AT&T Scsi Drive Specs

From: Bruce Lane <>
Date: Fri Jan 19 21:37:19 2001

At 02:48 20-01-2001 +0100, Iggy Drougge wrote:

>And now for the 10 000$ question: what /is/ differential SCSI? In what way
>does it differ?

        Differential is merely a type of electrical interface. Its use is not
limited to SCSI devices: Certain types of high-speed serial ports, such as
V.35, are also differential.

        Here's the details. Single-ended SCSI interfaces have one signal line, and
one matching ground return line, for each SCSI signal (REQ, ACK, D0-D7,
etc.) A signal line is considered to be active or 'asserted' when the line
goes to a logic high (TTL logic, +5V usually) in reference to the ground line.

        With differential, you have two active signal lines per signal. When said
signal is asserted, one line will be pulled to a given positive voltage
(often +12) while the other is pulled to the opposite negative voltage (-12
in this context).

        A differential interface only recognizes that a signal has gone active
when there is a significant difference in potential between the two lines.
This gives a diff interface much greater noise immunity than that of
single-ended setups, and it also allows much greater cable length than you
can have in SE (25 meters vs. 9, as I recall... someone check me on that,

        Critical point: You CANNOT mix diff and SE SCSI devices on the same bus
without an interface converter on the mismatched device. Without said
converter, one of two things will happen.

        1). At best, built-in protective features in the interface drivers will
detect the presence of a differential device (the DIFFSENSE line), and will
shut down the entire bus.

        2). If, for some reason, said features fail to function as designed,
you'll be reaching for a fire extinguisher pretty quick.

        Recommended reading:

Bruce Lane, Owner and head honcho, Blue Feather Technologies // E-mail:
Amateur Radio: WD6EOS since Dec. '77 (Extra class as of June-2K)
"I'll get a life when someone demonstrates to me that it would be
superior to what I have now..." (Gym Z. Quirk, aka Taki Kogoma).
Received on Fri Jan 19 2001 - 21:37:19 GMT

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