AT&T Scsi Drive Specs

From: Eric Smith <>
Date: Fri Jan 19 21:23:58 2001

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

It may be a $10K question, but you get a free answer, and hopefully worth
every penny. :-)

Differential SCSI is electrically completely incompatible with "normal"
(single-ended) SCSI. Differential is now called High-Voltage
Differential, or HVD, to distinguish it from the new Low-Voltage
Differential (LVD).

Single-ended signalling means that each signal pair has one signal that
is at TTL level (under 0.8V for logic 0, above 2.0V for logic 1), and a
signal return at ground level. Note that the active signal never carries
a negative voltage with respect to the return. (In reality there may
be short undershoots due to "ringing"; one of the functions of termination
is to minimize this.)

Differential signalling actively drives both signals of a pair. For a logic
0, one signal (called "true") is driven low, and the other ("complement")
is driven high. For a logic 1, the reverse is done. Note that this means
that the true signal voltage can be either positive or negative with
respect to the complement signal.

Differential signalling is preferred because it is more reliable,
especially when used with long cables. Many high-end servers and
workstations (e.g., IBM, HP, Sun) used HVD for this reason.

Unfortunately, HVD uses exactly the same connectors as single-ended SCSI,
though it will not interoperate unless you use a converter (generally
fairly expensive).

The newer LVD operates on the same principle but at lower voltage levels.
LVD signalling is not compatible with either HVD or single-ended. However
(before everyone jumps all over me), LVD devices will detect that they
are plugged into a single-ended SCSI bus, and switch to single-ended
signalling. Thus you can mix LVD and single-ended SCSI devices on one
bus, but you do not get the benefits of LVD signalling when you do so.
Received on Fri Jan 19 2001 - 21:23:58 GMT

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