LVD SCSI Differentail impedance values have too much tolerance

Walter Bridgewater x2371 wally at eng.adaptec.com
Wed Jan 31 18:22:05 PST 1996


* From the SCSI Reflector, posted by:
* wally at eng.adaptec.com (Walter Bridgewater x2371)
*

In reference to the X3T10 SPI-2 Study Group and 
document X3T10-95/ 315 /R04

In calculating values for a delta current for 
asymmetrical LVD SCSI output drivers, I realized that
there is a large change in current for the different corners
of differential impedance and the bias voltage.

The differential impedance can be 100-115 ohms
and the bias voltage can be 0.100-0.130 volts.

If you look at the current at the corners

0.100/115  =  0.869mA  &  0.130/100 = 1.3mA

This is a 50% change in current.  This is too much tolerance
to design an output driver to that has equal voltage swings.

I would like to make a proposal that the numbers be tighten
up too:
   0.100-0.104 volts for the bias voltage,

because this will minimize the current imbalance, and

 108-112 ohms for the differential impedance,

Because this is in the middle of the expect loaded cable
impedance range of 85-135 ohms.

The above proposal will tightened down the change in current too 8%,
which should help us set the ratio of different currents for the negation
and assertion states.

Wally






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