Using Circuits.UART, how do you read a specific amount of bytes?

bmitc

For Circuits.UART, I am looking for a way to read a specific amount of bytes where the number of bytes to read may vary from call to call. This is a common use case needed for when certain messages return a specific amount of bytes in the response but the number of bytes in the response may differ from message to message.

For example, I expected to be able to do something like this:

iex> {:ok, pid} = Circuits.UART.start_link
iex> Circuits.UART.open(pid, "COM2", speed: 115200, active: false)
iex> Circuits.UART.write(pid, "some message expecting 3 bytes in response\r\n")
iex> {:ok, three_byte_response} = Circuits.UART.read(pid, 3, 3000)
iex> Circuits.UART.write(pid, "some message expecting 5 bytes in response\r\n")
iex> {:ok, five_byte_response} = Circuits.UART.read(pid, 5, 3000)

This isn’t possible right now because the read/2 of course doesn’t have a number of bytes to read parameter. I originally expected there to be something like read/3 which would accept a specific number of bytes to read.

It seems a possible alternative is to create a framer behaviour, but that has two downsides: (1) the data isn’t actually framed, (2) this would require constantly calling configure like Circuits.UART.configure(pid, number_of_bytes: rx_framing_timeout: 500) before every write and read pair. I’m not entirely sure of the consequences of doing (2), but it’s the only possible way I could see doing this with the framing feature (if it would even work).

I think the Circuits.UART module should have a read/3 function like read(pid, bytes_to_read, timeout). This is quite common in the serial communication world:

So I’m curious, how have people handled this with Circuits.UART?

I asked this question a while back on the Circuits GitHub repo, but got a non-answer.

2 comments

/nerves #circuits_uart

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2023-12-07 08:58:46 UTC

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Sebb

I do not use circuits, but from the docs it seems, that this is not supported. So you either have to change the other side to a framed protocol or change circuits.uart or write your own - which is not too hard if you do not aim to be as comfortable as circuits.uart.

Open a Port — Elixir v1.12.3 and just take the bytes from the uart and write them to stdout (and vice versa).

I did this for a project of mine and it seems to work but its not battle tested. If you feel up to it I could make the code public for you to get it.

Note: I use a framed protocol, so you’d have to add the functionality you need. But my uart-adapter has just 200loc and should be easy to handle.

To give an idea how this works:

...

static void
forward(int const from, int const to, int const err_code)
{
  static char   buf[BUFSIZ];
  ssize_t const size = read(from, buf, BUFSIZ);
  require(size > 0, err_code + ERR_READ_ERROR_);
  ssize_t const write_size = write(to, buf, size);
  require(write_size == size, err_code + ERR_WRITE_ERROR_);
}

int
main(int argc, char* argv[])
{
  f_uart = config_uart(argc, argv);

  int const maxfd = f_uart + 1;
  fd_set    fds;

  for (;;) {
    FD_ZERO(&fds);
    FD_SET(f_uart, &fds);
    FD_SET(STDIN_FILENO, &fds);

    select(maxfd, &fds, NULL, NULL, NULL);

    if (FD_ISSET(f_uart, &fds)) {
      forward(f_uart, STDOUT_FILENO, ERR_FWD_STDOUT_FAILED_);
    }
    if (FD_ISSET(STDIN_FILENO, &fds)) {
      forward(STDIN_FILENO, f_uart, ERR_FWD_STDIN_FAILED_);
    }
  }

and on the Elixir side:

 def handle_info({_port, {:data, data}}, state = %{subscriber: subscriber}) do

    if subscriber do
      send(subscriber, {:data, data})
    end

    {:noreply, state}
  end

...

  @impl true
  def handle_cast({:write, data}, %{port: port} = state) do
    Port.command(port, data)
    {:noreply, state}
  end

...

Post #1

bmitc

Thank you for the response! I apologize for never responding, although I did read it at the time.

I haven’t been working on this project but may start it up again sometime in the new year. I will probably navigate away from Elixir Circuits, at least Ciruits.Uart since it doesn’t seem to be broadly supported for broad use cases.

In this case, as in most use cases of RS-232 and such communication, I have no control over the device’s serial protocol. One often wants to communicate with an external device that is delivered by someone else. I am considering writing my own Port library, likely using .NET’s cross-platform serial library as a starter.

Post #2

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