Should there be a bit in INQUIRY Data for Fast-20?

Thomas Tewell thomas.tewell at
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 
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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