the input pins.  I have been able to use pin 4 DTR for a very low current (20ma max) PIC processors, but not reliably.  I prefer to always supply external power and use pin 4 as a signal to turn on or off my device.  I'll attach pin 4 to a transistor that switches my power source to my PIC to turn it on or off. 
14. Q: What about ‘packets’?  Does RS-232 support any commands or data segregation?  OR  Data comes in at seemingly random times? 
    Serial data flow through RS-232 has nothing to do with ‘packets’.  It’s just a stream of bytes in and out.  There is no guarantee that data arrives together. 
    
    Packet Protocols 
    Any notion of data compartmentalization (packets) would have to be coded by you for your unique use.  Much of my time working with serial has been spent on defining useful packet like protocols, that usually include some type of header, command structure, and CRC check.  For example, the first two bytes are the packet length, the next two bytes is the command, next two bytes are parameters, and the last byte is a CRC.  Then my apps would buffer incoming data and look in the buffer for valid packets.  Of course it differs depending on the device you’re working with and your specific needs.  USB does have specific communications protocol defined, one of them being command based, like the little packet just mentioned.  But with USB, you’re able to get the whole command and parameter together at once, with serial you have to create the protocol yourself, buffer, and parse the data.  My previous serial post has more info under the header “Protocol Development”. 
    
    Buffering Incoming Data 
    Since bytes may come in at any time, buffering incoming data is critical.  For example, you may send a command out to your device, and the response back to the PC could trigger a single DataReceived event with all the 30 bytes of response data in the receive buffer.  Or more likely, it could be any number of separate triggers of the DataReceived (up to the number of bytes received), like 4 triggers, first with 2 bytes, then 15 bytes, then 1 byte, then 12 bytes.  Don’t look for a complete response in a single DataReceived call, instead: 
    1. buffer the incoming data 
    2. then scan your buffer to find complete data 
    3. remove the used data from the buffer 
    
    To buffer incoming data, use a coding pattern like this: 
    (download the code SerialComBuffering.zip) 
    

    using System;

    using System.IO.Ports;

    using System.Collections.Generic;

    namespace SerialComBuffering

    {

      class Program

      {

        SerialPort com = new SerialPort(SerialPort.GetPortNames()[0],

          9600, Parity.None, 8, StopBits.One);

        List<byte> bBuffer = new List<byte>();

        string sBuffer = String.Empty;

        static void Main(string[] args)

        { new Program(); }

        Program()

        {

          com.DataReceived +=

            new SerialDataReceivedEventHandler(com_DataReceived);

          com.Open();

          Console.WriteLine("Waiting for incoming data...");

          Console.ReadKey();

        }

        void com_DataReceived(object sender,

          SerialDataReceivedEventArgs e)

        {

          // Use either the binary OR the string technique (but not both)

          // Buffer and process binary data

          while (com.BytesToRead > 0)

            bBuffer.Add((byte)com.ReadByte());

          ProcessBuffer(bBuffer);

          // Buffer string data

          sBuffer += com.ReadExisting();

          ProcessBuffer(sBuffer);

        }

        private void ProcessBuffer(string sBuffer)

        {

          // Look in the string for useful information

          // then remove the useful data from the buffer

        }

        private void ProcessBuffer(List<byte> bBuffer)

        {

          // Look in the byte array for useful information

          // then remove the useful data from the buffer

        }

      }

    }

                                                 
15. Q: How do you detect when a device is connected or disconnected? 
    Simply put, the device usually starts or stops sending data.  There are no built in events when a device connects or disconnects.  But there are a few tricks you can do, if you’re creating the serial device yourself, then you have more options.  I’ve had my devices (and PC apps) send a constant “are you there” set of bytes, like 00 00 11 11 00 00 hex (I’d use a ‘are you there’ custom ‘packet’ as in Q13 above) till a device on the other end responds.  You could also use hardware, that’s what some of the other signals lines are for (CDC, DSR, RTS, CTS, RI), you can tie one of these high and then catch an event when the line goes high so you know there’s a device there, when it goes low the device is gone. 
     
