Subject: RE: narrow device connected to a wide scsi
From: Paul Bunn (paul@ultrabac.com)
Date: Fri Nov 26 1999 - 09:31:21 AKST
"T"s are actually in the SCSI spec, though they are called "stubs", and all
SCSI buses have them. The spec allows for a maximum stub length for SCSI-2
of 10cm. Ultra/LVD likely change this max length. Note that most devices
already have about 5cm stub length internally (the wiring from the SCSI
connector to the SCSI drivers on the PCB). This gives you a "usable" length
of about 5cm, or about 2 inches -- not much cable length at all ;-)
There are two different types of converters for going from 68->50 pins --
with or without termination. To answer the original posters's question, it
is likely that you'll need a converter that terminates the upper 8bits of
the wide bus. The best source I've found for excellent quality SCSI cables
and terminators is "Granite Digital" (http://www.scsipro.com) and they have
a 68->50 pin converter that automatically terminates the high byte of the
bus. If you use this cable, the converter MUST be placed at the physical
end of the cable. In the poster's case, this would mean that the LAST 68pin
connector must be connected to the backplane/external connector.
When you have a SCSI card that has 3 connectors -- only 2 may be used at any
one time to prevent violation of SCSI stub-length maximums.
Regards,
Paul Bunn, UltraBac.com, 425-644-6000
Microsoft MVP - WindowsNT
http://www.ultrabac.com
----- Original Message -----
From: Rafael Noas <noas@mediasoft.cz>
To: <axp-list@redhat.com>
Sent: Friday, November 26, 1999 7:50 AM
Subject: RE: narrow device connected to a wide scsi
> what happens if I use one of this scsi-backplane connector that connect
> directly to the internal wide scsi cable, not at the end of the bus, and
> make one more external connector, then I can put there my tape without
> terminator. I'm right?
>
> internal wide bus
> |
> |
> +- hdd wide
> |
> +- 68->50pin bakplane convertor -- narrow Tape drive (not
> terminated)
> |
> +- hdd wide
> |
> # terminator
>
>
The SCSI Cable must be a single linear chain. You are putting a T in it,
and this wont work reliably.
The only way the 68-50 pin adapter should be used is to plug directly into
the back of a 50 pin drive,
as there is no T that way. I would recommend using an external 68 pin with
and external 68 pin terminator at the end of the chain. Place the tape
drive in between the teminator and the system, and remove its termination
and use that of the terminator. Use the 68-50 to plug directlry into the
back of the tape unit.
This applies if you are trying to run any ultra speeds on the 68 pin cable.
If you arent, you might be able to get away with a less than compliant
solution.
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