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SerialPortStream is an independent implementation of System.IO.Ports.SerialPort and SerialStream for better reliability and maintainability. Default branch is 2.x and now has support for Mono with help of a C library.
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SerialPortStream is an independent implementation of System.IO.Ports.SerialPort and SerialStream for better reliability and maintainability, and now for portability to Mono on Linux systems.

The SerialPortStream is a ground up implementation of a Stream that buffers data to and from a serial port. It uses low level Win32API (on Windows) or Posix (on Linux) for managing events and asynchronous I/O, using a programming model as in the MSDN PipeServer example.

These notes are for version 2.x, which is a new design based on version 1.x that enhances portability and fixes bugs. See the end of these notes for differences.

Why another Serial Port implementation?

Microsoft and Mono already provides a reasonable implementation for accessing the serial port. Unfortunately, documentation is sparse. When one tries to find information about how to program the serial port one comes across instead many blogs and forums describing the issues that they've observed.

Through the implementation of SerialPortStream, I've used ILSpy to reverse engineer how the Microsoft implementation works, discovering many other subtle, but noteworthy implementation issues.


This project tries to achieve the following:

  • An implementation similar to the MS implementation of SerialPort. It's not meant to be 100% compatible, but instead provide similar functionality
  • Abstract the driver implementation and provide for a more reliable transport, by making writing serial data completely buffered. With the MS implementation, one can write data, but subsequently needs to check if all data is written or not. If it isn't written, then it needs to be retried. The SerialPortStream makes this easier.
  • Provide for reliable and consistent behaviour. See the next section.

Issues with MS Serial Port

The SerialPortStream tries to solve the following issues observed:

  • Zach Saw describes issues regarding behaviour of the fAbortError flag in the Serial DCB. The SerialPortStream defines this flag.
  • Closing a serial port, then reopening it generally causes problems. The SerialPortStream shouldn't have this issue.
  • The ReadTo() implementation can subtly change the byte stream buffer, when one switches from characters to bytes. This problem occurs because the MS implementation actually converts the characters back to bytes into its buffer. So if you have UTF8, decoded some invalid characters and then have a timeout, this results in the invalid characters being converted back to bytes, resulting in "lost" data. I take some care when decoding bytes to characters to ensure a seamless and accurate transition between bytes and characters.
  • Write() gives the data to the serial port. If the operation is asynchronous, the callback results in the number of bytes that were actually transferred to the driver. You need to check yourself if this is valid or not. In the synchronous case, the data is simply thrown away. The SerialPortStream method simply copies data to a local buffer and uses asynchronous writes in a different thread. It works hard in the background to send out the data you provided. If the data can't be sent, then you get a TimeoutException without any data being buffered at all. So you can implement reliable protocols and your code is simpler.
  • Disposing or Closing the serial port during a blocking write operation will not abort the write operation. This implementation will abort with an System.IO.IOException type.

System Requirements


Software has been tested and developed using:

  • .NET 4.5 on Windows 7 x86 and x64.
  • .NET 4.5 on Windows 8 Pro x64 and Windows 8.1 Pro x64.
  • .NET 4.6.x on Windows 10 Pro x64
  • Mono from Xamarin on Ubuntu 14.04 (32-bit) and 16.04 (64-bit)

I use this software for automation in another system (Windows) that runs for multiple days and it appears stable. The implementation for Mono is relatively new and has been tested with a battery of unit and component tests. It probably needs more testing in real world environments though.

See later in these notes for known issues and changes.


Mono Framework

You should use the latest version of Mono. Version 3.2.8 has significant bugs and will not work (Ubuntu 14.04 ships with this). Use the latest version of Mono that comes direct from Xamarin instead of your distribution where possible.

For instructions on how to install the latest Mono for your system, refer to Install Mono On Linux.

Microsoft Compact Framework

SerialPortStream is not designed for the Compact Framework.

Untested, but should work:

Theoretically it should work on the following, but it hasn't been tested.

  • .NET 4.0 on Windows XP
  • .NET 4.0 on Windows 7

If you have feedback that it works on these platforms, please let the author know! They can then update the documentation for the benefit of others.


Version 1.x

You can download the release for 1.x, which contains binaries compiled against Framework version 4.0 and 4.5. You can also install the NuGet package at See the v1.x branch for specific details and a 1.x specific version of these notes.

Version 2.x

There are no releases at this time. NuGet packages are difficult for non-Windows platforms due to the large number of Linux distribution variants. The preferred mechanism for Linux is to compile the code from sources.

I'm currently investigating making Debian packages for easy installation on Ubuntu and similar components.


On Windows, just reference the assembly in your project as you did with version 1.x.


You first need to compile the support library for your platform. To do that, you'll need a compiler (e.g. GCC 4.8 or later) and cmake.

