Jeff - I - barely - seem to recall either a web page, or an email exchange from years back, on an explanation of the probable cause of problems with (what were then) "newer" SCSI drives and compact Macs.
The explanation amounted to something like the proposed internal drive not supplying the "right kind" of termination (or was it the kind and/or amount of resistance value??) ...by adding the internal [passive] terminator to the end of the internal SCSI ribbon cable, the issues the compact Mac was having with the drive were abrogated. (At the time, I knew that the termination on the drive itself was working fine, as it worked properly with an Adaptec controller in a PC.) I have had a couple of drives (including the 9GB currently in my SE30) where the "magic solution" was hanging an internal terminator off the end of a modified (or 3 position/2 drive) cable. ..at least I carried forward into time with me the impression that some [modern] drives simply don't supply recognizable termination [values] for the compact Macs. ..but I don't think I ever tried just hanging a terminator on the Compacts external SCSI plug (and if I did try that, it was so long ago, I don't remember it). It would certainly be easy enough to give it a try ...a bit displeasing to my sense of aesthetics, perhaps. I _do_ remember having to have an external terminator either on the end of the SCSI device chain OR on the external SCSI plug of my old Mac IIfx though (but that was such an odd beastie, it probably just wouldn't do to draw any conclusions therefrom) ...somewhat akin to your use with the Jackhammer. --- brandon davis --- -- sacramento, ca -- ----- Original Message ----- > 1. Re: SCSI drives in compacts > ---------------------------------------------------------------------- > Date: Sat, 27 Dec 2003 22:19:37 -0600 > From: Jeff Walther <[EMAIL PROTECTED]> > Subject: Re: SCSI drives in compacts > > > During this discussion, I've been wondering if one might need a > terminator at the motherboard end of the cable. > > As I understand it, on these old machines the motherboard does not > supply any termination. Apple relied on the internal SCSI cable > being very short to allow termination of only the internal drive to > provide sufficient SCSI termination. So, in fact, only one end of > the SCSI chain is terminated, absent external drives. > > Could the problem be that with newer drives, the newer drive won't > tolerate the unterminated MB end of the cable, even with the very > short cable? This argument falls apart, if folks have tried this > with a properly terminated external SCSI chain connected and it still > didn't work. > > So, what I'm thinking is that when one does not have any external > SCSI devices connected (with the final one terminated) perhaps one > should try plugging a DB25 terminator into the SCSI port on the back > of the Mac and see if that solves the problem. Naturally, one > would also set termination on the internal drive to 'On'. > > Some illustrations: > > T: and :T are SCSI termination > D is a SCSI device > T:D and D:T are a device with termination installed or activated. > MB is the motherboard > ----- is internal SCSI cabling > ===== is external SCSI cabling. > > Original Mac config. with no external chain: > > T:D----MB Only one end termination, but short cable. > > Brandon's suggestion: > > T:---D---MB Not relying on drive termination, still only one end term. > > Original Mac configuration with external chain: > > T:D---MB====D:T Both ends terminated > > My proposal: > > T:D---MB:T Both ends terminated. > > If the internal drives under discussion truly have faulty on-board > termination, then this wouldn't help. But if the problem is that > the MB end of the SCSI chain is left flapping in the breeze, then a > DB25 terminator on the outside of the Mac would solve the problem. > And it has the advantage that you must remove it if you install an > external SCSI cable, so there's no danger of leaving SCSI termination > in the middle of the SCSI cable. > > I find this scheme useful with the NuBus Jackhammer card because it > has actual termination resistors on the card. It's a pain to open > the machine to move the resistors if one is often removing and > installing an external segment to the SCSI chain, so I just leave a > 68 pin terminator on the external port of the JackHammer and remove > it when I install an external device. > > Jeff Walther > ------------------------------ -- Compact Macs is sponsored by <http://lowendmac.com/>. Support Low End Mac <http://lowendmac.com/lists/support.html> Compact Macs list info: <http://lowendmac.com/lists/compact.shtml> --> AOL users, remove "mailto:"; Send list messages to: <mailto:[EMAIL PROTECTED]> To unsubscribe, email: <mailto:[EMAIL PROTECTED]> For digest mode, email: <mailto:[EMAIL PROTECTED]> Subscription questions: <mailto:[EMAIL PROTECTED]> Archive:<http://www.mail-archive.com/compact.macs%40mail.maclaunch.com/> --------------------------------------------------------------- >The Think Different Store http://www.ThinkDifferentStore.com ---------------------------------------------------------------
