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Can I leave my SCSI device powered off while the computer is on? - Mac Applications & Software

I read the question below in a FAQ on the LaCie site... The answer surprises me. Is it really true I can't use my computer with connected scsi devices turned off? I've been doing that for years...!! ------------------------------------------------------------------------ *Can I leave my SCSI device powered off (or disconnect it) while the computer is on?* No. All SCSI devices connected to the computer MUST be powered on while the computer is on. You want to turn on all SCSI devices before powering on the computer and leave the devices on until after you have done a proper shut down of the ...

  1. #1

    Default Can I leave my SCSI device powered off while the computer is on?

    I read the question below in a FAQ on the LaCie site... The answer
    surprises me. Is it really true I can't use my computer with connected
    scsi devices turned off? I've been doing that for years...!!

    ------------------------------------------------------------------------
    *Can I leave my SCSI device powered off (or disconnect it) while
    the computer is on?*

    No. All SCSI devices connected to the computer MUST
    be powered on while the computer is on. You want to turn on all SCSI
    devices before powering on the computer and leave the devices on until
    after you have done a proper shut down of the computer. Failure to run
    with SCSI devices powered on can cause the computer to not operate
    properly and may cause directory corruption.

    NOTE:Never connect or disconnect a SCSI device while the computer is
    powered on. You can cause hardware damage. Always power off the
    computer first and then power off all SCSI devices before connecting or
    disconnecting anything on the SCSI bus.

    ------------------------------------------------------------------------

    Steven

    --
    nl



    Steven Guest

  2. #2

    Default Re: Can I leave my SCSI device powered off while the computer is on?

    >I read the question below in a FAQ on the LaCie site... The answer 

    The answer is "It depends."

    If the SCSI device in question is the last device in the chain, is actively
    terminated and self-generates terminating power, then no; if you leave the
    device off, the SCSI chain is not terminated.

    If the device (or the entire chain) is passively terminated, then yes--as long
    as the device does is not providing term power for the last device in the
    chain.

    This is "SCSI voodoo" stuff--sometimes, your compute will work just fine if
    your SCSI devices are turned off, other times your entire SCSI chain will act
    funky if you have a SCSI device that's off. It depends on the device, the SCSI
    chain, the way it's terminated, the phase of the moon, and when the last time
    you waved a dead chicken over your computer was.

    --
    Rude T-shirts for a rude age: http://www.villaintees.com
    Art, literature, shareware, polyamory, kink, and more:
    http://www.xeromag.com/franklin.html

    Tacit Guest

  3. #3

    Default Re: Can I leave my SCSI device powered off while the computer is on?

    Steven <nl> wrote:
     

    Then you know the answer to your question. There used to be a few SCSI
    devices that would cause problems on the SCSI chain if they were not
    powered. Almost all recent SCSI devices (made in the past ten years or
    so) will pass SCSI signals and provide termination (if applicable) even
    when they are off.

    The only real limitation these days is that OS X will generally not
    recognize any SCSI devices, including optical and other removable-media
    drives, that are unpowered at startup.

     

    This part is true. Hot-plugging SCSI devices that are not designed for
    it is perilous.
    Neill Guest

  4. #4

    Default Re: Can I leave my SCSI device powered off while the computer is on?

    In article <supernews.com>,
    Steven <nl> wrote:
     

    LaCie would be selling bad hard drives if this was true. The only
    exception is if the device has an internally powered terminator. Often
    there's a switch that can be used to draw termination power from the
    SCSI bus instead, as an external terminator would.
    Kevin Guest

  5. #5

    Default Re: Can I leave my SCSI device powered off while the computer is on?

    net (Neill Massello) writes:
    [...] 

    Most computer equipment needs electricity flowing through it to
    properly operate. :>
     
    >
    > This part is true. Hot-plugging SCSI devices that are not designed
    > for it is perilous.[/ref]

    Did Apple take the camcontrol(8) command from FreeBSD?

    It's possible to send SCSI commands using it. Is there a 'power off'
    SCSI command? I know I could power-off Fibre Channel disks on a Sun
    box I had access to. AFAIK, Fibre Channel is just a sub-set of SCSI.

