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- Path: sparky!uunet!overload!dillon
- From: dillon@overload.Berkeley.CA.US (Matthew Dillon)
- Newsgroups: comp.sys.next.sysadmin
- Subject: Re: Turning off SCSI devices and Rel 2.1
- Distribution: world
- Message-ID: <dillon.0k2a@overload.Berkeley.CA.US>
- References: <1992Jul22.114709.9162@investor.pgh.pa.us> <4455@rosie.NeXT.COM> <1992Jul27.025718.22038@umbc3.umbc.edu> <nqab7h8@zuni.esd.sgi.com>
- Date: 27 Jul 92 09:34:57 PST
- Organization: Not an Organization
- Lines: 49
-
- In article <nqab7h8@zuni.esd.sgi.com> olson@anchor.esd.sgi.com (Dave Olson) writes:
- >In <1992Jul27.025718.22038@umbc3.umbc.edu> brian@umbc4.umbc.edu (Brian Cuthie) writes:
- >| In article <4455@rosie.NeXT.COM> amm@NeXT.COM (Alan M. Marcum) writes:
- >| >Bob Peirce #305 writes
- >| >> I have an HSD scanner at the end of my SCSI chain. Since I don't
- >| >> ...
- >| >The SCSI bus is actually an active bus. To function correctly,
- >| >all devices on the bus should be powered on, the cables should
- >| >....
- >| This is bunk. The SCSI bus is driven by open-collector devices. A
- >| properly behaved SCSI peripheral may be left in the powered down state without
- >| any ill effects.
- >
- >Ah, thus speaks someone who hasn't dealt with the real world. Many
- >(most?) SCSI devices will pull all the bus lines into the active state
- >when powered off. In fact SCSI 2 only has an implementors note (NOT a
-
- The problem is that nearly all IC's have protection diodes on their I/O
- lines. That is, a diode to VDD as part of their internal circuitry.
- When the unit is powered down VDD (the power bus) goes to 0V thus
- forward biasing the input protection diodes and forcing the bus line to
- 0-0.7V. ANY INPUT, INPUT-OUTPUT, OR TRI-STATE LINE HAS THIS. Many
- output-only pins also exhibit similar behavior depending on the
- process.
-
- Simply put, you have to use special IC's that do not have the
- aformentioned protection diodes or use the SCSI power pin to keep part
- of your board powered up to keep the input protection diodes out of the
- way.
-
- And before anyone asks why it is necessary to have input protection
- diodes... without them you blow up the IC when you touch it. Even
- *with* them it's relatively easy to blow up IC's through static
- discharge.
-
- -Matt
-
- >--
- >Let no one tell me that silence gives consent, | Dave Olson
- >because whoever is silent dissents. | Silicon Graphics, Inc.
- > Maria Isabel Barreno | olson@sgi.com
-
- --
-
- Matthew Dillon dillon@Overload.Berkeley.CA.US
- 891 Regal Rd. uunet.uu.net!overload!dillon
- Berkeley, Ca. 94708 ham: KC6LVW (no mail drop)
- USA
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-