After cloning the repository, execute the following:

$ git clone
$ cd serialportstream/dll/serialunix
$ ./

Binaries are built and put in the bin folder from where you ran the build script. You can add a reference to LD_LIBRARY_PATH to the library:

$ export LD_LIBRARY_PATH=`pwd`/bin/usr/local/lib:$LD_LIBRARY_PATH

and then run your Mono program from there.

Or you can build and install in your system:

$ cd serialportstream/dll/serialunix
$ mkdir mybuild
$ cd mybuild
$ cmake .. && make
$ sudo make install

Extra Features

The following features are in addition to the System.IO.Ports.SerialPort implementation:

  • You can obtain the RingIndicator pin status.
  • The Read() and Write() buffers are completely independent of the low level Windows driver.

Reading and Writing - Buffering

Why is it interesting to perform buffering? A driver might be configured to be 4096 or 8192 bytes (which is quite typical). Testing with the PL2303 chipset, one can't write more than about 12KB with a single write operation.

A Write buffer may be 128KB, which one writes to. The thread in the background will write the data and issue as many write calls as is necessary to get the job done. A Read buffer may be 5MB. The background thread will read from the serial port when ever data arrives and buffers into the 5MB.

So long as the I/O thread in .NET an execute every 100-200ms, it can continue to read data from the driver. Your own application doesn't need to keep to such difficult time constraints. Such issues typically arise in Automation type environments where a computer has many different peripherals. So long as the process doesn't block, your main application might sleep for 10 seconds and you've still lost no data. The MS implementation wouldn't be so simple, you have to make sure that you perform frequent read operations else the driver itself might overflow (resulting in lost data).

Improvements and Fixes over SerialPortStream 1.x

The following bug fixes have been made on top of SerialPortStream v1.x.

Windows and Mono:

  • Better clean up (Open now properly closes the native serial port if there was a problem starting the monitoring thread).
  • Fix conversion conversion of UTF8 to two UTF16 characters. In some cases, there might be sequences of bytes that actually convert to two UTF16 characters. Version 1.x would raise an exception. This version handles that scenario correctly.
  • Correct race conditions when using the character methods.
  • Flush() now raises a TimeoutException when flushing isn't completed in time. This may be a breaking change.
  • Properties CDHolding, CtsHolding, DsrHolding, RingHolding now return false if the serial port is not open, instead of raising an exception.
  • DtrEnable and RtsEnable can now be set without raising an exception if handshaking is enabled, the result is the property is shadowed. This provides more logical behaviour.
  • Trace resources now better handled. Having multiple SerialPortStream instances open may result in tracing stopping when another object is disposed.
  • Moved over to the nUnit test framework to be compatible with MonoDevelop IDE as well as Visual Studio 2012-2015 that support IDE plugins.

Windows Only:

  • The EOF event is now properly raised.

Code in general has been flattened, simplified to make it easier to read without losing functionality from v1.x.

Known Issues


The following issues are known:

  • This is not an issue, but when using the Com0Com for running unit tests, some specific test cases for Parity will fail. That is because Com0Com doesn't emulate data at a bit level.
  • .NET 4.0 to the currently tested .NET 4.6 has a minor bug in System.Text.Decoder that in a special circumstance it will consume too many bytes. The PeekChar() method is therefore a slower implementation than what it could be. Please refer to the Xamarin bug 40002. Found against Mono and later tested to be present since .NET 4.0 on Windows XP also.

Driver Specific Issues on Windows

Flow Control

Using the FTDI chipset on Windows 10 x64 (FTDI dated 09/Mar/2016) flow control (RTS/CTS) doesn't work as expected. For writing small amounts of data (1024 bytes) with CTS off, the FTDI driver will still send data. See the test case ClosedWhenFlushBlocked, change the buffer from 8192 bytes to 1024 and the test case now fails. This problem is not observable with com0com 3.0. You can see the effect in logs, there is a TX-EMPTY event that occurs, which should never be there if no data is ever sent.

Mono on non-Windows Platforms

Ubuntu 14.04 ships with Mono 3.2.8. This is known to not work.

  • [Mono-Dev] Mono 3.2.8 incompatibility with .NET 4.0 on Windows 7-10. The System.Text implementation for converting bytes to UTF8 don't work. If you don't use the character based methods, it may work. But the software has not been tested against this framework.
  • The DataReceived event doesn't fire for the EOF character (0x1A). On WIndows it does, as this is managed by the driver itself.
  • The test case for opening two serial ports simultaneously on Mono fails, meaning that it's possible to open the same device twice, which on Windows raises an UnauthorizedAccessException().
  • ListPorts is not implemented on Mono and uses the SerialPort implementation.
  • Mono (tested) has a minor bug in System.Text.Decoder as in the .NET references, that in a special circumstance it will consume too many bytes. The PeekChar() method is slower when this bug is detected. Please refer to the Xamarin bug 40002. Found against Mono and later tested to be present since .NET 4.0 on Windows XP also.