    --
    David Magda <dmagda at ee.ryerson.ca>, http://www.magda.ca/
    Because the innovator has for enemies all those who have done well under
    the old conditions, and lukewarm defenders in those who may do well
    under the new. -- Niccolo Machiavelli, _The Prince_, Chapter VI
    David Guest

  6. #6

    Default Re: Can I leave my SCSI device powered off while the computer is on?

    David Magda <dmagda+ryerson.ca> wrote:
     
    >
    > Most computer equipment needs electricity flowing through it to
    > properly operate. :>[/ref]

    Before OS X took away direct access to hardware by applications, it was
    possible to power up a SCSI CD burner, launch Toast, and burn a CD, all
    without restarting. In OS X, this works with FireWire and USB, but not
    with SCSI devices, which need to be powered on at startup. (If somebody
    knows a command line workaround for this, please post it.)

     
    > >
    > > This part is true. Hot-plugging SCSI devices that are not designed
    > > for it is perilous.[/ref]
    >
    > Did Apple take the camcontrol(8) command from FreeBSD?
    >
    > It's possible to send SCSI commands using it. Is there a 'power off'
    > SCSI command? I know I could power-off Fibre Channel disks on a Sun
    > box I had access to. AFAIK, Fibre Channel is just a sub-set of SCSI.[/ref]

    I can't find any file or man page for camcontrol in my 10.2.6
    installation.

    There is a spindown in the SCSI command set, but I don't see how that
    would prevent possible damage from hot-plugging, which comes from
    shorting and arcing when a connector is plugged or unplugged on a
    powered SCSI bus.
    Neill Guest

  7. #7

    Default Re: Can I leave my SCSI device powered off while the computer is on?

    In article <BB58700F.FEE4%com>,
    Henry <com> wrote:
     
    > >
    > > Before OS X took away direct access to hardware by applications, it was
    > > possible to power up a SCSI CD burner, launch Toast, and burn a CD, all
    > > without restarting. In OS X, this works with FireWire and USB, but not
    > > with SCSI devices, which need to be powered on at startup. (If somebody
    > > knows a command line workaround for this, please post it.)
    > >
    > > 
    > >
    > > I can't find any file or man page for camcontrol in my 10.2.6
    > > installation.
    > >
    > > There is a spindown in the SCSI command set, but I don't see how that
    > > would prevent possible damage from hot-plugging, which comes from
    > > shorting and arcing when a connector is plugged or unplugged on a
    > > powered SCSI bus.[/ref]
    >
    > Here's my two cents: I've got a very nice EPSON scanner that is (you
    > guessed it) SCSI. I use it very infrequently, so I want to leave the power
    > off. There seems to be no harm to powering it up while my G4 (MacOS 10.2.6)
    > is running, except of course, it's inaccessible -- there's only one scan for
    > devices, and that occurs during boot-up.
    >
    > I've located a system function that will apparently rescan the bus, but I
    > don't see installing the developer tools and learning how to use them for
    > the sake of calling one function. (Anyone want to volunteer?)
    >
    > One more thing: SCSI connector clips being unreliable, there are plenty of
    > time I've had a cable fall off while everything is powered, and I've never
    > had any damage. Likewise, I've been known to wiggle a connecter, thinking
    > (barely) "that looks a little loose" -- and finding that it was in fact
    > half-way off. Again, no damage. I'm not suggesting that people start
    > hot-plugging SCSI connectors, but just giving evidence that at least some
    > SCSI equipment isn't all that fragile.
    >
    > OK, I gotta say it. YMMV radically. DO NOT TAKE THIS AS A RECOMMENDATION
    > TO HOT-PLUG SCSI!!!
    >
    > Thanks,
    >
    > Henry
    >
    > com remove 'zzz'
    >[/ref]

    I have a Microtek ScanMaker4 hooked up to my G3. It's a SCSI connection.
    I never turn on that computer until all of my external SCSI devices are
    powered up, or never did until I got the scanner. I don't use it that
    often and found it works just fine if I turn it on when I need it and
    off when I'm finished. I'm not going to advise anyone else to do the
    same, because I've heard that you need to have all devices in the chain
    on or off. I'm surprised only one person has used the term "SCSI Voodo"
    so far. SCSI devices have always had minds of their own in spite of
    rules to the contrary.
    bux Guest

  8. #8

    Default Re: Can I leave my SCSI device powered off while the computer is on?

