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Grex Hardware Item 170: Fried SCSI bus with a side of grits. Help, please.
Entered by other on Sun Dec 5 05:03:27 UTC 1999:

I think my SCSI bus fried.  Help me figure this one out if you can.  Here are
the details:

Machine:
        Macintosh Centris 640
        Motorola 68040
        Mac OS 8.0

Symptoms:
        At startup, i get the disk icon with the flashing "?" symbol.
        When I tried to force boot from a CD-ROM, i got the black screen with
the sad mac face and these two lines under it:
                        00000F
                        000001

        The machine also will not seem to be willing to allow a force boot from
a floppy, because as soon as i put one in, it spits it right out again, or
if i start the machine with one in, it spits it out as soon as it powers up.

(I may have the force boot keys wrong.  i can't find a reference.  if you know
what they are, please post.)

        I swapped out the hard drive for another one, and nothing is different.
        (The drives i swapped have both successfully booted this machine
before.)

Any questions, comments, suggestions?  How can i determine with certainty
whether the problem is the SCSI bus?  Is there anything I can do about it,
or would i have to put in a new motherboard? (or buy a new computer, since
the replacement wouldn't be worth it for a 1993 machine.)

Thanks!

2 responses total.

#1 of 2 by mdw on Mon Dec 6 06:23:55 1999:

I think you need to find out what the numbers mean.  I don't believe the
floppy is on the scsi bus, so if you can't boot from floppy either, that
suggests some other more major problem.  Perhaps there's an error with
low memory, a fault with the DMA logic, or some other serious problem.

First thing you might check is cabling.  The next thing you might check
is memory -- you might start with taking out all the memory, then trying
with 1 module at a time.  I don't think the 68040 needs pairwise memory,
so this should be sufficient.  Try each memory chip by itself.  If you
have a voltmeter, something else you might check is the logic level for
+5 and +12.  If you have an oscilliscope, you might also check for
ripple on these lines.  A bad power supply could easily result in the
motherboard operating properly (since it only needs +5) but improper
operation of peripherals (which often need +12).

Unless this is your personal machine, it may not be worth spending a lot
of time on this.  If you want to fix it, you might find it worthwhile to
visit UM property disposition (or elsewhere) to pick up a used centris
and swap parts.  Of course, there's every chance if you found another
centris, that it might have the same dead part as yours - if there's
some sort of family weakness in the product.

#2 of 2 by n8nxf on Mon Dec 6 12:00:56 1999:

Check the internal battery.  Macs do odd things when their batteries go dead.

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