SerialPortStream was tested on Ubuntu 14.04 and Ubuntu 16.04. Feedback welcome for other distributions!

The main functionality on Linux is provided by a support C library. The issues are observed:

  • Custom baud rates are not supported. To know what baudrates are supported on your system, look at the file config.h after building.
  • DSR and DTR handshaking is not supported. You can still set and clear the pins though.

Patches are welcome to implement these features!

Driver Specific Issues on Linux

Tests have been done using FTDO, PL2303H, PL2303RA and 16550A (some still do exist!). The following has been observed:

Parity Errors

Some chipsets do not report properly parity errors. The 16550A chipset works as expected. Issues observed with FTDI, PL2303H, PL2303RA. In particular, on a parity error, more bytes are reported as having parity errors than there are in the stream. Tested using loopback devices with 'comptest'.

./nserialcomptest /dev/ttyUSB0 /dev/ttyUSB1`
  [ RUN      ] SerialParityTest.Parity7E1ReceiveError
/home/jcurl/Programming/serialportstream/dll/serialunix/libnserial/comptest/SerialParityTest.cpp:221: Failure
Value of: comparison
  Actual: false
Expected: true
Unexpected byte received with Even Parity
[  FAILED  ] SerialParityTest.Parity7E1ReceiveError (585 ms)
[ RUN      ] SerialParityTest.Parity7O1ReceiveError
/home/jcurl/Programming/serialportstream/dll/serialunix/libnserial/comptest/SerialParityTest.cpp:373: Failure
Value of: comparison
  Actual: false
Expected: true
Unexpected byte received with Even Parity
[  FAILED  ] SerialParityTest.Parity7O1ReceiveError (584 ms)
[ RUN      ] SerialParityTest.Parity7O1ReceiveErrorWithReplace
/home/jcurl/Programming/serialportstream/dll/serialunix/libnserial/comptest/SerialParityTest.cpp:427: Failure
Value of: comparison
  Actual: false
Expected: true
Unexpected byte received with Even Parity
[  FAILED  ] SerialParityTest.Parity7O1ReceiveErrorWithReplace (572 ms)

Garbage Data on Open

On Linux Kernel with Ubuntu 14.04 and Ubuntu 16.04, we observe that some USB-SER drivers provide extra data depending on what a previous process was doing. It shows itself as garbage zero's appearing at the beginning of a stream when reading, and may be visible in your application also. There's a test case comptest/kernelbug that shows this behaviour on a Lenevo T61p. Affected is PL2303H and FTDI chipsets. Chipsets that don't show this behaviour are 16550A and PL2303RA chipsets. Invocate the test program twice and you'll see the error. This is reported to Ubuntu

$ kernelbug /dev/ttyUSB0 /dev/ttyUSB1
Offset: 4
Writing Complete...
Reading complete...
Comparison MATCH                    <---- PASS
Reading complete...

$ kernelbug /dev/ttyUSB0 /dev/ttyUSB1
Offset: 108
Flush 2 bytes
Writing Complete...
Reading complete...
ERROR: Comparison mismatch          <---- ERROR
Flush 510 bytes
Reading complete...

Monitoring Pins and Timing Resolution

Monitoring of pins CTS, DSR, RI and DCD is not 100% reliable for some chipsets and workarounds are in place. In particular, the chips PL2303H, PL2303RA do not support the ioctl(TIOCGICOUNT), so on a pin toggle, we cannot reliably detect if they have changed if the pulse is too short. For 16550A and FTDI chips, this ioctl() does work and so we can always detect a change. To check if your driver supports the TIOCGICOUNT ioctl() call, run the small test program comptest/icount.

$ ./icount /dev/ttyS0
Your driver supports TIOCGICOUNT

or in the case it's not supported:

$ ./icount /dev/ttyUSB0
Your driver doesn't support TIOCGICOUNT
  Error: 25 (Inappropriate ioctl for device)

Close Times with Flow Control

Some times closing the serial port may take a long time (observed from 5s to 21s) if it is write blocked due to hardware flow control. In particular, the C-library function serial_close() appears to take an excessive time when calling close(handle->fd) on Ubuntu 16.04. This issue appears related to the Linux driver and not the MONO framework.

The .NET Test Cases that show this behaviour are (blocking on write):

  • ClosedWhenBlocked
  • CloseWhenFlushBlocked
  • DisposeWhenBlocked
  • DisposeWhenFlushBlocked

This issue is not reproducible with the 16550A UART when it is write blocked. In this case, the times for closing are usually not more than 20ms.

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