At 2:53 AM 11/18/97, SamMaEl wrote:
>        A question though... if the internal drive is terminated.... would
>that block me from being able to add other drives onto the chain later?
>Like if I fix the APS case and throw a 170 MB drive into it that I have
>lying around? Thanks!

The SCSI spec says you need exactly two terminators, one at either end of
the chain.  For many Mac's this means you have one internal drive and the
last external drive terminated, either with termination on the drive itself
or with a seperate terminator.

If you have not external devices you probably don't need the second
terminator since the length of the internal portion of the SCSI bus is
short enough.  The real electrical requirement is that noise from signal
reflections off an unterminated end be damped quickly enough.  If the bus
is very short then a reflection from one end will travel the length of the
bus and die at the terminator on the other end very quickly and everything
works.

No one can really tell you what the limits of your chain will be, aside
from the fundamental limits of a few meters total length and 8 SCSI ID's.
Some things that will give you some extra margin are:  using an active
terminator instead of the passive one usually sold for Apple's, using
high-quality cables like Apple or Ultra-Spec, and using the shortest cable
possible for the configuration.  It is also a good idea to put multiple
devices in a single external box when possible.

Some people advise minimizing the number of connector types in the cable
chain, but you will want to do that for other reasons as well since it's a
real pain dealing with multiple types of SCSI connectors.  In general you
can get reflections anywhere there is a change in the wiring:  a connector,
a device, whatever.  The bigger the change, the bigger the reflection.

I have never successfully used more than two external boxes on a SCSI
chain.  On the other hand I have been able to put 6 devices on a bus
several times (counting the motherboard controller that is) with only a
little bit of fiddling as long as I use active terminators.  My current
NetBSD box has two disks in an old Sun box and two internal disks, plus an
internal CD-ROM drive.  It seems solid, but I had a hard time getting the
internal cables in the Sun box seated properly.

Hard as I've tried, I've never had enough devices around to use up all 7
SCSI ID's. ;-)

To get back to your question:  the internal drive is *supposed* to be
terminated and that does not limit your external expansion options.  In
fact if it isn't it may cause problems since there is usually no
termination on the motherboard.  You want that end of the chain terminated
somehow.

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