Comments on Ultra SCSI
Steve Finch
TFINCHS%A1.VAX2T.mrouter at vax2t.tus.ssi1.com
Fri Feb 11 09:28:04 PST 1994
Is Ultra SCSI for differential only? I think it will have to be, as
single ended has significant problems.
For Single Ended operation, Ultra SCSI isn't just an easy divide by two
and go as illustrated by some. (See Jim McGrath's Email as an example.)
To help explain, here's a reference diagram:
0 __________________ ______________
/| |\ /
REQ* 1 _____/ | | \___________________/
->| |<--T1 ->| |<--T2 |
| | | |
|<------T3------>| |<-------T4-------->|
We all know that the period for the fastest "fast SCSI" is a minimum of
100 ns and is made up of T1+T2+T3+T4. We have specifications for these:
T1 5 ns minimum
T2 5 ns minimum
T3 30 ns minimum at transmitter
T4 30 ns minimum at transmitter
---------
70 ns of 100 ns budget.
30 ns slop, fairly easy to do! This is OK.
Now for Ultra SCSI, the numbers are:
T1 5 ns minimum
T2 5 ns minimum
T3 15 ns minimum at transmitter
T4 15 ns minimum at transmitter
---------
40 ns of 50 ns budget.
Hmm... 20% slop! Sounds like maybe we can do it, but close examination
shows some major problems.
The primary one is slew rate control of the rise and fall times. A
design to guarantee a 5 ns slew in the best case results in 15 ns or
more in the worst case. So taking this into account, it gives you:
T1 15 ns maximum
T2 15 ns maximum
T3 15 ns minimum
T4 15 ns minimum
----------
60 ns of 50 ns budget! OOPS!
Tightening down the slew rate control may be possible, but I doubt that
anyone would be able to do better than: 5ns <= slew <= 10ns, and even
this would be very expensive. Even if we IC designers can control the
maximum variance in slew rate to this range, the result would be:
T1 10 ns maximum
T2 10 ns maximum
T3 15 ns minimum
T4 15 ns minimum
-----------
50 ns of 50 ns budget!
This can't be done, as no one's perfect!
One solution is to scale the 5ns slew rate requirements down to 2.5 ns.
We should be able to control the maximum slew to around 7.5ns. But if
we do this, what happens to the signal quality? Wasn't this the reason
for 5ns slew originally?
If we can't scale the slew rate, then room for the process variance must
be found somewhere. What this means is that the transmit assertion
period and transmit negation period of 15 ns must be decreased to at
least 10 ns, or preferably even smaller. Of course, this creates a whole
new set of problems. As an example:
T1 10 ns maximum
T2 10 ns maximum
T3 10 ns minimum
T4 10 ns minimum
-----------
40 ns of 50 ns budget!
We might be able to do this.
If Ultra SCSI is for differential only, are we need to investigate the
whole picture like was done in SPI Annex C. This shows skew variance of
up to 19 ns.
T1+T2 19 ns Skew budget
T3 15 ns minimum
T4 15 ns minimum
----------
49 ns of 50 ns budget!
The problems are still here to haunt us....
------------------------------------------------
Out of curiosity, is anyone out there coming to the March 3 Ultra SCSI
meeting thinking this idea is a done deal?
COMMENTS?
===================================================================
Steve Finch, Silicon Systems, Inc.
Ph. 714 573-6808, Fx. 714 573-6916
email: 5723283 at mcimail.com
===================================================================
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