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From: gwyn@smoke.brl.mil (Doug Gwyn)
Newsgroups: comp.sys.apple2
Subject: Re: What's wrong with my hard drive
Message-ID: <16075@smoke.brl.mil>
Date: 8 May 91 02:02:22 GMT
References: <48477@ut-emx.uucp> <52478@apple.Apple.COM> <1991May6.010943.5215@utstat.uucp>
Organization: U.S. Army Ballistic Research Laboratory, APG, MD.
Lines: 29

In article <1991May6.010943.5215@utstat.uucp> philip@utstat.uucp (Philip McDunnough) writes:
>Bottom line: should I remove the terminator from my drive or not!?

We've been through this before, so to simply summarize BRIEFLY:
Electrically, there should be termination at both ends of the SCSI bus and
nowhere in the middle of the bus.  The "Rev C" SCSI Card did not provide
termination; the "High-Speed SCSI Card" does provide termination.  There
was thus no ideal way to configure the Rev C SCSI subsystem, and Apple's
recommendation was to put one terminator at the big end of the "Apple
SCSI System Cable", which was as close to the Rev C Card as one could
install an Apple SCSI terminator.  In the case of only one device other
than the SCSI interface attached to the SCSI bus, e.g. a single HD20SC,
following that recommendation would put a single terminator at what
happened to already be the "far" end of the bus, hardly ideal, but
better than the alternatives.  For "Rev C" subsystems with more than one
SCSI peripheral, Apple recommended a second terminator at the far end of
the bus (in addition to the one somewhat near the Rev C SCSI Card).

The High-Speed (DMA) SCSI Card eliminates this nonsense, because it
includes termination at its end of the SCSI bus, so regardless of the
peripheral complement attached to a High-Speed SCSI subsystem, one adds
just one terminator at the far end of the SCSI bus, always.  (In all these
descriptions I have assumed the devices are not supplying termination.
Once one determines where the terminator(s) SHOULD be, then if there is
a device plugged in at that spot one can use the drive's internal
SCSI bus termination, if it provides it (normally a couple of flat
resistor packs).  Termination should be removed from all devices that are
not being deliberately configured with termination to satisfy the "both
ends terminated" SCSI bus requirement.)