LVDS-SCSI

Kevin Gingerich S=Kevin_Gingerich%S=Gingerich%G=Kevin%TI at mcimail.com
Wed Dec 20 12:21:00 PST 1995


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

 
  To  SCSI REFLECTOR   X400
 
>From  Kevin Gingerich  GING
 
/pn=scsi/mbx1=scsi at symbios.com/ems=internet/admd=mci/c=us/
 
Subj  LVDS-SCSI
 
Give a guy a spreadsheet and some time and anything is possible. I generated
the following table of worst-case scenarios for the LVDS-SCSI signal levels.
The circuit uses a three resistor divider (ala RS485 SCSI) connected to VTERM.
 
VODMIN and VODMAX were the parameters used for maximization or minimization
respectively. The data set shown gave me the highest VODMIN (MIN column) and
lowest VODMAX (MAX column) with 1% resistors, 2% VTERM, Cable Z0/2 between 55
and 67.5 ohms inclusive, minimum node spacing of 0.05 m, and Cable C no less
than 40 pF/m and no greater than 140 pF/m.
 
I had to tighten the receiver thresholds to 50 mV and lower the maximum ground
potential difference to 300 mV to get a viable solution. I also included the
highest VODMIN analysis for RS-485 for reference.
 
Observation:    Worst-case is just that. Without some statistical
                tolerancing, 30 mW drivers will be required for LVDS-SCSI and
                RS-485 SCSI would not work! (Which we know it does, and well.)
 
Merry Christmas!
/Kevin/
 
p.s. If the columns get misaligned during transmission, good luck!
 
 
Scenario Summary        --------LVDS---------
                        NOM     MIN     MAX     WC 485 MIN
Changing Cells:
R1              OHMS    920     910.8   910.8   313.5
R2              OHMS    120     121.2   121.2   157.5
R3              OHMS    920     910.8   929.2   313.5
VTERM           VOLTS   2.4     2.448   2.448   5.25
Iin-            AMPS    10E-6   -20E-6  -20E-6  000.0E+0
Iin+            AMPS    -10E-6  20E-6   -20E-6  000.0E+0
CONSTRAINT #1- MIN/MAX VOLTAGES:
MAX V1 or V2    VOLTS   2.5     2.5     2.5     12
MIN V1 or V2    VOLTS   0       0       0       -7
VGND            VOLTS   0       0.3     0.3     7
CONSTRAINT #2- MINIMUM NOISE MARGIN:
Crosstalk       VOLTS   000E+0  35E-3   35E-3   35E-3
Terminators     VOLTS   000E+0  20E-3   20E-3   20E-3
Rcvr VT         VOLTS   000E+0  50E-3   50E-3   200E-3
Cable Length    m       6       15      15      25
Cable Z0/2      OHMS    60      55      55      55
Cable C         pF/m    43      40      40      40
Diff Cin        pF      7.5     7.5     7.5     25
Nodes           #       8       16      16      32
Node spacing    m       0.2     0.05    0.05    0.05
CONSTRAINT #3- DRIVER POWER DISSIPATION:
PWR/CH          WATTS   0.05    0.05    0.05    0.2
VCC             VOLTS   3.3     3.6     3.6     5.25
 
Result Cells:
Iin-            AMPS    80E-6   -320E-6 -320E-6 000E+0
Iin+            AMPS    -80E-6  320E-6  -320E-6 000E+0
V1              VOLTS   1.3     1.3     1.6     3.2
V2              VOLTS   1.1     1.1     1.5     2.1
V1-V2           VOLTS   138E-3  189E-3  151E-3  1E+0
(V1+V2)/2       VOLTS   1.2     1.2     1.5     2.6
Diff Zterm      OHMS    113     114     114     126
CM Rterm        OHMS    460     455     460     157
Noise Margin    VOLTS   000E+0  55E-3   55E-3   55E-3
MIN VOD1        VOLTS   138E-3  294E-3  256E-3  1E+0
Attenuation     VOLTS   9E-3    47E-3   41E-3   327E-3
Loaded bus Z/2  OHMS    44      25      25      15
Reflections     VOLTS   25E-3   173E-3  151E-3  1.8E+0
MIN VOD         VOLTS   172E-3  514E-3  447E-3  3.4E+0
MAX VOD         VOLTS   1.2E+0  787E-3  519E-3  1.3E+0
MIN IOUT        AMPS    2.9E-3  9.3E-3  8.1E-3  61.6E-3
MAX IOUT        AMPS    19.4E-3 14.3E-3 9.4E-3  24.0E-3
MIN DVR PWR     WATTS   9E-3    29E-3   26E-3   115E-3
RANGE                   74%     21%     7%      -44%
 
Notes:  1. V1 is the SIGNAL- and V2 is the SIGNAL+ lines.
        2. Iin is the input current to a connector pin referenced to ground.
        3. Attenuation is based upon -0.1 dB/m and a minimum bus voltage of MIN
        VOD1.
        4. Reflections are calculated using the cable ZO/2, Loaded Bus Z/2, and
        instantaneous voltages no lower than MIN VOD1.





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