LVD SCSI Common Mode Resistance

Kevin Gingerich S=Kevin_Gingerich%S=Gingerich%G=Kevin%TI at
Mon Jan 22 07:32:00 PST 1996

* From the SCSI Reflector, posted by:
* <Kevin Gingerich <S=Kevin_Gingerich%S=Gingerich%G=Kevin%TI at>>

  To  SCSI Reflector   X400
>From  Kevin Gingerich  GING
/pn=scsi/mbx1=scsi at
Subj  LVD SCSI Common Mode Resistance
Dennis, the relationship I derive for the common-mode resistance of a three
resistor divider is
Rcm = Rdiff/4 x (Vref/Vb -1).
In any case, this only lowers Rcm to 690 ohms, using your inputs. This would
add +/-220 mV of steady-state common-mode voltage to the bus if we continue to
model each receiver input's leakage as a +/-20 uA current source and worst-case
tolerance stack up. (Rcm/2 ohms x Ii Amperes/input x 2 inputs/node x 16 nodes)
The trade off is the maximum allowable driver differential output level that
will keep the bus steady-state voltage between 0 V and 2.5 V. An additional
220 mV of steady-state common-mode voltage subtracts directly from the maximum
differential steady-state output voltage and tightens the spread between the
minimum and maximum output levels. The final determination of Rcm will depend
on what maximum-to-minimum steady-state output voltage range can be held on the
line drivers and the leakage model.
The ac constraint for the terminator common-mode impedance (not resistance)
will depend upon the allowable driver common-mode output voltage and media
common-mode impedance. I suspect that the dc constraint will be the more
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To: Kevin_Gingerich
From: /PN=HAYNES_DENNIS/MBX1=HAYNES(u)DENNIS#s#error(u)bbc#s##s##s##s##s##s##s#
Subject: LVD SCSI Common Mode Resistance
* From the SCSI Reflector, posted by:
* <HAYNES_DENNIS/error_bbc////////HPMEXT1/DENNIS#b#HAYNES#o#TUC#o#06 at u2.bbrown.
I have been working on equations to describe
the relationship between the various parameters
on the three resistor terminator for Low Voltage
Differential SCSI. I will show the results at the
next working group session Jan 29/30.
One of the relationships I have found is that
       Rcm  =  Rdiff/4 x Vref/Vb
        Rcm is the desired common mode impedance
        created by the terminator.
        Rdiff is the desired differential
        impedance created by the terminator
        Vref is the power supply at the top of
        the three resistors that make up the
        Vb is the desired bias voltage generated
        by the terminator
If this equation is met the terminator
can be implemented with a single supply.
If the equation is not met, then two supplies
will be needed.
When I use the worst case numbers for the
various elements in the equation I get a higher
value for Rcm than we have discussed before.
        Rcm = 115/4 x 2.5/0.1 = 719 ohms
This says that unless we can raise the Rcm value
to 719 ohms we will need to go to a more complex
implementation.  The circuit can be realized with
two voltage sources, on at the top and one at the
bottom of the resistor string.
We now have models that give us quantifiable
effects on the values of Rdiff, Vb and Vref.  So
far we have not gotten similar models that would
give us insights on the impact on system performance
as the value of Rcm is changed.
If any of you have resources that you can
determine the implications of changes in
Rcm on system performance please use them.
If we unnecessarily keep the value of Rcm too low
it will raise the cost of termination without
providing a system benefit.
Dennis Haynes
Burr-Brown Corp.
Tucson, AZ
Phone: 520 746 7262
EMail: haynes_dennis at

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