martya@cpl-mfh.UUCP (Marty Adelman) (03/08/90)
A friend of mine has a high capacity external rodime hard drive. They just upgraded to a //ci from an SE and took their hard drive with them. They began to have problems with the machine crashing excessively and making work impossible. After going over his problems and what he had tried to do to correct them, I suggested he call Rodime and check if there were new drivers for the hard disk. Sure enough they sent him updated drivers for his //ci and this helped clear up most of the problem, but his machine was still bombing occasionally. He called up Rodime tech support again and they suggested to doubly terminate the SCSI interface, seems that the new higher speed SCSI was causing RFI? noise to bounce back and might be causing his problem. He tried this and sure enough his problems went away. I have personally never heard of this problem, has anyone else heard about this and is the explanation plausible??? Any comments welcome. ----------------------------------------------------------------------------- Marty Adelman, PH.D. Phone: (716) 887-4572 Director Computer Systems Internet: cpl-mfh!martya@acsu.buffalo.edu Clinical Pharmacokinetics Laboratory UUCP: !sunybcs!cpl-mfh!martya State University of New York at Buffalo -----------------------------------------------------------------------------
dplatt@coherent.com (Dave Platt) (03/10/90)
In article <-531597662@cpl-mfh.UUCP> martya@cpl-mfh.UUCP (Marty Adelman) writes: > ... He called up Rodime tech support again and > they suggested to doubly terminate the SCSI interface, seems that the new > higher speed SCSI was causing RFI? noise to bounce back and might be > causing his problem. He tried this and sure enough his problems went away. > I have personally never heard of this problem, has anyone else heard about > this and is the explanation plausible??? I've heard similar reports. Yes, the explanation is entirely plausible. The SCSI specification calls for the SCSI bus to be terminated precisely twice... once at each end of the bus. This ensures that the "not-busy" voltage levels on the bus lines are correct, and that signals travelling down the bus are absorbed by the terminators, rather than reflecting back along the bus and interfering with later signals. The majority of Macintosh hard-disk setups are actually _not_ in conformance with the SCSI specification... they are terminated only once, at the internal or external disk drive. The Mac doesn't include a terminator, even though the Mac is often the device at one end of the bus. Up until now (Mac IIci) we've usually been able to get away with a singly-terminated bus... the reflections haven't been enough of a problem to cause bus-jamming, I guess. Apparently, the IIci's SCSI interface is fast enough to be troubled by the bouncing signals in some cases. If you have a IIci and an external hard disk, you'd be best off to have two terminators: one attached to the external hard drive (either on the drive controller or plugged into the drive case's SCSI port), and one at the IIci end of the bus. Unfortunately, the Mac does not use the standard 50-pin SCSI jack... it uses a DB-25... and it's not easy to find SCSI terminators that use DB-25 connectors. I understand that one of the big Mac disk-sellers (MicroNet Technology, I think) is making a terminator module which can be connected _inside_ the IIci case... it plugs into the internal 50-pin SCSI header, I think. This is probably the best way to go, if you can get such a beast... it places the termination at the near end of the bus, minimizing reflections. Putting a second terminator at the external drive case is a possible alternative... it'll help regulate the voltage levels, but I don't know that it would really do a good job of damping down reflections on the bus. It might help to use a _very_ short SCSI cable to the external drive case... a 1' cable rather than a 2' or 3' cable. -- Dave Platt VOICE: (415) 493-8805 UUCP: ...!{ames,apple,uunet}!coherent!dplatt DOMAIN: dplatt@coherent.com INTERNET: coherent!dplatt@ames.arpa, ...@uunet.uu.net USNAIL: Coherent Thought Inc. 3350 West Bayshore #205 Palo Alto CA 94303
teener@apple.com (Michael Teener) (03/10/90)
In article <49073@coherent.coherent.com> dplatt@coherent.com (Dave Platt) writes: > If you have a IIci and an external hard disk, you'd be best off to have > two terminators: one attached to the external hard drive (either on the > drive controller or plugged into the drive case's SCSI port), and one at > the IIci end of the bus. Unfortunately, the Mac does not use the > standard 50-pin SCSI jack... it uses a DB-25... and it's not easy to > find SCSI terminators that use DB-25 connectors. This is generally correct, but it is important to note that if your IIci has an *internal* hard disk, it is already terminated once (we include a termination in all our internal drives). Do not put *three* terminators (internal and two external) on the SCSI bus, this will overload the 5380 line drivers. Also, if the IIci doesn't have an internal hard disk and only has *one* external SCSI device, then a single termination at that device will work ... providing that the SCSI system cable (25-to-34 pin) is nice and short (like the one we sell ... now you know why the sucker is so inconveniently short). If you have more than one external SCSI device and no internal drive, then put terminators at the first and last device in the chain. Since we are now guaranteed to have adequate termination, we can make the SCSI peripheral cable (34-to-34 pin) fairly long. ---- Michael Teener -- 408-974-3521 ---------------------------------+ ---- Internet teener@apple.com, AppleLink TEENER | ---- Apple may know my opinions, but *I* am responsible for them | ---------------------------------------------------------------------+ Transportation by Cheetah N9900U, a loyal beast for the past 6 years.