Discover and use UARTs and serial ports in Elixir
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Circuits.UART allows you to use UARTs, serial ports, Bluetooth virtual serial port connections and more in Elixir. Some highlights:

  • Mac, Windows, Linux, and Nerves
  • Enumerate serial ports
  • Receive input via messages or by polling (active and passive modes)
  • Add and remove framing on serial data - line-based framing included for use with GPS, cellular, satellite and other modules
  • Unit tests (uses the tty0tty virtual null modem on Travis)

Looking for Nerves.UART? Circuits.UART is the new name. Everything else is the same. Update your project by replacing all references to nerves_uart and Nerves.UART to circuits_uart and Circuits.UART and you should be good.

Something doesn't work for you? Check out below and the docs. Post a question on the Elixir Forum or file an issue or PR.

Example use

Discover what serial ports are attached:

iex> Circuits.UART.enumerate
%{"COM14" => %{description: "USB Serial Port", manufacturer: "FTDI", product_id: 24577,
    vendor_id: 1027},
  "COM5" => %{description: "Prolific USB-to-Serial Comm Port",
    manufacturer: "Prolific", product_id: 8963, vendor_id: 1659},
  "COM16" => %{description: "Arduino Uno",
    manufacturer: "Arduino LLC (", product_id: 67, vendor_id: 9025}}

Start the UART GenServer:

iex> {:ok, pid} = Circuits.UART.start_link
{:ok, #PID<0.132.0>}

The GenServer doesn't open a port automatically, so open up a serial port or UART now. See the results from your call to Circuits.UART.enumerate/0 for what's available on your system.

iex>, "COM14", speed: 115200, active: false)

This opens the serial port up at 115200 baud and turns off active mode. This means that you'll have to manually call to receive input. In active mode, input from the serial port will be sent as messages. See the docs for all options.

Write something to the serial port:

iex> Circuits.UART.write(pid, "Hello there\r\n")

See if anyone responds in the next 60 seconds:

iex>, 60000)
{:ok, "Hi"}

Input is reported as soon as it is received, so you may need multiple calls to read/2 to get everything you want. If you have flow control enabled and stop calling read/2, the port will push back to the sender when its buffers fill up.

Enough with passive mode, let's switch to active mode:

iex> Circuits.UART.configure(pid, active: true)

iex> flush
{:circuits_uart, "COM14", "a"}
{:circuits_uart, "COM14", "b"}
{:circuits_uart, "COM14", "c"}
{:circuits_uart, "COM14", "\r"}
{:circuits_uart, "COM14", "\n"}

It turns out that COM14 is a USB to serial port. Let's unplug it and see what happens:

iex> flush
{:circuits_uart, "COM14", {:error, :eio}}

Oops. Well, when it appears again, it can be reopened. In passive mode, errors get reported on the calls to and Circuits.UART.write/3

Back to receiving data, it's a little annoying that characters arrive one by one. That's because our computer is really fast compared to the serial port, but if something slows it down, we could receive two or more characters at a time. Rather than reassemble the characters into lines, we can ask circuits_uart to do it for us:

iex> Circuits.UART.configure(pid, framing: {Circuits.UART.Framing.Line, separator: "\r\n"})

This tells circuits_uart to append a \r\n to each call to write/2 and to report each line separately in active and passive mode. You can set this configuration in the call to open/3 as well. Here's what we get now:

iex> flush
{:circuits_uart, "COM14", "abc"}   # Note that the "\r\n" is trimmed

If your serial data is framed differently, check out the Circuits.UART.Framing behaviour and implement your own. Circuits.UART.Framing.FourByte is a particularly simple example of a framer.

You can also set a timeout so that a partial line doesn't hang around in the receive buffer forever:

iex> Circuits.UART.configure(pid, rx_framing_timeout: 500)

# Assume that the sender sent the letter "A" without sending anything else
# for 500 ms.

iex> flush
{:circuits_uart, "COM14", {:partial, "A"}}

Sometimes it's easier to operate with the pid of the UART GenServer rather than using the name of the port in active mode. An example of this is when you want to send an acknowledgment back after a receive and you are using more than one serial port at a time. You can do this with the id: :pid option to open/1 or configure/1.

iex> Circuits.UART.configure(pid, id: :pid)

# Assume some data was received

iex> receive do
...>   {:circuits_uart, pid, _} ->
...>     Circuits.UART.write(pid, "ack")
...> end


To install circuits_uart:

  1. Add circuits_uart to your list of dependencies in mix.exs:
def deps do
  [{:circuits_uart, "~> 1.2"}]
  1. Check that the C compiler dependencies are satisified (see below)

  2. Run mix deps.get and mix compile

C compiler dependencies

Since this library includes C code, make, gcc, and Erlang header and development libraries are required.

