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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