Subject: Re: SCSI CD changers supported?
To: Tom I Helbekkmo <>
From: Michael L. Hitch <>
List: current-users
Date: 09/10/1996 10:18:24
On Sep 10,  1:56pm, Tom I Helbekkmo wrote:
} There are still a couple of problems with the DRM-610, though.  The
} first is that it takes 5-6 seconds for it to change CDs, which
} wouldn't be too bad except that the whole SCSI bus is locked during
} this time, which is sort of silly.  It gets especially bad when I

  Since your DRM-610 does not accept the LUN in the IDENTIFY message, it
may very well not accept the IDENTIFY message period [I have a Chinon
CDROM drive that does that].  If the device does not accept or honor the
IDENTIFY message, it won't be able to disconnect from the bus while the
device is busy, and nothing else can use the SCSI bus at that time.

} exercise it a bit harder, by working actively on two CDs at the same
} time: after a couple of changes, the system will panic with a timeout
} on one of my magnetic disk drives.  I'm guessing that a command to the
} disk drive happens to be sent to the controller after commands have
} been queued that will demand two CD changes before the command for the
} disk will be processed, and the jump from 5-6 seconds to 10-12 causes
} some timeout limit in the SCSI driver to be reached.  Is there no way
} around this sort of thing?  Couldn't the driver ask the controller how
} the command is faring, and be told "hold your horses; we're getting
} around to it"?

  I don't see any way around this other than to increase the timeouts on
the disk access long enough to deal with the CD delays.  If the DRM-610
is really unable to disconnect, there isn't anything the driver can do
except wait for the operation to complete or a timeout to occur.  As long
as the device is busy, the SCSI bus will be busy, and the driver can't
do anything.  [I suspect the timeout values are set up for fixed disk
access times, which should never take very long.  A CD changer could have
much longer delays which the timeout values don't take into account.
Perhaps there needs to be something that can dynamically adjust the timeout
values to longer times when access the DRM-610.]


Michael L. Hitch			INTERNET:
Computer Consultant
Information Technology Center
Montana State University	Bozeman, MT	USA