Apple SCSI TermPWR; Was Re: This is funny (ebay)

From: Carl Lowenstein <>
Date: Sat Mar 2 19:18:39 2002

> Date: Sun, 24 Feb 2002 02:18:26 -0600 (CST)
> From: Doc <>
> To:
> Subject: Apple SCSI TermPWR; Was Re: This is funny (ebay)
> In-Reply-To: <>
> OK, maybe my deal is related to that, maybe not. It's been bugging me
> for a while, though.
> I have a number of older Seagate Hawk narrow SCSI 1G-2G drives. I have
> a couple of LC 475s, the SE/30, and a IIci. I've never gotten one of
> the Seagates to work as the internal drive on any of the older Macs
> without using a cable terminator. I've tried term power to drive, to
> bus, from bus, and IIRC, one of the 1G Hawks allows term power both to
> the drive and from the bus. Never works. Terminated cable always
> works.
> "Somebody 'splain this wonderment to me!"

Easy. Some computers with internal SCSI drives do not have a separate
terminator at the source (host adapter) end of the SCSI bus. They
depend on a terminator being present in/on the first drive.

With no terminator at all, the resting signal level on the SCSI bus is
not pulled up (to about +3V) so the open-collector bus drivers on the
host adapter or on the hard drive can't produce any detectable signal
by pulling the bus lines down to ground.

Computers that are built like this include some Macs, all NeXTs, and
probably others that I don't know about. Not SparcStations, they have
internal bus termination built onto the motherboard.


        carl lowenstein   marine physical lab   u.c. san diego
Received on Sat Mar 02 2002 - 19:18:39 GMT

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