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Ken Thompson kthompso at smtplink.wichitaks.ncr.com
Mon Oct 3 12:20:46 PDT 1994


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From: Ken Thompson <kthompso at smtplink.wichitaks.NCR.COM>
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Subject: Re[2]: SCAM and Differential


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Subject: RE: SCAM and Differential

I discussed SCAM and the properties of differential signals with Bill 
Ham over a year ago.  Based on both engineering design knowledge and 
what Bill has seen/measured in the lab, SCAM should work just fine in
differential.  When a signal is released, it may take a while (relatively 
speaking, perhaps a few microseconds) for the terminators to pull it to
a negated state, but it will occur.  And since the SCAM protocol is totally 
asynchronous it will work, albeit perhaps slightly slower than in a singel 
ended environment.  Oh yes, the receivers need a little hysterisis to
avoid oscillation on the slow negation edge, but apparently existing 
receivers already have this.

So, differential SCAM should work, whether it is practical is another matter. 
Everyone will make up their own mind as to whether SCAM is useful in a 
differential environment.  However, to the best of my knowledge every single 
SCSI protocol chip does not provide sufficient control of differential 
transceivers to implement SCAM.  In particular they are missing the ability 
to disable active negation and operate in an acctive-assert / passive-release 
/ wired-or mode.  Thus SCAM should work in differential, but there is no 
existing protocol silicon that can implement it, and I'm not aware of anyone 
developing any.

Ed Gardner

I think he is wrong here.  We have individual enables for each data line on the 
NCR53c825 (I do not have a 7xx book handy).
Ken Thompson





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