SCAM on LDV (round 2)

Tak Asami asami at itc.adaptec.com
Thu Oct 24 09:29:52 PDT 1996


* From the SCSI Reflector (scsi at symbios.com), posted by:
* Tak Asami <asami at itc.adaptec.com>
*
George Penokie said:
> What, no comments on SCAM being usable on LDV!!!  Did everyone leave the
> country or what.  Could it be that there just is no interest in SCAM on LVD.

Well, I don't have much comment on LDV, but I think I'll address a little
bit about LVD SCAM.
As you know, not all LVD signals are strictly differential; there is
(unless someone changed that while I'm not looking) such thing as passive
negation LVD.  This means only SIG+/SIG- pair is driving actively only 
when the signal is being asserted.  For negation, the signal pins are 
left tri-stated.
This type of arrangement is needed for some of the signals on the bus 
that is defined to be Wired-OR (such as BSY, SEL, RST).

This works only if the signal frequency is low and the bus settle time
qualification requirement is applied.
This is true for the above three signals, and it certainly can be true
for all data pins for SCAM protocols.  The hardware only needs to be 
able to support passive negation mode for data bus pins.

As far as I know, passive negation mode never existed for HVD, so someone
declared SCAM unworkable on HVD system some time ago.

I think SCAM is workable for LVD, as it is defined in SPI, as long as
the pins operate on passive negation mode.
NOT that I personally think it is important for SCAM to work for the
product acceptance, but that's another story.

But if those of you who is reading this reflector think I'm incorrect,
please advise.  I rather be corrected now than after I was burned.

Tak Asami =============================================================
Adaptec Inc / ITC MSE Architecture Group
9701 Jeronimo Rd  Irvine, CA 92618
Phone: (714) 455-8202
Fax:   (714) 455-8102
email: asami at itc.adaptec.com
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