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Newsgroups: comp.sys.apple2 Path: news.uiowa.edu!chi-news.cic.net!newsxfer.itd.umich.edu!news.mathworks.com!zombie.ncsc.mil!simtel!news.sprintlink.net!in2.uu.net!comp.vuw.ac.nz!actrix.gen.nz!dempson From: dempson@atlantis.actrix.gen.nz (David Empson) Subject: Re: Will SCSI2 work with Apple 2? Message-ID: <DG16u3.AAo@actrix.gen.nz> Sender: news@actrix.gen.nz (News Administrator) Organization: Actrix - Internet Services Date: Fri, 6 Oct 1995 14:27:39 GMT References: <44tsd6$l1g@usenetz1.news.prodigy.com> X-Nntp-Posting-Host: atlantis.actrix.gen.nz Lines: 88 In article <44tsd6$l1g@usenetz1.news.prodigy.com>, Phil Abel <TTSH35A@prodigy.com> wrote: > Will a SCSI2 hard drive work with an Apple 2 system? Usually, yes. I'm on my second Quantum drive that is described as "SCSI-2". For hard drives, "SCSI-2" basically means that the drive supports a stricter command set. The physical interface is usually identical. For other device types, "SCSI-2" means a lot more, because the original SCSI standard didn't define much in the way of device types and command sets, so most devices use proprietary command sets. SCSI-2 standardises the command sets for most types of devices. There are three special types of interface that you might see mentioned: - "Fast SCSI" supports data transfer at twice the speed of the original SCSI standard (10 MB per second vs 5 MB per second). This will not be a compatibility issue, as it is just the maximum transfer speed supported by the drive. The Apple II cannot transfer more than one megabyte per second. - "Wide SCSI" uses a different cable arrangement to double the width of the data path (16 bits instead of 8 bits). A wide SCSI drive cannot be used with an Apple II, unless it can also operated in "narrow" mode with the original 50-pin connector. (There is also "Fast Wide SCSI", which doubles the data rate and the width of the bus.) - "Differential SCSI" involves a different type of interface to the computer, where every data signal has a balanced positive and negative pair of wires, rather than a single wire and a ground line. I believe it has a different type of connector. Differential SCSI drives cannot be used with an Apple II. > I have a plain ol' Rev. C SCSI Card, which I would be using > initially, though I plan to work up to a faster one eventually. My Quantum LPS240 is working fine on an original Apple SCSI card (I haven't used my new drive on it). > Also, do the SCSI2 drives have a different kind of connector than the > standard 50 pin that the good 'ol SCSI1 drives use? Some drives use a proprietary connector, but the standard (narrow, non-differential) SCSI bus uses the same 50-pin connector for SCSI-1 and SCSI-2. The only significant problem you might run into is termination, and supply of termination power. SCSI-2 devices tend to be fussier about termination than older devices. The normal arrangement is to have terminator after the last device on the SCSI bus (either an external terminator or termination resistors or an active terminator on the drive mechanism). With the original Apple SCSI card, the card itself is not terminated, so if you are connecting more than one device, you need to add a second terminator between the computer and the first drive (using a "pass-through" external SCSI terminator, or internal termination on the first drive). At least one device (computer or any drive) must provide power for the SCSI terminators, by feeding 5 volts onto the TERMPWR line on the SCSI bus (usually through a diode, to prevent backfeeding from a higher voltage device; a good implementation will also have a fuse to protect against shorts and a capacitor to cope with a sudden rise in termination power drain). The Apple SCSI cards do not provide termination power (though recent Apple High-speed SCSI cards were modified by Apple to provide termination power). The RamFast has a switch (?) to select whether it supplies termination power. I do not recommend getting the computer to supply termination power unless you have a heavy duty power supply, as the TERMPWR line can potentially use up the entire available current capacity of the +5V rail going to the slots. Both of my Quantum drive mechanisms provide termination power to the SCSI bus, avoiding the need to supply it from anywhere else. -- David Empson dempson@actrix.gen.nz Snail mail: P.O. Box 27-103, Wellington, New Zealand