From:  ()
Subject: Re: Quadra SCSI Problems???
Organization: Apple Computer Inc.
Lines: 28

> ATTENTION: Mac Quadra owners: Many storage industry experts have
> concluded that Mac Quadras suffer from timing irregularities deviating
> from the standard SCSI specification. This results in silent corruption
> of data when used with some devices, including ultra-modern devices.
> Although I will not name the devices, since it is not their fault, an
> example would be a Sony 3.5 inch MO, without the special "Mac-compatible"
> firmware installed. One solution, sometimes, is to disable "blind writes"
> 
To the best of my knowledge there aren't any problems with Quadras and
blind transfers.  Trouble with blind transfers usually means the programmer
screwed up the TIBs or didn't test their driver with the device in question.
Well designed TIBs poll or loop at every point where delays of >16µsec occur.
This usually occurs at the first byte of each block of a transfer but some
devices can "hiccup" in the middle of blocks.  If this happens in the middle
of a blind transfer there is the possibility of losing or gaining a byte
depending on which direction the tranfer was going.  In anycase the SCSI Manager
will eventually return a phase error at the end of the transaction because
it is out of sync.  Actual data loss would only occur if the driver didn't
pay attention to the errors coming back.

Note that this effect is not caused by anything actually on the SCSI Bus but
rather by the transfer loops inside the SCSI Manager.  The problem occurs when
the processor bus errors trying to access the SCSI chip when the next byte
hasn't been clocked yet.  Also note that the Bus Error is dealt with by a bus
error handler and doesn't crash the machine...

Clinton Bauder
Apple Computer