On 16 May 98 at 19:54, ferdy wrote:

> I have an Adaptec 2940UW, and along the lines of the SCSI thread
> of a few days ago, I have a question concerning cabling. I would
> like to know if the controller card has to be the first device,
> that is on the END of the cable. I am running from the controller
> (50 pin) to one internal device then to an internal to external
> adapter (as David G. spoke of) out to a jaz drive. The cable isn't
> long enough if I put the controller on the end. So I wanted to put
> the second connector on the card, one end on the adapter then run
> the long length to the internal device. Four connectors total on
> cable. Is this OK? Thanks.

  Generally, SCSI devices are identified by ID, and may be attached
along the bus in any order.  The bus is terminated at each end.

  The Adaptec controller provides "auto termination" -- which you can
turn off if you want -- which is supposed to terminate the bus at the
controller card if it finds that it is the "end" device in that
direction.

  When you say "four connectors", I believe you are counting the
internal-to-external adapter; that doesn't count.  [I may be
misunderstanding you.]  So the situation you're asking about looks
like this:

----------                  ---------   -------
[Internal]========+=========[Adaptor]===[ Jaz ]
[ Device ]        L _       ---------   [Drive]
----------   ------------               -------
             [  Adaptec ]
             [Controller][
             ------------

  This is perfectly legitimate; when the Jaz drive is disconnected or
unpowered, the Adaptec should detect that case and terminate the bus
on that side.  The internal device should always be terminated in
this arrangement.

  If the internal device were between the controller and the adaptor,
then you'd have to enable termination on the device when the Jaz is
removed/unpowered, but disabled when you actually want to use the
Jaz.


Disadvantage:
  Note that in the above case, there are 50-pin devices on the bus on
both sides of the controller.  This means that you cannot use either
of the other two (wide) connectors on the Adaptec, since that would
try to make a "Y"-shaped SCSI bus.  Since we are using UltraWide SCSI
hard drives, we connect the 50-pin connector only to the adaptor, and
connect all internal devices to a 68-pin connector instead.

David G