RE: XAUI IO specs
Ali,
I think this statement just adds to the confusion again.
The fact is that these ARE intended to be differential inputs to a
differential receiver which is just like A-B on the scope (except for the
fact that the scope is supposed to be linear). A differential measurement is
made as A-B. There is no other way. To think of these as a pair of receivers
is wrong. This is a differential input!!!
For a logical 1 measurement you get A = +200 mV; B = -200 mV; A-B = +400
mV_diff
For a logical 0 measurement you get A = -200 mV; B= +200 mV; A-B = -400
mV_diff
The peak to peak (logic 1 - logic 0) is +400 mV_diff - -400 mV_diff = +800
mV_pp-diff.
It is just that simple. Draw a picture if you must, but it is just that
simple.
If these numbers are different than you intend, then simply change them.
Don't muck with the definition!
jonathan
p.s. just one address on the note please.
> -----Original Message-----
> From: ghiasi [mailto:Ali.Ghiasi@Eng.sun.com]
> Sent: Friday, April 28, 2000 9:27 AM
> To: kdemsky@us.ibm.com; stds-802-3-hssg@ieee.org; t11_2@dpt.org;
> sbrorson@nexabit.com
> Cc: Ali.Ghiasi@Eng.sun.com
> Subject: RE: XAUI IO specs
>
>
>
> 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
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> >
> > > 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
> >
>