At 19:54:05 16 May 1998 -0700, Brad wrote:
>I have an Adaptec 2940UW...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 out to a jaz drive. The cable isn't
>long enough if I put the controller on the end...


This sounds okay in principle, provided you follow the rules for termination
and linearity. You must terminate the devices at the ends of the cable. That
is, the jaz drive and the internal device you mention must both be terminated
and the controller must be unterminated. Also, this method would preclude
using either of the other two connectors on the controller, since to do so
would result in a "Y" shaped SCSI bus.

But...

I wonder if doing this might confuse the 2940UW controller. Obviously the
controller expects to see devices on either one or two of it's connectors.
When it sees devices on one connector only, the controller might logically
expect to be at the end of the chain. Certainly you would not be able
to rely on the automatic termination feature of the 2940UW in this case.
Whether there would be problems with manual termination, or problems with
the internal circuitry of the controller, remains to be seen. There might
be stub length problems. For example, suppose the distance from the 2940UW's
connector to the 78xx chip is four inches. When the controller is at an end
of the bus, or if the bus goes through the 78xx chip in the case where two
of the 2940UW connectors are used, this four inches is part of the SCSI bus.
But in your case, this four inches is a stub leading off the SCSI cable
chain. This *might* be a problem, particularly at Ultra SCSI frequencies.
(Please note that the four inch distance I mentioned is an example only.
It may be more or less. There may also be other problems that I have not
considered.)

This is an interesting idea, analagous to plugging the middle connector of
the IDE cable into the IDE controller to provide smaller distances between
the controller and either IDE device connected to it. This SCSI idea might
work, or it might work intermittently or in some cases only, or it might
fail to work at all. Since this is a non-standard setup, you might expect
to get non-standard results.

But if you don't try this, how will we know whether it's an acceptable
setup?

Regards,
Bill