From mboxrd@z Thu Jan 1 00:00:00 1970 From: Grant Edwards Subject: Re: Enabling auto RS485 half-duplex control Date: Thu, 18 Dec 2008 15:51:36 +0000 (UTC) Message-ID: References: <200812172224.13608.mfuchs@ma-fu.de> <20081218033200.GB9871@mit.edu> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Return-path: Received: from main.gmane.org ([80.91.229.2]:39823 "EHLO ciao.gmane.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750917AbYLRPvu (ORCPT ); Thu, 18 Dec 2008 10:51:50 -0500 Received: from list by ciao.gmane.org with local (Exim 4.43) id 1LDLA8-0008SP-It for linux-serial@vger.kernel.org; Thu, 18 Dec 2008 15:51:44 +0000 Received: from 64.251.14.41 ([64.251.14.41]) by main.gmane.org with esmtp (Gmexim 0.1 (Debian)) id 1AlnuQ-0007hv-00 for ; Thu, 18 Dec 2008 15:51:44 +0000 Received: from grante by 64.251.14.41 with local (Gmexim 0.1 (Debian)) id 1AlnuQ-0007hv-00 for ; Thu, 18 Dec 2008 15:51:44 +0000 Sender: linux-serial-owner@vger.kernel.org List-Id: linux-serial@vger.kernel.org To: linux-serial@vger.kernel.org On 2008-12-18, Theodore Tso wrote: > On Wed, Dec 17, 2008 at 10:24:13PM +0100, Matthias Fuchs wrote: >> Hi, >> >> some Exar UARTs like the XR16C2850 support a feature >> called auto RS485 half-duplex control. In this mode the >> RTS line has a special role when using the UART for >> RS485 communication. > > So I haven't maintained the serial driver for a long time, but > when I did, I refused to try to implement hardware half-duplex > control because there was absolutely no standards in this > space. Which RS-232 lines are involved wasn't standardized, Yes there are. The standard is that RTS is asserted when one whishes to transmit (and de-asserted when one has finished transmitting). Optionally, one waits for CTS to be asserted before transmitting. Isn't that the definition of RTS and CTS that's in the standard? > and there was often very strange timing restrictions as well. > Some devices required there to be a minimum delay of XX > milliseconds between when RTS is raised and when DTR is > raised. Other devices have a maximum time between when one > side raises RTS and the others side raises DTR. I've been doing half-duplex stuff for 30 years, and I've never seen DTR used in half-duplex flow control. > So the challenge is designing an interface for half-duplex > which is sufficiently general that it will actually work for > all/most of the half-dozen people in the world who still care > about half-ruplex RS-232 communications. :-) -- Grant Edwards grante Yow! I'm in direct contact at with many advanced fun visi.com CONCEPTS.