One more agenda item (actually a bunch) for SPI-2 meeting

RMoore at RMoore at
Wed Oct 30 14:21:00 PST 1996

* From the SCSI Reflector (scsi at, posted by:
* RMoore at
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Text: (24 lines follow)

I have another item for the agenda. I think there is an error in the 
thresholds specified in Fig. 24, Definition of timing measurement points.

For signals crossing from high to low, I think the upper threshold should be 
30 mV and conversely, for signals crossing from low to high, the lower 
threshold should be -30 mV. Reason: the +/- 60 mV is the dynamic threshold 
and as such represents the point at which the receiver MUST switch, at the 
latest. This limit is incorporated in the definitions of Vtan and Vtna. For 
a setup or hold measurement we want the interval between the time at which 
the first signal's transition MUST be seen (latest threshold) to the point 
at which the second signal's transition MAY be seen (earliest threshold). 
The second transition, since it is from a (more or less) steady-state value, 
will not be seen until the 30 mV (falling edge) or -30 mV (rising edge) 
level is reached, since these are the steady-state sensitivity levels 
defined in Table 19. The 30 mV steady-state sensitivity should guarantee 
that a steady-state signal of 62 mV will not cause the receiver to switch if 
noise drives the voltage down to 58 mV, for example -- so we cannot use +/- 
60 mV for these reference points.

	Richard Moore
	Adaptec Irvine Technology Center

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