Re: Powered-down IBM drive hangs bus

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#80

From: KurtChan
Date: Thu, 07 Dec 1995 21:27

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Some SCSI chips have ESD protection diodes. One diode tries to prevent the
SCSI
signal from rising above VDD and the other diode tries to prevent the SCSI
signal
from falling below GND. As a result, the VDD clamp diode will "load" the
SCSI signal
if VDD is lost. This can make it difficult for signals to be deasserted...
which on
open-collector SCSI is difficult to do cleanly anyhow (which is the
premise behind
active termination).

SCSI-2 recommended that devices not load the SCSI bus on power-down
conditions,
but there are still plenty of  SCSI-1 (or poorly designed SCSI-2) devices
which do
not comply with this recommendation.

Regards,
Kurt Chan
kurtchan@aol.com

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From: kurtchan@aol.com (KurtChan)
Newsgroups: comp.sys.mac.hardware.storage,comp.sys.mac.system,comp.periphs.scsi
Subject: Re: Powered-down IBM drive hangs bus
Date: 7 Dec 1995 16:27:22 -0500
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