I have a similar task, read/write to a MODBUS fieldbus (RS485).

I can successfully read with this function:

let rec really_read ?(timeout = 0.5) fd buffer start length =
  let open Unix in
  if length <= 0 then () else begin
    match Unix.select [ fd ] [] [] timeout with
    | [], [], [] -> raise (Timeout timeout)
    | fdl, _, _ -> begin
        match read fd buffer start length with
        | 0 -> raise End_of_file
        | r -> really_read fd buffer (start + r) (length - r)
      end
  end

The code in which I use really_read looks like this:

  Unix.write ctx.descriptor packet 0 packet_len |> ignore;
  let buffer = String.make 2048 (Char.chr 0) in
  let timeout = 1.0 in
  let expected_bytes = 13 in
  really_read ~timeout ctx.descriptor buffer 0 expected_bytes;

Before reading/writing I set a bunch of terminal variables:

let modbus_connect device_name baud csize parity cstopb =
  let open Unix in
  let descriptor = openfile device_name [ O_RDWR; O_NOCTTY; O_NONBLOCK; O_EXCL; O_CLOEXEC; ] 0o660 in
  let old_terminal_settings = tcgetattr descriptor in
  let slave_id = None in
  let ctx = { device_name; baud; parity; csize; cstopb; descriptor; old_terminal_settings; slave_id; } in

  let term_settings = tcgetattr descriptor in
  term_settings.c_obaud <- baud;
  term_settings.c_ibaud <- baud;
  term_settings.c_cread <- true;
  term_settings.c_clocal <- true;
  term_settings.c_csize <- csize;
  term_settings.c_cstopb <- cstopb;
  let () = match parity with
    | Parity_None -> begin
        term_settings.c_inpck <- false;
        term_settings.c_parenb <- false;
        term_settings.c_parodd <- false;
      end
    | Parity_Even -> begin
        term_settings.c_inpck <- true;
        term_settings.c_parenb <- true;
        term_settings.c_parodd <- false;
      end
    | Parity_Odd -> begin
        term_settings.c_inpck <- true;
        term_settings.c_parenb <- true;
        term_settings.c_parodd <- true;
      end in
  term_settings.c_icanon <- false;
  term_settings.c_echo   <- false;
  term_settings.c_echoe  <- false;
  term_settings.c_echok  <- false;
  term_settings.c_echonl <- false;
  term_settings.c_isig   <- false;
  term_settings.c_ixon   <- false;
  term_settings.c_ixoff  <- false;
  term_settings.c_opost  <- false;
  term_settings.c_vmin   <- 0;
  term_settings.c_vtime  <- 0;

  let () = tcsetattr descriptor TCSANOW term_settings in
  ctx

ctx is a type in which I keep the context for the connection.

Hope this may help.


--
Paolo