FAST-20 Question

Lohmeyer, John JLOHMEYE at cosmpdaero.ftcollinsco.ncr.com
Mon Jan 23 16:30:00 PST 1995


Tak wrote:
>Why in the (bleep) does a driver have to supply 22mA at any time at all?
>This line in Fast-20 is different from the similar line in SPI that reads
>
>  "VOH < 3,0V at IOH = 20mA".
>
>This line was added to insure that an active negation driver won't burn
>out active terminators that may be present. This was no problem since most
>of the active negation drivers didn't source 20mA.
>I remember fighting against that line because it reads like a driver has to
>supply 20mA or more, but was assured that it didn't mean that at the time.
>
>But this line (6.1.2.e) in Fast-20 reads
>
>  "IOH > 22mA @VOH < 2,0Vd.c."
>
>which says a driver MUST supply 22mA when the output voltage is 2.0V or 
less.
>This is a total reversal of what SPI had.  Is there any rational for this?

Yes, there was.  From the June 2nd, 1994 Fast-20 working group minutes 
(94-131):

"Kevin Gingerich stated that a minimum current for active negation
also needs to be specified to assure that the first step to 2.0
volts is made in the face of a impedance mismatch at the middle
of the cable.  The calculation assumed a 3.24 high voltage, a 0.2
low voltage, a 20 ohm impedance mismatch on an 84 ohm impedance,
maintaining the 0.8 volt step.  This resulted in a 62 ma driver
requirement which was then reduced by the current input from the
active terminators.  The result is a 22 ma minimum active
negation strength driver is required at 2.0 v D.C. "

John

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