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Date: Wed, 13 Aug 2014 09:23:09 +0200
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From: "Edouard Evangelisti" <edouardevangelisti@gmail.com>
X-Mailer: Sympa 6.1.17
Subject: [Caml-list] Unix library and serial port

Dear all,

I encountered a problem while trying to exchange informations with a ZStepper
Motor connected to the computer through a serial/USB adapter. I can send
commands and move the motor, but I cannot retrieve the informations which are
sent by the motor directly after completion. For this I am using the Unix
library as follows.

First of all, I get a read/write file descriptor :

let usb0 = "/dev/ttyUSB0"
let flags = Unix.([O_RDWR; O_NOCTTY; O_NONBLOCK])
let perm = 0o666
let fd = Unix.openfile usb0 flags perm

I then set the parameters using Unix.tcgetattr/Unix.tcsetattr (for instance
baud and parity check). Most of the default values are fine. Then I am able to
send commands :

let write_serial () =
 let cmd = "/2P1000R\r\n" in (* one command as  an example. *)
 let len = String.length cmd in
 let ret = Unix.write fd cmd 0 len in
 assert (ret = len); (* not the real code here for error management *)
 Unix.tcdrain fd

The call to write_serial triggers ZStepper motion as expected. Then, the motor
send back some data. I have tried to read them using :

let read_serial () =
   let len = 10 in
   let buf = Buffer.create len in
   let str = String.create len in
   let rec loop () =
   let n = Unix.read fd str 0 len in
   if n > 0 then (
     Buffer.add_substring buf str 0 n;
     loop ()
   ) else Buffer.output_buffer stdout buf in
   try loop () with Unix.Unix_error (Unix.EAGAIN, _, _) -> ()

I always got EAGAIN error and no data available. When using
Unix.in_channel_of_descr, I can retrieve some data but asynchronously.

I have tried to launch simultaneously my program and GtkTerm configured with
the same settings and GtkTerm is able to retrieve the informations sent by the
motor after completion. The source code of GtkTerm looks very similar to mine.
I probably do something wrong but I cannot see where is the problem.

Would you have an idea ?

Many thanks in advance for your precious help.