From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from snark.thyrsus.com (static-71-162-243-5.phlapa.fios.verizon.net [71.162.243.5]) by huchra.bufferbloat.net (Postfix) with ESMTP id 73ADF200BFD for ; Wed, 14 Mar 2012 21:24:04 -0700 (PDT) Received: by snark.thyrsus.com (Postfix, from userid 1000) id 6370340617; Thu, 15 Mar 2012 00:23:43 -0400 (EDT) Date: Thu, 15 Mar 2012 00:23:43 -0400 From: "Eric S. Raymond" To: Patrick Maupin Message-ID: <20120315042343.GA32483@thyrsus.com> References: <20120314104920.697EA40617@snark.thyrsus.com> <20120314184258.GA30210@thyrsus.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Organization: Eric Conspiracy Secret Labs X-Eric-Conspiracy: There is no conspiracy User-Agent: Mutt/1.5.21 (2010-09-15) Cc: thumbgps-devel@lists.bufferbloat.net Subject: Re: [Thumbgps-devel] USB handshake signals and Linux X-BeenThere: thumbgps-devel@lists.bufferbloat.net X-Mailman-Version: 2.1.13 Precedence: list Reply-To: esr@thyrsus.com List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 15 Mar 2012 04:24:04 -0000 Patrick Maupin : > The latency timer on the FTDI parts can be dropped to a millisecond. > If we find a USB dongle that has a USB converter with similar > capabilities and a GPS chipset that has rock-solid first-character > timing, we might avoid needing the RS232 handshake altogether. There's a reason I'm a bit wary of a non-1PPS design based on timing the leading edge of the $GPZDA or whatever. That is this: unless the device is self-identifying, there'll be no way to know when the time information can be trusted. Remember that the bufferbloat deployment is planned to use GPSD and that we *cannot* assume that GPSD knows anything hand-configured about the USB devices it talks to. By "self-identifying" that it ether (a) has a recognizable USB vendor/ID pair, (b) ships a unique sentence type that GPSD can recognize, or (c) there's a probe string GPSD can ship it that elicits a unique response. For reasons of history and vendor stupidity most GPS-chip/adapter combos will fail all three tests. One of the advantages of 1PPS is that it is self-identifying in this way. If you're talking to a GPS, nothing but 1PPS produces DCD or RI transitions, so when you see a 1PPS pulse you know what it is. -- Eric S. Raymond