Fast-20 timing

Tak Asami Asami x7621 asami at
Sun Dec 4 15:45:18 PST 1994

Excuse me for asking an obvious question, but I am looking for an obvious
For Fast-20, I suppose the synchronous transfer period (i.e., REQ to REQ
or ACK to ACK interval) is 50nsec minimum, correct?
Where does it say that in Fast-20 working draft X3T10/1071D rev 1?
I don't see it anywhere except in foreword stating "20, 40, or 80 megabytes...".
On SPI document, in section 9 before Table 10 (that's "SCSI bus timing
values") starts, there is a paragraph of text that basically says 100nsec
to 200nsec is fast, and 200nsec and above is slow.
That text is not there for Fast-20 document, and I want it in.
Actually, I want it expanded for more details.
When you say "cycle time" is 50nsec, which edge is it referring to?
I think it's down going edge, since that's when the data is supposed
to be latched.
If so, is the waveform below legal for REQ or ACK signal?
                        15nsec 15nsec
       |<-- 35nsec ->|<-   ->|<-   ->|<-- 35nsec ->|
_______|             |_______|       |_____________|
       \_____________/       \_______/             \___
Excuse teletype graphics...  From falling edge to falling edge it's
50nsec, but from rising edge to rising edge, it's 30nsec.
I think this is still legal, but something to watch out for, during
the implementation.
Any takers??
Tak Asami ==========================================================
Western Digital Corp.
I/O Product Engineering
(714) 932-7621 : Voice
(714) 932-6496 : Fax
asami at

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