On Linux systems, this usually requires you to install the build-essential and erlang-dev packages. For example:

sudo apt-get install build-essential erlang-dev

On Macs, run gcc --version or make --version. If they're not installed, you will be given instructions.

On Windows, if you're obtaining circuits_uart from, you'll need MinGW to compile the C code. I use Chocolatey and install MinGW by running the following in an administrative command prompt:

choco install mingw

On Nerves, you're set - just add circuits_uart to your mix.exs. Nerves contains everything needed by default. If you do use Nerves, though, keep in mind that the C code is crosscompiled for your target hardware and will not work on your host (the port will crash when you call start_link or enumerate. If you want to try out circuits_uart on your host machine, the easiest way is to either clone the source or add circuits_uart as a dependency to a regular (non-Nerves) Elixir project.

Building and running the unit tests

The standard Elixir build process applies. Clone circuits_uart or download a source release and run:

mix deps.get
mix compile

The unit tests require two serial ports connected via a NULL modem cable to run. Define the names of the serial ports in the environment before running the tests. For example,


If you're on Windows or Linux, you don't need real serial ports. For linux, download and install tty0tty. Load the kernel module and specify tnt0 and tnt1 for the serial ports. Check the tty0tty, but this should looks something like:

cd tty0tty/module
sudo cp tty0tty.ko /lib/modules/$(uname -r)/kernel/drivers/misc/
sudo depmod
sudo modprobe tty0tty
sudo chmod 666 /dev/tnt*


On Windows, download and install com0com (Look for version if the latest hasn't been signed). The ports on Windows are CNCA0 and CNCB0.

Then run:

mix test

If you're using tty0tty, the tests will run at full speed. Real serial ports seem to take a fraction of a second to close and re-open. I added a gratuitous delay to each test to work around this. It likely can be much shorter.


Do I have to use Nerves?

No, this project doesn't have any dependencies on any Nerves components. The desire for some serial port library features on Nerves drove us to create it, but we also have host-based use cases. To be useful for us, the library must remain crossplatform and have few dependencies. We're just developing it under the Nerves umbrella.

How can I use the serial port on Linux without sudo?

Serial port files are almost always owned by the dialout group. Add yourself to the dialout group by running sudo adduser yourusername dialout. Then log out and back in again, and you should be able to access the serial port.

Debugging tips

If you're having trouble and suspect the C code, edit the Makefile to enable debug logging. See the Makefile for instructions on how to do this. Debug logging is appended to a file by default, but can be sent to stderr or another location by editing src/circuits_uart.c.

If you're on Linux, the tty0tty emulated null modem removes the flakiness of real serial port drivers if that's the problem. The serial port monitor jpnevulator is useful for monitoring the hardware signals and dumping data as hex byte values.

On OSX and Windows, I've found that PL2303-based serial ports can be flakey. First, make sure that you don't have a counterfeit PL2303. On Windows, they show up in device manager with a warning symbol. On OSX, they seem to hang when closing the port. Non-counterfeit PL2303-based serial ports can pass the unit tests on Windows 10, but I have not been able to get them to pass on OSX. FTDI-based serial ports appear to work better on both operating systesm.

ei_copy why????

You may have noticed Erlang's erl_interface code copy/pasted into src/ei_copy. This is only used on Windows to work around issues linking to the distributed version of erl_interface. That was compiled with Visual Studio. This project uses MinGW, and even though the C ABIs are the same between the compilers, Visual Studio adds stack protection calls that I couldn't figure out how to work around.

How does Circuits.UART communicate with the serial port?

Circuits.UART uses a Port and C code. Elixir/Erlang ports have nothing to do with the serial ports of the operating system. They share the same name but are different concepts.


When building this library, node-serialport and QtSerialPort where incredibly helpful in helping to define APIs and point out subtleties with platform-specific serial port code. Sadly, I couldn't reuse their code, but I feel indebted to the authors and maintainers of these libraries, since they undoubtedly saved me hours of time debugging corner cases. I have tried to acknowledge them in the comments where I have used strategies that I learned from them.