LVD Release Glitches

Kevin Gingerich S=Kevin_Gingerich%S=Gingerich%G=Kevin%TI at
Thu Jan 11 14:53:00 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 Release Glitches
Here's another vote against trying to eliminate a negation-to-release "glitch"
by reducing the negation current of a LVD-SCSI driver. The peak voltage upon
releasing a driver from negation can be approximated by
1) Vp = In/2(Zo-Zt) - Vbias, where In is the negation current.
Substituting the worst-case values for the receiver thresholds (Vp = -0.03), a
loaded Vbias (Vbias = 0.058), media characteristic impedance (Zo = 135), and
termination impedance (Zt = 100) gives,
-0.03 > In/2 (135-100) - 0.058.
Solving for In gives In < 1.6 mA.
The minimum negation current required is determined by the
assertion-to-negation transition at the loaded media and can be approximated by
2) Vp = 0.5 Ia Zt - Vbias - 0.5 (|Ia| + |In|) Zl, where Ia is the assertion
Solving for In, substituting the values above, using the minimum loaded media
impedance of 85 ohms (Zl), the maximum Zt, and solving gives,
In = 2/Zl ((Ia/2(Zl-Zt)) - Vp + Vbias)
In > 2/85 (Ia/2(85-115)) + 0.03 + 0.058)
In > -0.353 Ia + 0.002.
Assume that a maximum assertion current of 10 mA is allowed, then the
constraints become 1.5 < |In| < 1.6 ma.
The real answer to eliminating the "glitch" is revealed in equation 1) above
and is to more closely match Zo and Zt. The trade-offs are the
negation-to-assertion minimum level and output current.

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