Date: Sun, 14 Jan 1996 04:22:28 -0500 (EST) From: Peter Dufault <dufault@hda.com> To: nate@sri.MT.net (Nate Williams) Cc: mmead@Glock.COM, hackers@freebsd.org Subject: Re: SCSI Scanner anybody? Message-ID: <199601140922.EAA13846@hda.com> In-Reply-To: <199601140240.TAA28309@rocky.sri.MT.net> from "Nate Williams" at Jan 13, 96 07:40:56 pm
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> > [ Do SCSI scanners needs residual reads ] > > > Apparently some scanners don't care. > > > > > Of the supported cards, the only ones I know of that do the residual > > > reads correctly (which is a prerequisite for attach/detach in target > > > mode, I think, and the source of my confusion) is the Adaptec 1542B. > > > The 1542A and 1542C won't work. > > > > Hmm. So does this mean that my AHA2940 will not work with an HP > > ScanJet IIcx? Thanks for the info... > > If the only knowns drivers are the Adaptec then my scanner shouldn't > work, since it uses the NCR controller. I suspect it's not necessary > for the HP scanners, so your AHA2940 should work. Residuals let you detect when less data is transferred than requested. This isn't a prerequisite for attach/detach in target mode - it lets you try to read more than the peripheral will send and detect what is actually sent. This is most important talking to a processor-type device since other devices (e.g., tapes) handle it differently in a way supported by all the drivers. When the driver doesn't support residuals the upper levels assume it read the requested amount, so the scanner should work fine. A quick scan of the drivers shows that only the 1542 and the aic7xxx (which includes aha2940) set the SCSI_RESID_VALID flag that tells the upper levels of the software that the driver properly sets the "xs->resid" field. -- Peter Dufault Real Time Machine Control and Simulation HD Associates, Inc. Voice: 508 433 6936 dufault@hda.com Fax: 508 433 5267
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