LVD SCSI Termination Questions

Siegfried Schmalz Siegfried.Schmalz at dalsemi.com
Wed Feb 28 13:09:16 PST 1996


* From the SCSI Reflector, posted by:
* Siegfried.Schmalz at dalsemi.com (Siegfried Schmalz)
*

QUESTIONS REGARDING THE LVS SCSI BUS TERMINATION AS
DESCRIBED IN SPI-2 REV 04 (JANUARY 23, 1996)


REGARDING: TERMINATION BALANCE TEST
###################################

On pages 17-18, a "Termination Balance Test" is described. 
Questions:

     1.	In Table 3 "Parameters for termination balance test", 
	Vref is specified to range from 1.2V to 1.3V.  However,
	Vref is not a parameter drawn in either Figure 6 or Figure 7.
	I presume this has to do with the 1.25V common mode that the 
        termination is to be centered about, but can someone explain 
        more precisely what is meant here?  In particular,
	if one uses a termination such as I have in the HSPICE deck
	below, where there is no explicit Vref, it is unclear to
	me how I should interpret this. 
     2. In Figure 6 "Termination balance test configuration", delta_V
	is defined as (V-) - (V+) at the termination for the test
	circuit shown.  Table 3 says that this parameter may not
	exceed +/-10mV when the test voltage is swept over its specified
	range.  Has this changed since the draft spec was written?
	Or has the termination balance test been modified? If not,
	I believe some of the proposed termination configurations 
	would fail the test.  The HSPICE deck below uses one
	of the proposed terminations, and its delta_V stays right at
	70.5mV +/-.02mV with these ideal elements.  Could it be that
	that the delta_V is actually defined so that instead of
	delta_V = (V-) - (V+), we define Vdif = (V+) - (V-), and define
	delta_V as the *variation* of Vdif as the test voltage is swept?
	In other words, in the HSPICE deck below, 70.5mV would be Vdif
	and .02mV would be delta_Vmax (or delta_Vmin) in this particular
	simulation with ideal elements.
 	Or am I misunderstanding the test altogether?

The following HSPICE run illustrates my question.
------------------------------------------------------------------------------
       *** one of the proposed termination schemes ***
       vtop top 0 dc 1.5
       vbot bot 0 dc 1.0
       rtop top minusig 240
       rmid minusig plusig 130
       rbot plusig bot  240
       ***************************
     
       rx1 minusig com 100.01
       rx2 plusig com 99.99
       vsw com 0 dc 0
    
       .dc vsw 0.6 1.8  0.1
    
       .print dc
       +       vdif=par('v(plusig)-v(minusig)')
    
       .end
    
       ******  dc transfer curves               tnom=  27.000 temp=  27.000
    
             volt       vdif
   
       600.00000m    -70.5119m
       700.00000m    -70.5099m
       800.00000m    -70.5079m
       900.00000m    -70.5059m
         1.00000     -70.5039m
         1.10000     -70.5019m
         1.20000     -70.4999m
         1.30000     -70.4979m
         1.40000     -70.4959m
         1.50000     -70.4939m
         1.60000     -70.4919m
         1.70000     -70.4900m
         1.80000     -70.4880m
    
               ***** job concluded
------------------------------------------------------------------------------



REGARDING: NAMING CONVENTION 
############################

This is essentially a repeat of the question I sent to the reflector
on 20 Feb 96 and to which there were no replies.
Question:

1. It would be customary to define
   positive current as flowing from the positive node of a
   voltage source, through the source, to the negative node
   of that voltage source. Also, one would expect that
   Vdiff = (V+) - (V-).  Using these conventions, however, I
   am unable to duplicate Figure 3 (page 15). 
   There are, therefore, two possibilites:

	a) I am mistaken in my understanding of the naming
	   conventions that are to be used for this spec.
	b) The plot of Figure 3 is to be drawn as a
	   mirror image (about the Y axis) from where it
	   is presently drawn.

   The following HSPICE runs illustrate my question:
   ----------------------------------------------------------
        *** One of the proposed termination schemes ***
	vtop top 0 dc 1.5
        vbot bot 0 dc 1.0    
        rtop top minusig 240
        rmid minusig plusig 130
        rbot plusig bot  240
	************************************************

        vdc dcoff 0 dc 1.25v    
        vsw plusig dcoff dc 0
        esw minusig dcoff plusig dcoff -1.0

       .dc vsw -0.5 0.5 0.1
        
       .print dc
       +      vdif=par('v(plusig)-v(minusig)')
       +      i1=par('i(vsw)')
    
       .end    

      ******  dc transfer curves               tnom=  25.000 temp=  25.000
     
             volt       vdif          i1
    
      -500.00000m     -1.0000       8.7340m
      -400.00000m   -800.0000m      6.7788m
      -300.00000m   -600.0000m      4.8237m
      -200.00000m   -400.0000m      2.8686m
      -100.00000m   -200.0000m    913.4615u
         0.            0.          -1.0417m
       100.00000m    200.0000m     -2.9968m
       200.00000m    400.0000m     -4.9519m
       300.00000m    600.0000m     -6.9071m
       400.00000m    800.0000m     -8.8622m
       500.00000m      1.0000     -10.8173m
   
              ***** job concluded
 
  
   As one can see, when this HSPICE deck is run, a mirror image of Figure 3
   (about the Y-axis) is produced.  


  Similarly, when the common mode impedance test is done (Figures 4 & 5),
  a mirror image (about V=1.25) of Figure 5 is produced:

        *** One of the proposed termination schemes ***
	vtop top 0 dc 1.5
        vbot bot 0 dc 1.0    
        rtop top minusig 240
        rmid minusig plusig 130
        rbot plusig bot  240
	************************************************

 	rshort plusig minusig .00001
        vswp plusig 0 dc 0
        .dc vswp .7 1.8 0.1
  
        .print dc
        +     i1=par('i(vswp)')
      
        .end

        ******  dc transfer curves               tnom=  25.000 temp=  25.000
    
              volt        i1
     
        700.00000m      4.5833m
        800.00000m      3.7500m
        900.00000m      2.9167m
         1.00000       2.0833m
         1.10000       1.2500m
         1.20000     416.6666u
         1.30000    -416.6667u
         1.40000      -1.2500m
         1.50000      -2.0833m
         1.60000      -2.9167m
         1.70000      -3.7500m
         1.80000      -4.5833m
     
           ***** job concluded
   ----------------------------------------------------------

Am I correct in my understanding of the naming conventions to be used?



Siegfried Schmalz
	
Dallas Semiconductor				
schmalz at dalsemi.com
(214) 450-3764






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