Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!think!mintaka!ogicse!blake!milton!whit
From: whit@milton.acs.washington.edu (John Whitmore)
Newsgroups: comp.sys.mac.hardware
Subject: Re: Can the SCSI bus be terminated w/o powering up noisy device?
Message-ID: <2031@milton.acs.washington.edu>
Date: 20 Feb 90 01:14:48 GMT
References: <17008@boulder.Colorado.EDU> <1990Feb15.204213.29062@chaos.cs.brandeis.edu> <47013@improper.coherent.com>
Reply-To: whit@milton.acs.washington.edu (John Whitmore)
Organization: University of Washington, Seattle
Lines: 18

In article <47013@improper.coherent.com> dplatt@coherent.com (Dave Platt) writes:
>
>The SCSI-1 specification forbids having powered-off devices attached to
>the bus... so these "unfriendly" devices are not really in violation of
>the spec, I think.  I'd much rather buy devices that have a robust
>terminator-power circuit, however!
>-- 
	There is a strong likelihood that the SCSI receiver circuits are
diode-clamped to the power supply, so that a powered down SCSI device
can act as an improper termination.  Integrated circuits don't just have
switches that turn "off"; they use diodes and ONLY WHEN POWER IS APPLIED
AS DESIGNED do those diodes turn off.  So, a powered down disk drive
should be unplugged, regardless of whether it contains termination
resistors or not.  SCSI drive/receive chips will vary, and some varieties
may be inert when powered down.  Don't bet on it.

I am known for my brilliance,             John Whitmore
 by those who do not know me well.