Should there be a bit in INQUIRY Data for Fast-20?
thomas.tewell at seqadvtech.com
Thu Aug 25 09:23:14 PDT 1994
> Does anyone know what will happen to current SCSI devices if one
> sends them an SDTR message with a proposed Transfer Period less than
> 100 ns? While they _should_ simply negotiate up to a supported
> value, I wouldn't want to bet too much on it. Should we add a bit
> to the INQUIRY Data to say the target supports Fast-20 to avoid any
> nasty surprises?
This is a very interesting question. It relates to the _expected_
behavior of a target device versus the assumed behavior of an
initiator device driver. This problem became very evident during the
development of device drivers for SCSI-2 Fast SCSI devices. If you
negotiated a synchronous rate above 5 MB/sec to some devices, they
would _accept_ the period and then hang in the data phase due to the
fact that they could not support Fast SCSI. One would have assumed
that if the target reported it was a SCSI-2 device AND the device
supported synchronous data transfers that it would know how to handle
requested rates above 5 MB/sec but alas, this was not to be. We have
been forced to build a table of devices based on Vendor ID and Model
to determine whether to negotiate above 5 MB/sec or not. This means
that there are certain devices out there that are not in our table
that are sure to fail. So much for 'plug and play'.
The same will hold true, I suspect for Fast-20. There has been some
'talk' on the SCSI refelector about whether the target should be
burdened with features to make initiator life easier or should we
assume good and proper behavior from an initiator to correct
the behavior of wayward targets. The issue here is, of course, one of
_mandatory_ features for a target to prevent confusion on the
initiator side. The issue for the committee should be, in my opinion,
" Anything that can be done to make SCSI easier to use for the
customer should be done. A puritanical attitude towards the
specification doesn't mean beans to an OEM deciding whether to
incorporate SCSI on a system or not. The bottom line is _DOES IT
WORK AND WILL IT WORK FOR MY CUSTOMER_ "
To this end the issue should not be one of whether a Fast-20 bit
should be added to the inquiry field but 'Where should it go?'
It should be there and it should be mandatory. The number of SCSI
targets out there that act 'weird' due to non-mandatory issues in the
SCSI specification is immense. The committee should act now to
eliminate these sort of ambiguities for SCSI-3.
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