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On Saturday, August 25, 2001, at 04:46 PM, der Mouse wrote:

>> I am having trouble with one SCSI tape drive.  No idea why.  [...]
>
>> The tape PowerMacinstosh 6500/300 has the classic Mac 25 pin SCSI.
>
> I have a Power Macintosh 4400 with 25-pin SCSI.  I once opened it up
> and was somewhat astonished to see that internally it's a normal 50-pin
> Berg, but they simply drop half the lines (grounds) for the 25-pin
> connector.  If your cabling doesn't do grounding very carefully, you
> may be suffering from unfortunate transmission-line effects (which can
> look a lot like termination problems).  SCSI really really likes to
> have signal and ground _pairs_; I've never understood why Apple felt it
> worth the dollar or two difference to cut corners on the ground pins.

My _guess_ would be that it was more an issue of:

1.  Space on the back panel.  By a factor of two or more.
2.  Not having a connector on the back of your computer that breaks 
easily.

#2 being the critical one.  It's way too easy to bend or break those 
little metal snap locks on the Centronics connectors.  Your 4400 would 
probably be hard to use by now if Apple had used the 50 pin connectors.  
While I'm not sure if I agree with dropping grounds, I would have to 
agree that the decision to use a more sane connector design was a good 
one.

That having been said, I obviously wasn't at Apple at the time, and I 
speak entirely for myself, not my employer.  ;-)


>> It strikes me that the behavior is similar to other times I've had
>> termination problems, but as far as I can tell the SCSI bus is
>> correctly terminated.  Would I have better luck if it were not?
>
> Maybe.  I am not a high-frequency transmission-line engineer; what
> little I know of the field indicates that in situations like these it
> borders on black magic.

Yup.  If you're getting really flaky behaviour, verify that the 
termination voltage being supplied is sufficient (I don't remember the 
correct voltage, do a google search), and if not, consider blocking the 
term power from the computer and then enabling the hard drive's jumper 
to provide term power.  Also, I've actually found that about one out of 
every two inexpensive SCSI cables is insufficiently shielded and just 
plain doesn't work reliably.  Yeah, that bad.  Try a different cable, 
shorter if possible.  If you can get it under a foot or so, IIRC, you 
can actually go with no termination, but I don't recommend this.  :-)



>> Should I just pack it in and forego any attempt to use the tape drive
>> as part of this set up.
>
> If you aren't afraid to get your hands dirty, you might open up your
> machine and see if yours too has 50-pin ribbon cable internally; if so,
> you might remove the cable and the DB-25 and run ribbon cable out to
> the external devices.

Better to try to mount the tape drive internally if your machine has a 
spare bay that'll hold it.  Maybe move the CD-ROM externally.  Also make 
sure you aren't getting doubly terminated.


David

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On Saturday, August 25, 2001, at 04:46 PM, der Mouse wrote:


<excerpt><excerpt>I am having trouble with one SCSI tape drive.  No
idea why.  [...]

</excerpt>

<excerpt>The tape PowerMacinstosh 6500/300 has the classic Mac 25 pin
SCSI.

</excerpt>

I have a Power Macintosh 4400 with 25-pin SCSI.  I once opened it up

and was somewhat astonished to see that internally it's a normal 50-pin

Berg, but they simply drop half the lines (grounds) for the 25-pin

connector.  If your cabling doesn't do grounding very carefully, you

may be suffering from unfortunate transmission-line effects (which can

look a lot like termination problems).  SCSI really really likes to

have signal and ground _pairs_; I've never understood why Apple felt it

worth the dollar or two difference to cut corners on the ground pins.

</excerpt>

My _guess_ would be that it was more an issue of:


1.  Space on the back panel.  By a factor of two or more.

2.  Not having a connector on the back of your computer that breaks
easily.


#2 being the critical one.  It's way too easy to bend or break those
little metal snap locks on the Centronics connectors.  Your 4400 would
probably be hard to use by now if Apple had used the 50 pin
connectors.  While I'm not sure if I agree with dropping grounds, I
would have to agree that the decision to use a more sane connector
design was a good one.


That having been said, I obviously wasn't at Apple at the time, and I
speak entirely for myself, not my employer.  ;-)



<excerpt><excerpt>It strikes me that the behavior is similar to other
times I've had

termination problems, but as far as I can tell the SCSI bus is

correctly terminated.  Would I have better luck if it were not?

</excerpt>

Maybe.  I am not a high-frequency transmission-line engineer; what

little I know of the field indicates that in situations like these it

borders on black magic.

</excerpt>

Yup.  If you're getting really flaky behaviour, verify that the
termination voltage being supplied is sufficient (I don't remember the
correct voltage, do a google search), and if not, consider blocking
the term power from the computer and then enabling the hard drive's
jumper to provide term power.  Also, I've actually found that about
one out of every two inexpensive SCSI cables is insufficiently
shielded and just plain doesn't work reliably.  Yeah, that bad.  Try a
different cable, shorter if possible.  If you can get it under a foot
or so, IIRC, you can actually go with no termination, but I don't
recommend this.  :-)



<color><param>0000,0000,DEB7</param>

</color><excerpt><excerpt>Should I just pack it in and forego any
attempt to use the tape drive

as part of this set up.

</excerpt>

If you aren't afraid to get your hands dirty, you might open up your

machine and see if yours too has 50-pin ribbon cable internally; if so,

you might remove the cable and the DB-25 and run ribbon cable out to

the external devices.

</excerpt>

Better to try to mount the tape drive internally if your machine has a
spare bay that'll hold it.  Maybe move the CD-ROM externally.  Also
make sure you aren't getting doubly terminated.



David


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