FAST-20 Question

Tak Asami Asami x7621 asami at
Mon Jan 23 14:16:20 PST 1995

Ronald DeRemer of IBM says:

>  Perhaps a better wording would be:

>   e) Ioh (high-level output current > 22 mA (signal negated) @
>      Voh = 2,0 V. d.c.

I'm just so happened to be looking at the same line, and trying to interpret.
My question is even more fundamental.

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?

If the way it is in draft is correct, that there is a good reason to do so,
the here's the interesting question.
Most (if not all) Fast-20 device operates both in Fast-20 domain as well as
fast and slow SCSI domain.  Is it implied that we have to switch the driver
parameter depending on which operatin a device is performing at the time?

I appreciate some help on this subject.

Tak Asami =================================================================
Western Digital Corp.
I/O Product Engineering
asami at
(714) 932-7621 : Voice
(714) 932-6496 : Fax

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