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*To*: kdemsky@us.ibm.com, stds-802-3-hssg@ieee.org, t11_2@dpt.org, sbrorson@nexabit.com*Subject*: RE: XAUI IO specs*From*: ghiasi <Ali.Ghiasi@Eng.sun.com>*Date*: Fri, 28 Apr 2000 09:26:58 -0700 (PDT)*Cc*: Ali.Ghiasi@Eng.sun.com*Reply-To*: ghiasi <Ali.Ghiasi@Eng.sun.com>*Sender*: owner-stds-802-3-hssg@ieee.org

Hi Sturat The intention of Hari was 800 mV differential p-p when you use the scope A-B. As you said this voltage is fictious and the receiver only sees 1/2 of swing. For the case of Hari XAUI the pick-pick swing across the base of transistor would 400 mV ((VCM+200mV) - (VCM-200mV)). Thanks, Ali Ghiasi > From: Stuart Brorson <sbrorson@nexabit.com> > To: kdemsky@us.ibm.com, stds-802-3-hssg@ieee.org, t11_2@dpt.org > Subject: RE: XAUI IO specs > Date: Fri, 28 Apr 2000 09:56:36 -0400 > MIME-Version: 1.0 > X-Resent-To: Multiple Recipients <stds-802-3-hssg@majordomo.ieee.org> > X-Listname: stds-802-3-hssg > X-Info: [Un]Subscribe requests to majordomo@majordomo.ieee.org > X-Moderator-Address: stds-802-3-hssg-approval@majordomo.ieee.org > > > > By your own statement below, a static Logic 1 is when Vo+ is 400 mV above > > Vcm, and Vo- is 400 mV below Vcm, then the differential voltage (Vo+) - > > (Vo-) is 800 mV (positive peak). > > By your own statement below, a static Logic 0 is when Vo+ is 400 mV below > > Vcm, and Vo- is 400 mV above Vcm, then the differential voltage is (Vo+) - > > (Vo-) is -800 mV (negative peak). > > Then peak to peak is (positive peak - negative peak) = 1600 mV, and not > > 800 > > mV as you have in your response. So, I agree with Mike Dudek, Ed Grivna, > > that your statement below is confusing. I am agreeable to any way of > > defining logic levels with or without common mode, single ended or > > differential. I agree with Ed Grivna that Vcm is not necessary to define > > a > > differential signal, and I am only responding to a thread that is using > > Vcm. > > > I recently had exactly this entire argument with several of my colleauges at > Nexabit Networks. Of course, we did not settle it, since the debate hinges > upon the definition of the term "differential voltage swing", and semantic > arguments can never be resolved. > > However, permit me to make one point about signal levels and swings before I > go back to lurking: From the standpoint of the receiver of the differential > signal, the important quantity is the voltage difference between the Vin+ > and Vin- inputs. The input stage of the receiver is almost always a diff > amp, and it is the difference | (Vin+) - (Vin-) | = 800 mV which appears > across the bases of the two input transistors. When a logic 1 is > transmitted, then the diff amp sees (Vin+) - (Vin-) = +800 mV between the > transistors' bases. When logic 0 is transmitted, the diff amp sees (Vin+) - > (Vin-) = -800 mV. As others have pointed out, the voltage difference > between these two logic states is 1600 mV. > > But the question is: what is the important voltage to quote, 800mV or 1600 > mV? As I see it, the phyiscally important voltage is 800 mV because this > is the signal amplitude which much defeat any physical imperfections at the > receiver input diff amp, e.g. input offset voltage, noise, etc. The 1600 mV > difference between logic 1 and logic 0 is a meaningless number since the > diff amp never sees any physical voltage of 1600 mV. > > The situation is different in single-ended logic, where the voltage > difference between logic 1 and logic 0 is actually seen by the input > amplifier; it is this voltage difference (e.g. 5V in old-fashoined TTL) > which must defeat the input amp imperfections. > > > Given all this confusion, it may be prudent to include pictures of logic > > signals, and define the swings in an already standardized way. > > > Agreed. The best thing to do is draw a picture. > > > Stuart Brorson > Nexabit Networks/Lucent Technologies >

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