🚀 go-pugleaf

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