# Circuits.UART

[![Hex version](](
[![API docs]( "API docs")](
[![Build status](](
[![Build status](](

`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

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

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

## Installation

To install `circuits_uart`:

  1. Add `circuits_uart` to your list of dependencies in `mix.exs`:

  def deps do
    [{:circuits_uart, "~> 1.3"}]

  1. Check that the C compiler dependencies are satisified (see below)

  1. 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.

On MacOS, download and install [socat]( You can install it via Homebrew. Once you have it installed and ready to go, run the following command. You will need to change `<USERNAME>` to your current system username

sudo socat -d -d -d -d -lf /tmp/socat pty,link=/dev/dummy1,raw,echo=0,user=<USERNAME>,group=staff link=/dev/dummy2,raw,echo=0,user=<USERNAME>,group=staff

Once that opens, in a separate terminal emulator, set the Circuits ENVars, and go about your testing

export CIRCUITS_UART_PORT1=/dev/dummy1
export CIRCUITS_UART_PORT2=/dev/dummy2
mix test

## FAQ

### 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.

## Acknowledgments

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.