README.md

# Nerves.UART

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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)
* 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](https://github.com/freemed/tty0tty) virtual
  null modem on Travis)

Something doesn't work for you? Check out below and the
[docs](https://hexdocs.pm/nerves_uart/). Chat with other users on the nerves
channel on the [elixir-lang slack](https://elixir-slackin.herokuapp.com/), or
file an issue or PR.

## Example use

Discover what serial ports are attached:

```elixir
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:

```elixir
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.

```elixir
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:

```elixir
iex> Nerves.UART.write(pid, "Hello there\r\n")
:ok
```

See if anyone responds in the next 60 seconds:

```elixir
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:

```elixir
iex> Nerves.UART.configure(pid, active: true)
:ok

iex> flush
{:nerves_uart, "COM14", "a"}
{:nerves_uart, "COM14", "b"}
{:nerves_uart, "COM14", "c"}
{:nerves_uart, "COM14", "\r"}
{:nerves_uart, "COM14", "\n"}
:ok
```

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

```elixir
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`

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 `nerves_uart`
to do it for us:

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

This tells `nerves_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:

```elixir
iex> flush
{:nerves_uart, "COM14", "abc"}   # Note that the "\r\n" is trimmed
:ok
```

If your serial data is framed differently, check out the `Nerves.UART.Framing`
behaviour and implement your own. `Nerves.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:

```elixir
iex> Nerves.UART.configure(pid, rx_framing_timeout: 500)
:ok

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

iex> flush
{:nerves_uart, "COM14", {:partial, "A"}}
```

## Installation

To install `nerves_uart`:

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

  ```elixir
  def deps do
    [{:nerves_uart, "~> 1.0.0"}]
  end
  ```

  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:

```sh
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 `nerves_uart` from `hex.pm`, you'll need MinGW
to compile the C code. I use [Chocolatey](https://chocolatey.org/) and install
MinGW by running the following in an administrative command prompt:

```sh
choco install mingw
```

On Nerves, you're set - just add `nerves_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 `nerves_uart` on your host machine, the easiest way is to
either clone the source or add `nerves_uart` as a dependency to a regular
(non-Nerves) Elixir project.

## Building and running the unit tests

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

```sh
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,

```sh
export NERVES_UART_PORT1=ttyS0
export NERVES_UART_PORT2=ttyS1
```

If you're on Windows or Linux, you don't need real serial ports. For linux,
download and install [tty0tty](https://github.com/freemed/tty0tty). Load the
kernel module and specify `tnt0` and `tnt1` for the serial ports. Check the
`tty0tty` README.md, but this should looks something like:

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

export NERVES_UART_PORT1=tnt0
export NERVES_UART_PORT2=tnt1
```

On Windows, download and install
[com0com](https://sourceforge.net/projects/com0com/) (Look for version 2.2.2.0
if the latest hasn't been signed). The ports on Windows are `CNCA0` and `CNCB0`.

Then run:

```sh
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.

## 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/nerves_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](https://jpnevulator.snarl.nl/) 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 Nerves.UART communicate with the serial port?

Nerves.UART uses a [Port](https://hexdocs.pm/elixir/Port.html) 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](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.