LVDS-SCSI

Kevin Gingerich 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/
 
Subj  LVDS-SCSI
 
>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 Cudzewicz
>Fermilab
 
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
receiver.
 
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.
 
Regards,
/Kevin/





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