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*