# Nerves.UART
[](https://travis-ci.org/nerves-project/nerves_uart)
[](https://ci.appveyor.com/project/fhunleth/nerves-uart/branch/master)
Nerves.UART allows you to access UARTs, serial ports, Bluetooth virtual serial
port connections and more in Elixir. Feature highlights:
* Mac, Windows, and desktop and embedded Linux
* Enumerate serial ports
* Receive input via messages or by polling (active and passive modes)
* Unit tests (uses the [tty0tty](https://github.com/freemed/tty0tty) virtual null modem on Travis)
** This library is new. Expect API changes (hopefully minor) and bugs, but we'll get there!! If you try it out, please consider helping out by contributing documentation improvements, fixes, or more unit tests. **
## Example use
Discover what serial ports are attached:
iex> Nerves.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 (www.arduino.cc)", product_id: 67, vendor_id: 9025}}
Start the UART GenServer:
iex> {:ok, pid} = Nerves.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 `Nerves.UART.enumerate/0` for what's
available on your system.
iex> Nerves.UART.open(pid, "COM14", speed: 115200, active: false)
:ok
This opens the serial port up at 115200 baud and turns off active mode. This means that
you'll have to manually call `Nerves.UART.read` 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> Nerves.UART.write(pid, "Hello there\r\n")
:ok
See if anyone responds in the next 60 seconds:
iex> Nerves.UART.read(pid, 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> Nerves.UART.configure(pid, active: true)
:ok
iex> flush
{:nerves_uart, "COM14", "a"}
{:nerves_uart, "COM14", "b"}
{:nerves_uart, "COM14", "c"}
:ok
It turns out that `COM14` is a USB to serial port. Let's unplug it and see what
happens:
iex> flush
{:nerves_uart, "COM14", {:error, :eio}}
Oops. Well, when it appears again, it can be reopened. In passive mode, errors
get reported on the calls to `Nerves.UART.read/2` and `Nerves.UART.write/3`
## Installation
If [available in Hex](https://hex.pm/docs/publish), the package can be installed as:
1. Add `nerves_uart` to your list of dependencies in `mix.exs`:
def deps do
[{:nerves_uart, "~> 0.0.1"}]
end
2. List `:nerves_uart` as an application dependency:
def application do
[applications: [:nerves_uart]]
end
3. Check that the C compiler dependencies are satisified (see below)
4. 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, you will need MinGW. Assuming that you installed Erlang and
Elixir via [Chocolatey](https://chocolatey.org/), install MinGW by
running the following in an administrative command prompt:
choco install mingw
## Building and running the unit tests
The standard Elixir build process applies. Clone `nerves_uart` or
download a source release and run:
mix deps.get
mix deps.compile
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,
export NERVES_UART_PORT1=ttyS0
export NERVES_UART_PORT2=ttyS0
If you're on Linux, you don't need real serial ports. Download and install
[tty0tty](https://github.com/freemed/tty0tty). Load the kernel module and
specify `tnt0` and `tnt1` for the serial ports.
Then run:
mix test
## FAQ
### A feature is missing
Yes, I haven't gotten to a couple really important ones for some use cases.
See `TODO.md` for now. Please ping me if you'd like to help.
### 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 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.
## Acknowledgments
When building this library, [node-serialport](https://github.com/voodootikigod/node-serialport)
and [QtSerialPort](http://doc.qt.io/qt-5/qserialport.html) 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.
