From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-vx0-f171.google.com (mail-vx0-f171.google.com [209.85.220.171]) (using TLSv1 with cipher RC4-SHA (128/128 bits)) (Client CN "smtp.gmail.com", Issuer "Google Internet Authority" (verified OK)) by huchra.bufferbloat.net (Postfix) with ESMTPS id B0B5620036C for ; Sat, 10 Mar 2012 10:40:55 -0800 (PST) Received: by vcbfl15 with SMTP id fl15so4801153vcb.16 for ; Sat, 10 Mar 2012 10:40:54 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=mime-version:date:message-id:subject:from:to:content-type :content-transfer-encoding; bh=yLeEoHqxsq0RpgCWboyNHfepsU4UFikzgNJW5GVosT4=; b=QDouhaj/V9M6SSS5vP4x+ksJM12PJu61kckBOun5caX6wGk3gA8WTDMKN25/eudCLK 5HrltocS9ClWQEdHg6nsQIY4wKM1hjGAgro9lKza9q2kKAJw5Vo9HvSGIA+TiZ/rkNiT mwGFMzcutuB7tWo1Yn6a9HM7VqKJcVp1WfQumjCgkjtUMq/32y1FtyKo4ijvOJnQM5+w 48n8vIqmCm+VmNuJ76A8vKIIIygI1gnpeNKsYAQxkl+UgZiribb1M1ULywTh0Am1C9zU 4jiMe1QqNmHJ+ZudpNeHn0AW7hBSqFEKc2Q5b6FF0qw5AXoM0/vxCc86MNQwGCqQ78fb 13sg== MIME-Version: 1.0 Received: by 10.52.36.226 with SMTP id t2mr10248202vdj.52.1331404854651; Sat, 10 Mar 2012 10:40:54 -0800 (PST) Received: by 10.220.140.132 with HTTP; Sat, 10 Mar 2012 10:40:54 -0800 (PST) Date: Sat, 10 Mar 2012 12:40:54 -0600 Message-ID: From: Patrick Maupin To: thumbgps-devel@lists.bufferbloat.net Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: quoted-printable Subject: [Thumbgps-devel] Responses to comments from Dave Taht X-BeenThere: thumbgps-devel@lists.bufferbloat.net X-Mailman-Version: 2.1.13 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sat, 10 Mar 2012 18:40:56 -0000 This is a response to comments made by Dave on esr's blog here: http://esr.ibiblio.org/?p=3D4171&cpage=3D1#comment-374799 > A full uart emulation chip for usb is just slightly > more expensive than a 4 wire one. yes but that may be irrelevant because even the 4 wire one will do what we need (none of the GPS modules use CTS, and the FTDI will interrupt on a change in CTS, and you don't have to use RTS/CTS flow control.) > Which I think sort of begins to explain why the PPS > devices have been vanishing from the planet. No, I think there are three reasons: (a) most people don't care, and (b) wobbling the RS232 control signals could actually confuse some software. Many fewer support calls to not connect it up to the converter. I said three reasons and I listed two. What's (c)? Well, I could be all wet, but I've worked for chip manufacturers most of my adult life now, and I know how they think. Building chips is like flying a plane. The first seat costs a lot, the rest are practically free. Like airlines, at some point when the market gets big enough, manufacturers like to try to extract the maximum value that each customer will pay. If this means only supporting PPS for extra money, this will happen. If this means making sure that the customers who don't buy the PPS version can't extract that information from the RS232 message timing, THEY MIGHT ACTUALLY GO OUT OF THEIR WAY TO WOBBLE THE RS232 MESSAGE TIMING AROUND. This is not conspiracy theory talking. This is experience from being around where you can make something for $1 and your largest customer will only give you $1.50 but others might give you $3.00. It is in your interest to make sure that, if the $1.50 product leaks on the open market, it doesn't perform as well as the $3.00 product. Blow a fuse, attach an extra bond wire, have a different firmware load, whatever. A really cheap to implement modification can mask the fact that, underneath, the two parts came from the same silicon wafer. Case in point. I used to work for a company that made a DAA (phone interface for voice and modems). They wanted to price-differentiate the market into 1200/2400 baud and above, so they actually added hardware to add distortion to the cheap part to make it not work so well at 56K. Their goal is to achieve economies of manufacturing scale, and keep as much of that as surplus as possible. Our goal is to engage in arbitrage to figure out which devices will work well enough for us. > It does strike me as kind of odd [that there is not a native GPS device c= lass] Yeah, that's a lot of engineering AND political work. Plus, personally, I think there are almost too many device classes. Unlike the internet, USB was defined as a monolithic whole. Sure, it's been updated multiple times since, but I believe there are device classes that probably nobody uses. > USB2 has several modes of =91native=92 operation that seem more > suitable=85 interrupts, and isochronous mode. =91Interrupts=92 are > interesting in that they aren=92t actually interrupts, but are polled for= . Yes, that's how all the RS232 devices send data into the host. > isochronous mode supports data streams of up to 480Mbit Not quite. That's the total bus bandwidth including all overhead... > and individual =91packets=92 of up to 1024 bytes. Which might be dropped. isochronous is analogous to UDP in that there are no guarantees. So it's actually good for timestamps, bad for content other than real-time streaming stuff where being late is as bad as not being delivered.