16. Q: How do I transfer files? 
    If you’re interested in sending entire files between computers or to a device, you have two primary choices.  First choice is to write your own protocol for breaking a file into manageable packets, detecting errors, sending/receiving, etc.  Second choice is to use a protocol already created, like XModem, YModem, ZModem, etc.  These protocols were created long ago specifically to send files over a serial connection with features like error correction, being able to resume transfers, compression, etc.  I’m only aware of commercial implementations for .NET right now, like Sax Software’s CommStudio or Scientific Component’s PortController.NET.  If anyone find an open source implementation, please let me know! 
17. Q: Where can I get USB-to-Serial cables? 
    USB-to-Serial adapter cables allow you to get a standard RS-232 DB9 cable by plugging in the adapter to your USB port.  You can use several of these to get multiple DB9 ports.  Many laptops, even desktops, don’t have serial ports these days which makes these particularly handy. 
      
    Every so often I source a good cable for a decent price, typically around $10.  Here is the deal I’m currently using. http://www.iofast.com/product_info.php/products_id/4681 
      
    Be forewarned, not all cables are created equally!  There are two issues to contend with…
    1. There are a number of companies that create USB chipsets that emulates an RS-232 port.  Some are much more compatible with PCs and Macs than others.  For example, the Prolific and FTDI chipset are used in the most compatible adapters (unlike Ark Micro, or others).  Those companies produce WHQL (Windows Hardware Quality Labs) certified drivers and the drivers are built into most Windows and Mac systems already.  So when shopping for a cable, be sure to check the chipset manufacture.  Here are some handy links to drivers in case you need them.
       1. Prolific Chipset Drivers
       2. FTDI Chipset Drivers
    2. Standard RS-232 signals range from 3 to 15 volts (plus or minus), 0 is not a valid signal.  The USB-to-Serial interface chips used in these cables produce 0 to 5 volts, standard TTL level logic.  While this works for some PCs, and certainly most microcontrollers, it is not 100% compatible.  The ‘correct’ USB-to-Serial cables incorporate an additional line level driver, like the MAX232, that take the 0 to 5 volts and convert it to an appropriate +/- 12 volts for RS-232.  The problem is that many low-quality cables exist out there without this line level driver chip. 
18. Q: How can I best interface with my microcontroller?  USB-to-Serial Bridge Chipsets 
    If you are building your own devices, usually with some type of microcontroller, and want to interface with the PC, you can certainly use RS-232 either plugged straight into your PC or via a USB-to-Serial adapter cable, but another option is to use an USB-to-Serial bridge chipset directly in your project (what is inside of the USB-to-Serial adapter cables).  This allows your device to use RS-232 internally, but connect to a computer via USB and you as a project designer never need worry about the USB interface or drivers (your device and PC software just see serial RS-232).  You could use a native USB microcontroller (without an RS-232 bridge), such as the PIC18F2450, but that requires writing your own USB drivers and a much deeper understanding of the USB infrastructure (both in hardware and software).  This article is focused on serial RS-232, so I’m covering the USB-to-Serial bridges here. 
      
    These modern ICs don’t require external crystals, can power your device from USB Power, provide clock sources for your micro if you like, and come with Windows Certified (WHQL) drivers that are actually included in Windows Vista and beyond (also built in Mac drivers).  Here are a few…
    1. FTDI FT232RL 
       The FTDI chips are most popular in electronics projects and used by companies like Parallax and the Arduino Duemilanove.  They also have the best support in terms of evaluation modules, well written datasheet, and product availability. 
       Links: product info, datasheet, evaluation modules, buy from Mouser, drivers 
    2. Prolific PL-2303HX 
       These are the chips used in the most popular USB-to-Serial cables.  They are slightly less costly than the FTDI chips in bulk, but are not quite as well supported in the US market.  
       Links: product info, datasheet, buy from Saelig, drivers 
    3. TI TUSB3410 
       Another option from Texas Instruments.  I haven’t used this chip.  Product Info
    p.s. For trying it out and prototyping, I personally recommend this particular FTDI evaluation board, the UM232R since it plugs into typical solderless protoytpe breadboards and has most of the pins easily accessible.  Last priced it was $20 from Mouser. 
    
     
19. Q: May I have permissions to use your code? 
    Yes!  The code and samples here are released under the MIT License.  
20. Q: How can I get more support?  What are my support options?

• Read this FAQ section!  :) 
• You can try leaving a comment below.  Perhaps someone knows the answer
• Read the official MS documentation, MSDN on SerialPort
• See if the article Wikipedia on RS-232
• Try doing a search and look for other content.  " SerialPort C#", "Serial Com C#", "serial port .net"
• Post a question in the .NET Base Class Library in MSDN Forums