jim@jagubox.gsfc.nasa.gov (Jim Jagielski) (05/28/91)
I've noticed on some SCSI drives, like the 335MB Wren Runners (ST4385NM) that
one can select whether you want the terminator power for that drive to come
from the SCSI cable OR from the drive's own power cable. Now I know that Apple
says NOT to have more than 2 sets of terminators or else you may overtax the
line drivers on the SCSI chip, but what if you had these terminators powered
via the power cord?
For example, say you have 4 SCSI disks... ALL terminated except that disks #2
and #3 (#1 is the 1st and #4 is the last) are configured to take their termi-
nator power from the wall and NOT the SCSI cable. Now would THIS configuration
do any damage to the Mac (recall that as far as the Mac is concerned, it's
providing term. power to only 2 sets)? Would this config. cause any SCSI
communications problems (i.e. do "overterminated" SCSi lines get less
reliable)?
Finally, assume that you have a disk which is terminated and set up to take
term. power from the wall... THEN you add an external term. block to it...
What then?
Finally, has anyone noticed problems with the IIfx-type terminator block? That
is, can one rest assured that, assuming all else is Okay with your SCSI bus and
devices, you will have no problem when you switch from the standard ext. term.
block to the IIfx-type? (My question arises from the fact that my MacII is
being upgraded to a IIfx. My system works fine now with the standard issue
external terminators, but what will happen when I start using the IIfx-type?
I'm sure that all will STILL work after all, it's just a coupla' cap's, right?)
:)
--
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#include <std/disclaimer.h>
=:^)
Jim Jagielski NASA/GSFC, Code 711.4
jim@jagubox.gsfc.nasa.gov Greenbelt, MD 20771
"If we increase the size of the penguin until it is the same height as a man
and then compare the relative brain size, we know find that the penguin's
brain is still smaller. But, and this is the point, it is larger than it WAS!"stanfiel@testeng1.misemi (Chris Stanfield) (05/29/91)
In article <5440@dftsrv.gsfc.nasa.gov> jim@jagubox.gsfc.nasa.gov (Jim Jagielski) writes: > >I've noticed on some SCSI drives, like the 335MB Wren Runners (ST4385NM) that >one can select whether you want the terminator power for that drive to come >from the SCSI cable OR from the drive's own power cable. Now I know that Apple >says NOT to have more than 2 sets of terminators or else you may overtax the >line drivers on the SCSI chip, but what if you had these terminators powered >via the power cord? > This isn't an Apple spec., it's part of the SCSI standard. You may not have more than two terminators and they must be connected to opposite ends of the bus, it doesn't matter where they are powered from. If you have too many terminators, the bus impedance is wrong and the bus drivers won't work properly. People will tell you they have had too many terminators on and the bus worked - they were just lucky - it's not guaranteed. The same thing applies to having only one terminator - this will usually work if the bus is short. This mode may actually be supported by the spec - I can't remember and we just moved and I can't find my copy of said spec. Normally, term pwr is supplied via diodes and is automatically supplied from the device rather than the host, although the host will automatically supply power if the device is turned off - that is what the diodes are for. Chris Stanfield, Mitel Corporation: E-mail to:- uunet!mitel!testeng1!stanfiel (613) 592 2122 Ext.4960 We do not inherit the world from our parents - we borrow it from our children.