Dear Paolo and Anders,
Thank you for your messages. I tried to use Paolo's code and the exception Timeout was raised. I was not able to read char by char without O_NONBLOCK : the program is just waiting forever for something to read.
I was able to retrieve some data using Unix.in_channel_of_descr, but the data are not transmitted synchronously, which is required in my case. Thus, I will probably rewrite this part in C using the same strategy and see if it works. I cannot understand why this strategy works for HyperTerminal and not for me with the same material.
Thank you again.
Edouard