From: Bill Cohane :Subject: Re: [PCBUILD] SCSI drive install problems-
update

Original message (slightly trimmed) below

> At 00:05 1/15/2000 , Brad Loomis wrote:
> >In answer to some previous questions, the drives are seen in the Adaptec
> >Bios scan as well in the SCSI set up. <snip>

> Hi Brad
>
> You didn't mention the Zip drive and scanner. Your total
> cable length is waaaaay too long.
>
> Your SCSI bus starts at the scanner and goes to the Zip
> disk and then to the Adaptec controller and then to the
> three hard drives. This is one SCSI bus and the length
> of the bus is the total of the lengths of all the cables.
>
> In my thinking, the 68 pin cable to your hard drives is
> 3.5 feet long and the 50 pin internal cable is at least
> 2.5 feet long and the external cable to your scanner is
> probably at least 3 feet long (or maybe even 6 feet).
>
> Thus your total cable length is at least 9 feet...probably
> more. You are WAY over the 5 foot limit. With 5 devices
> connected to your adaptec controller, even the most liberal
> interpretation of maximum length is going to be 5 feet.
> (Check Adaptec's installation guide. If you don't have it,
> you can download the installation guide from
> <http://www.adaptec.com/support/overview/aha2940uw.html>.
>
> The other problem (and this one may be even worse) is
> that you appear to be using the scanner to terminate the
> external part of the SCSI bus. (You didn't say if you
> are using a separate Active terminating module plugged
> into the scanner.) The scanner will have a passive
> terminator on board and you cannot use a passive terminator
> ANYWHERE on your SCSI bus if you have Fast or Ultra hard
> drives. (Also, I wouldn't use the termination in a Zip
> drive either. I doubt it is good enough to use at Ultra
> speeds.)
>
> I cannot emphasize this too much. Pull the 50 pin cable
> off the Adaptec controller (make sure that the Adaptec's
> termination is set on AUTO) and see how your hard drives
> work. (You may need to FDISK them again.)
>
> If the drives work when you only have one cable connected
> to the 2940UW, you are going to have to either lose the
> Zip drive and scanner permanently or get another SCSI
> controller just for these two devices. <SNIP>
> Lastly, your SCSI BIOS version 1.34.3 is a good one. No
> problems there.
>
> Regards,
> Bill
>
Brad,

I disagree with Bill on the total length of the cables for the following
reasons:

From Adaptec's manual on the 2940 UW :

"When one or more Fast SCSI Devices are connected to the host adapter, the
total length of all cables (internal and external) must not exceed 3 meters
(9.8ft) to ensure reliable operation.  If no fast SCSI devices are
connected, the total length of all cables must not exceed 6 meters (19.7ft).
"

From Iomega's SCSI insider manual:

"For any SCSI chain, the combined chain length (the total of all cables in
the SCSI chain, including internal cables) should not exceed 6 meters (19.6
feet)."

Unless your scanner is a fast SCSI device then the total length of your
cables should not exceed 6 meters. (19.6 ft).  Not all SCSI scanners cost
less then cables. I have an HP 4C scanner as the last device on my external
SCSI bus connected to an external zip, connected to an external jaz,
connected internally to a Plextor CD Rom and a Plextor CDR then to the host
adapter.  I have not used any of the cheaper scanners around so I can't say
what kind of termination they use. Internally I have 3 UW SCSI drives.  The
scanner has termination enabled on the scanner and the last drive on the UW
chain has termination enabled.  The total length of the cables on my system
definitely exceeds 5 ft. and my system has been running with no problems for
over 2 years.

Check your scanners documentation to see what kind of termination it has and
what kind of SCSI interface it has.  You can then determine what total cable
length applies to your setup.

Mary Wolden

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