AT&T Scsi Drive Specs

From: Iggy Drougge <>
Date: Sat Jan 20 06:45:59 2001

Eric Smith skrev:

>> 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.

Thank you for the exhaustive explanation. But does this mean that differential
SCSI doesn't have any ground pins, instead using these for the complement?

En ligne avec Thor 2.6.
Vi m?ste vara r?dda om varandra
                              - det ?r det enda reciproka pronomen vi har.
Received on Sat Jan 20 2001 - 06:45:59 GMT

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