linux-kernel - Re: Serial port blues

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Date:	Sun, 21 Jan 2007 13:52:18 -0500
From:	Theodore Tso <tytso@....edu>
To:	Grzegorz Jaśkiewicz <gryzman@...il.com>
Cc:	Willy Tarreau <w@....eu>, Johannes Stezenbach <js@...uxtv.org>,
	Joe Barr <joe@...rimer.com>,
	Linux Kernel mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Serial port blues

On Sun, Jan 21, 2007 at 04:10:58PM +0100, Grzegorz Jaśkiewicz wrote:
> you're right, I used wrong term to describe.
> But the problem still exists. Nowadays it should be possible to run many
> serial ports fully accurate "at the same time". It is also true that the
> same problem exists in windows, and bsd worlds. So it is not only Linux
> problem.

It's not so much a "problem", but rather, most people aren't
particularly interested in using the millions and millions of
transisters of a modern CPU to generate a square wave.  So OS's aren't
optimized to do that.  Linux has no problems running many serial
ports; but in their normal designed function, which is to send and
receive characters, using the UART's FIFO's and interrupts to do so.  

The question of manually toggling DTR/CTS RS-232 lines to generate a
tone is something that you *can* do, but you won't be doing anything
else --- and that's simply because fundamentally that's a very silly
thing to do.  Most people will use a tiny amount of dedicated hardware
--- like the sound care that is built into every single modern PC ---
rather than manually waggling the RS-232 lines in order to generate a
tone.

> Like I said previously, 30$ board (usb+avr+max232) would do it accurately +
> over 300$ PC to control it :D funny...

You *can* do it, and we've described how to do it.  It won't be
efficient (you won't be doing much else), but that's because a PC and
Linux is optimized for different set of tasks.  Sometimes dedicated
hardware is the far superior option to a general purpose OS and a
general-purpose hardware.  This should not at all be surprising.

						- Ted
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