JLOHMEYE at cosmpdaero.ftcollinsco.ncr.com
Tue May 23 15:41:00 PDT 1995
Jim McGrath wrote:
>Actually, the original 6 m single ended SCSI limit was probably a
>REQ/ACK type limit as well, so maybe we can go longer. Does anyone
>remember (John?) and/or have done empirical tests?
SASI and SCSI-1 specified 6 m for single-ended because of signal quality
issues -- not timing problems. So if we use LPD on all the signals, we
should be able to go as far as we want up to the signal quality limits we
design into LPD or the arbitration limits (around 125 feet), whichever is
If we elect to save some pads and have a mixture of LPD and single-ended,
then we will have to determine which driver/receiver technology becomes the
limiting factor: LPD on the fast signals or single-ended on the slower
I recall that Bill Spence once ran single-ended SCSI on very long cables at
T.I. (over 60 feet, I think). He had a carefully controlled environment.
So the 6 m limit is not etched in stone for single ended. It really
depends on a lot of factors.
I believe that the data and parity signals must use the same technology
drivers/receivers as for REQ/ACK. Otherwise, it will be extremely difficult
to control the setup and hold times.
I tend to favor a total LPD solution because it gives us better common-mode
noise immunity. And, yes, DIFFSENSE, which is an SE signal, enters into
this equation, but we can filter it extensively -- it is extremely low
John Lohmeyer E-Mail: john.lohmeyer at symbios.com
Symbios Logic Inc. Voice: 719-573-3362
1635 Aeroplaza Dr. Fax: 719-573-3037
Colo Spgs, CO 80916 SCSI BBS: 719-574-0424 300--14400 baud
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