SPI Tolerance

Gerry Houlder Gerry_Houlder at notes.seagate.com
Tue May 31 11:28:35 PDT 1994

I agree with Jeff Stai's position. The existing words put 100 ns as a lower
limit (on the period) that cannot be exceeded, and the upper limit is
unlimited. This is needed to allow the target to add long pauses (seconds, if
needed) between REQ pulses so the offset isn't exceeded. In fact, many devices
that agree to 10 Mtransfers/sec may not be able to send data faster than 8.0
Mtransfers/sec but can receive data at 10 Mtransfers/sec. This is allowed under
SCSI negotiation rules, and even encouraged so that a target and initiator that
implement different frequency increments can agree on the "best" available rate
in one pair of messages.
Its a shame that someone's target silicon works at 100.000 ns but doesn't at
99.776 ns. The target behavior is within the SCSI spec, however, while the
initiator's isn't. A critical designer might say "If it doesn't work at 99.776
ns period, maybe it doesn't work very well at 100.000 ns either". This is a
"robust design" matter, not a spec conformance issue.
Gerry Houlder -- Gerry_Houlder at notes.seagate.com
Seagate Technology   -   920 Disc Drive   -   Scotts Valley, CA 95066 USA
Main Phone 408-438-6550   -   Email Problems postmaster at notes.seagate.com
Technical Support: BBS 408-438-8771  Fax 408-438-8137  Voice 408-438-8222  

### OGATE Version 8 message trace and attachment information:
### MsgFileName: m:\mgate\outbound\14.MSG
### Org Date:    05-31-94 08:23:33 AM
### From:        Gerry Houlder at SEAGATE
### To:          scsi @ WichitaKS.NCR.COM @ internet
### Subject:     SPI Tolerance
### Attachments: none

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