SUMMARY: scsi cables

From: David Way (
Date: Thu Jun 25 1992 - 01:49:01 CDT

On Monday, June 22, I wrote:

> Do scsi cables need to be shielded ? I'm trying to troubleshoot a problem
> with our scsi disk subsystem on our 4/490 (4.1.1), which I suspect might be
> that we've exceeded the maximum bus length for non-differential scsi. Can
> anyone verify for me what this length is, according to whatever standard
> there is for scsi? I've always heard 14-15 feet is the max, and it appears
> that we are very close to that if you include cables that are inside
> enclosures. Two of the cables we are currently using are ribbon cables
> with connectors press-fitted onto them, unshielded. It looks like there
> is plenty of excess length in all the external cables, so I'm going to
> replace all of them with custom-made shorter ones, but I'm not sure if I
> should request shielded cable.

I got quite few responses. Just about everyone mentioned that the maximum
bus length for single-ended scsi is 6 meters. Most people also said that
shielded cables SHOULD be used, although there wasn't a consensus on whether
the scsi standard requires this.

Art Hayes had an interesting suggestion, which I may end up trying:

< 1.) Use a 110ohm active terminator (this is a terminator that has a
< voltage regulator inside that adjusts the termination voltage optimally
< for cable conditions). Can be supplied by Andataco, Corey Preville,
< (301) 963-9191. They are made by a company named Methode Electronics,
< (800) 323-6858. I dont know if this company is the only supplier.

A few people said that 6 meters is highly optimistic for maximum bus length,
and 12-15 feet is more likely to work reliably in practice. This seems to
bear out what I've seen. At this point I'm going to buy the shortest
shielded cables I can get my hands on (and possibly one of those active
terminators) and be done with it, hopefully.

Thanks to all who responded: (Daniel Strick)
kalli!glenn@fourx.Aus.Sun.COM (Glenn Satchell)
rick%pgt1@Princeton.EDU (Rick Mott) (Mike Pearlman) (Hanh Vu - xt 2801 ) (Russ Poffenberger)
era@niwot.scd.ucar.EDU (Ed Arnold)
lsr-vax!art@uunet.UU.NET (Art Hays - PSTAFF)
Jim Guyton <> (Dale Wellborn)
Dave Wilmot <>
Stefan Mochnacki <>
Forrest Cook <cook@stout.atd.ucar.EDU>
kevin@ukw.uucp (Kevin Sheehan {Consulting Poster Child}) (William LeFebvre)
trdlnk!mike@uunet.UU.NET (Michael Sullivan)

David Way  McDonald Observatory/Astronomy Dept.- Univ. of Texas, Austin 
(office) RLM 16.206 (voice) 471-7439 (internet)

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