S=Kevin_Gingerich%S=Gingerich%G=Kevin%TI at mcimail.com
Tue Oct 10 14:26:00 PDT 1995
To SCSI Reflector X400
>From Kevin Gingerich GING
/pn=scsi/mbx1=scsi at symbios.com/ems=internet/admd=mci/c=us/
>I've probably missed something, but is there any difference between what
>eveyone is calling LVDS and the present Differential SCSI we're using now? We
>have a lot of Fast and Wide differential disk drives spinning at our site
>today. Just wondering if LVDS is going to use the same signaling levels, etc.
Ron, among other differences, the minimum output level of LVDS is 250 mV versus
1000 mV for RS-485 SCSI. LVDS does not have the noise margin for the cable and
stub lengths, bus loading, and live insertion as does RS-485. A LVDS signal can
be received by a RS-485 receiver and a RS-485 signal may be received by an LVDS
The current connector pin assignments are defined to accomodate Single-ended
SCSI compatibility and not the existing Differential SCSI.
>Yes, Ron. LVDS is a lot different from conventional differential scsi.
>LVDS is signal level compatible to industry standards such as IEEE-1596.3,
>EIA-422, 485 or 612. Diff scsi is EIA 485 compatible.
>To make the long story short, its main motivations are to:
>1) Take advantage of signal integrity of differential scsi to enable
> faster data tranfer rate (Fast-40 and Fast-80).
>2) Reduce the driving power on the signal so the drivers can be built
> in to the LSI device for lower cost.
>LVDS is being documented in the new SPI-2 project in ANSI X3T10 and it
>is expected to have ability to automatically detect the signal levels,
>(it uses the same cable pin-outs as its predecessor), but they cannot
>talk the different levels (single ended, differential, LVDS) on the bus.
>Yes, it does have the potential of obsoleting the existing differential scsi.
> Tak Asami
Tak, be careful, LVDS (EIA/TIA-644) is compatible with the standards you
reference only under very restricted conditions of common-mode voltage, output
voltage levels, and input thresholds.
As I wrote of above, you will not have the same noise margins to work with
under LVDS that RS-485 enjoys. You do get less EM emissions and
susceptibilility compared to single-ended signals at the same speeds and the
cost advantage of integration. These features indicate to me that LVDS-SCSI is
a replacement for Single-ended SCSI and not Differential.
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