PnP SCSI Industry Meeting, Jan. 10, South Lake Tahoe
Steve Timm (SYS)
stevetim at microsoft.com
Tue Feb 7 14:54:36 PST 1995
This industry meeting was called to discuss and identify any changes
required to update the Plug and Play SCSI Rev. 1.0 spec. dated
March 30, 1994. The review was prompted by feedback from implementers
following a recent PnP SCSI Plugfest and updates to Parallel SCSI-3 SPI.
In addition, new proposals for sustaining or extending PnP SCSI's definition
and applications were discussed.
ATTENDEES: Hyundai MPD (formerly NCR MPD); Adaptec; Conner; Digital Equipment;
Quantum; IIX; Zadian Technologies; Seagate; Sun Microsystems; Unitrode;
IBM; HP; Future Domain;
FSI; Lawrence Livermore; Qlogic; Hitachi; Burr Brown; Dallas
Semiconductor; Apple; Tandem;
Western Digital; Fujitsu
Much of the discussion centered around design recommendations and
clarification of the SCSI-3 SPI Annex B "SCAM" protocol spec. Results include:
- Discussion and recommendations for PnP SCSI implementation including
considerations for support of a 25 pin connector, 16 bit wide SCSI,
and PCMCIA SCSI.
- Agreement on SCAM protocol design recommendations for submission to
X3T10 for approval and subsequent update to SPI Annex B. Specific items
for submission include input from Conner and Quantum summarized below.
Most of the recommendations provide excellent clarification for PnP SCSI
design. Implementers are advised to consider these recommendations
for their designs until final requirements are approved for
inclusion in SPI.
RECOMMENDATIONS for UPDATE of PnP SCSI 1.0 :
1. PnP SCSI requires use of a standard external 50 pin high density
connector. PCMCIA cards
however are limited to a 32 pin cable connector adapting to a 50
pin high density, possibly
violating PnP SCSI. Recommendation is to require bundling the
bridge cable for PnP compliance.
2. Proposal to define an optional low cost PnP SCSI profile based on a
25 pin connector was discussed
and deferred. Most felt that a second connector option weakens
PnP SCSI, but growth of PnP into
the high end and low end markets may warrant further review.
Other related PnP connector issues
discussed included: mixing narrow and wide SCSI and insufficient
grounding for 25 pin fast SCSI timing.
3. Some CD-ROM drives do not supply termination power and thus do not
work when attached to a
PCMCIA SCSI card (reference section 6.2 of PnP SCSI version 1.0).
Recommendation: Low power
battery operated PnP SCSI peripherals must supply termination power.
4. ID assignment to mixed bus width devices (ie; 8 bit external, 16
bit internal) needs clarification for
5. Industry/X3T10 and Microsoft ownership of PnP SCSI spec. was raised
as an open item.
Microsoft's recommenation is Industry ownership as a technical
report under X3T10. X3T10 Action is
needed to publish this report. (Note: Microsoft currently
references the PnP SCSI profile as the
basis for logo certification of SCSI products designed for Windows 95.)
SCSI-3 SPI ANNEX B Revision 15 SCAM Protocol Recommendations:
CONNER SCAM DESIGN RECOMMENDATIONS
Note: Items 2,
1. Various editorial comments
- Section B4.1.1 item 4 should read: "Read and latch data from
the DB(4-0) signal. All devices assert
the DB(6) signal.
- Section B4.1 rewrite is needed to clarify use of vague terms
such as "device" when this may
be either a target or initator or a SCAM or non SCAM device.
Also need some clarification
on when to assert the MSG signal.
- Use of SCSI terminology such as SCAM Initiator or SCAM Target
instead of master or slave.
2. Section B5.1.2 should mandate a 32 byte ID string to avoid
3. Section B5.1.3: SCAM initiators and targets should support the
"Configuration Process Complete"
function code to close out ID selection until the next reset.
This function code is strongly encouraged
to avoid potential ID conflicts such as the rare case of more
than 7 attached peripherals.
4. B.6.1.1 Polling of the bus by a SCAM initiator by attempting a
selection should be followed by
a specific command such as Inquiry or Test Unit Ready to close
out the selection gracefully.
Recommendation is to choose one of these commands and make it a
5. B5.1.1: Wired-OR glitch filtering must be implemented properly to
6. Section B5.3.1: Some SCAM targets do not support some action codes
such as "locate on" and
may stop participation in SCAM selection after receiving this
action code even though they have not
yet received an "assign ID" action code. Drives must not
misbehave in response to unsupported
funciton/action codes. Target support for all function/action
codes is recommended.
7. Timeouts and error reporting for common bus hang conditions is
lacking in the current specification
and is recommended for enhanced PnP SCSI design. Additional
standardization is required here.
8. B18.104.22.168 states SCAM tolerant reset to selection delay is 250ns.
Failure of the initiator to honor this
delay can result in failure to acknowledge a SCAM tolerant
legacy device resulting in ID conflicts.
This delay is mandatory to ensure interoperability.
QUANTUM SCAM DESIGN RECOMMENDATIONS
1. SCAM initiators must execute SCAM selection in response to a bus
reset since some SCAM
peripherals revert to their ID assignable state following a
reset. Failure to reassign IDs following
a reset condition can result in an ID conflict or the inability
to address devices on the SCSI bus.
1.1 Also, if a SCSI bus reset does occur during SCAM ID assignment,
set IDs are cleared and
on completion of SCAM (assuming it is not restarted in response
to reset) some devices
may revert to their original ID. Therefore, it is recommended
that bus resets not be prohibited by the
firmware or SCAM device drivers during SCAM selection. For new
designs, it is recommended that
SCAM devices not default to their hard ID on SCSI reset.
Note: A followup Plug and Play SCSI plugfest is being planned for
Please send mail to stevetim at microsoft.com if you have
questions about this event.
More information about the T10