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

...