Newsgroups: comp.sys.apple2 Subject: Re: The sad Truth about iMac: From: dempson@actrix.gen.nz (David Empson) Date: Thu, 11 Mar 1999 05:13:26 +1300 Message-ID: <1doheyf.6f6sqe10yoasmN@dempson.actrix.gen.nz> References: <7brmt4$stb$1@alexander.INS.CWRU.Edu> <7bs458$eu4$2@opal.southwind.net> <19990307145827.10460.00003765@ng67.aol.com> <36e34c04.9104363@news.bconnex.net> <36e3f9c3.1762885@news.bconnex.net> Organization: Empsoft X-Newsreader: MacSOUP 2.3 NNTP-Posting-Host: 202.49.157.176 X-Trace: 11 Mar 1999 05:10:59 NZST, 202.49.157.176 Lines: 70 Path: news1.icaen!news.uiowa.edu!NewsNG.Chicago.Qual.Net!129.79.6.160!news.indiana.edu!logbridge.uoregon.edu!su-news-hub1.bbnplanet.com!lsanca1-snf1!news.gtei.net!news.netgate.net.nz!news.xtra.co.nz!news.iprolink.co.nz!news.actrix.gen.nz!dempson Xref: news1.icaen comp.sys.apple2:145856 coaxial wrote: > In article , david@uow.edu.au (David Wilson) wrote: > > SCSI-3 has not been released so where did they get SCSI-5? > > I don't know. I found it in my most recent version of "inmac" catalog, > which is published by MicroWarehouse (the Canadian version). Despite the > name of the catalog, it has absolutely nothing to do with Mac's, it's a > catalog for PC hardware, software, networking & supplies. > > Here's how they (MicroWarehouse) listed it: > > SCSI I ...............up to 5Mbps > SCSI II (wide)........up to 10Mbps > SCSI III (fast).......up to 20Mbps > SCSI V (ultra)........up to 40Mbps > > Perhaps they screwed up? Note that these speeds are megabytes per second (written MBps), not megabits (written Mbps). They've invented their own names for the higher speeds, and they have also made at least two errors. The names I'm more familiar with: Standard SCSI 5 MB/s Standard/Wide 10 MB/s Fast SCSI 10 MB/s Fast/Wide 20 MB/s Ultra SCSI 20 MB/s Ultra/Wide 40 MB/s Ultra2 SCSI 40 MB/s Ultra2/Wide 80 MB/s All the narrow variants use a 50-pin cable and the centronics-style connector. The wide variants use a 68-pin cable and the corresponding connector. Wide does 16-bit transfers and can support 16 devices on the bus, while narrow does 8-bit transfers and supports 8 devices. The higher speeds impose stricter rules on the cables, presumably affecting the length, electrical characteristics of the wire, internal shielding requirements, etc. There is also a third categorisation: "single-ended", "differential" and "low voltage differential" (LVD). I think this just affects the interpretation of the signals on the cable: a signal and ground pair are replaced with a signal+ and signal- pair, for example. We use the single-ended standard narrow SCSI variety on the Apple II, though the computer limits the transfer rate to a maximum of 1 MB/s. The terms "SCSI-1" and "SCSI-2" refer to revisions of the standard, which includes details like the command sets and device types. SCSI-1 was rather limited (hard drives were one of the few device types that was clearly documented). SCSI-2 standardised a lot more devices including CD-ROM, and defined some of the faster speeds. There is a SCSI-3 standard, but I don't think the final version has been released yet. The "fast", "wide" and "differential" variants are all documented under SCSI-2 (possibly even under SCSI-1). I think "ultra", "ultra2" and the LVD option are later refinements, but I haven't gone looking at my (draft) SCSI-2 standard for a while, so I can't be sure of this. -- David Empson dempson@actrix.gen.nz Snail mail: P.O. Box 27-103, Wellington, New Zealand