    In article <supernews.com>,
    bux <com> wrote: 

    8600/250 upgraded with G3/450, copious RAM, running 9.1.

    I have a SCSI scanner on the end of my SCSI chain, which has two other
    devices on the chain. I keep the scanner on all the time because it goes
    to sleep when not in use. the rest I keep turned off unless I am using
    them. So far never had any problems, just turn on what I need when I
    need it. I use Silverlining Lite to mount device after turning it on.
    Have to re-boot to use the scanner after I burn a disk, as the toast
    extension seems somehow to disable the scanner. Other than that, smooth
    like creamery butter.
    --
    Trashy Girl

    Trashy Guest

  9. #9

    Default Re: Can I leave my SCSI device powered off while the computer is on?

    net (Neill Massello) writes:
     

    I'm wondering whether OS X uses FreeBSD's CAM [1] at all. Under
    FreeBSD it's used for just about all dynamically allocated storage
    devices. Most notably SCSI, FireWire (FW) and USB devices.

    You can rescan a SCSI bus for new devices by simply doing a
    'camcontrol rescan'. Also, when FW drives are plugged in they are
    automagically detected and can be used right away.

    I have a SCSI tape library hooked up all the time to a SCSI card but
    it's power off most of the time. When I to use it I just flip the
    switch and rescan the bus. When done, I turn it off and rescan. The
    tape/loader devices are automatically added and deleted. (This may
    not be the smartest thing to do thought. :)

    I'm sure there's a Apple developer doc on this somewhere. :>
     

    ACK.

    [1] http://www.freebsd.org/cgi/man.cgi?query=cam

    --
    David Magda <dmagda at ee.ryerson.ca>, http://www.magda.ca/
    Because the innovator has for enemies all those who have done well under
    the old conditions, and lukewarm defenders in those who may do well
    under the new. -- Niccolo Machiavelli, _The Prince_, Chapter VI
    David Guest

  10. #10

    Default Re: Can I leave my SCSI device powered off while the computer ison?

    Thanks for all the comments! I have 8600 with two external SCSI devices,
    a Linotype flatbed scanner and a Nikon film scanner. I *never* turn
    either of them on unless I need to and I usually turn on only one of the
    two. That was my practice until I upgraded to Mac OS X. I never had a
    problem with it. Too bad Mac OS X isn't as SCSI friendly as Mac OS 9...
    I guess this means switching to FireWire scanners eventually...

    Steven


    --
    nl


    Steven Guest

  11. #11

    Default Re: Can I leave my SCSI device powered off while the computer is on?

    Steven <nl> writes: 

    This is not strictly true, but it is effectively true for some drives.

    An external drive with built-in termination may not be providing
    power to the terminator when it is switched off. An unpowered
    terminator won't terminate the bus, resulting in flaky behavior.

    Since LaCie can't know how you're terminating your bus (external
    terminator, internal bus-powered terminator or internal drive-powered
    terminator), they tell you to leave all drives on at all times. This
    avoids the problem and eliminates the need for their support people
    ot have to think about the problem.
     

    This is absolutely true. While I've never seen this cause hardware
    damage, I have seen this result in system crashes and drive
    corruption.

    -- David

    David Guest

  12. #12

    Default Re: Can I leave my SCSI device powered off while the computer is on?

    David Magda <dmagda+ryerson.ca> writes: 

    Both still require a voltage supply. The difference is that one has
    a regulator in the terminator and the other doesn't.

    In either case, some device on the SCSI bus (doesn't have to be the
    one nearest the terminator) must be providing termination power to
    the bus. Most SCSI adaptors will provide this, but some do not.
     

    I am aware of this commonly cited statement, but I completely
    disagree. I've seen plenty of flaky non-ultra SCSI busses that
    became stable after replacing passive terminators with active ones.

    As far as I'm concerned, the entire concept of passive termination is
    an evil monstrosity that should be completely banned from existance.

    -- David
    David Guest

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