Fast-20 active negation current
S=Kevin_Gingerich%S=Gingerich%G=Kevin%TI at mcimail.com
Wed Jan 25 05:15:00 PST 1995
To SCSI Reflector X400
>From Kevin Gingerich GING
Subj Fast-20 active negation current
The load that the single-ended SCSI bus presents to a driver is time varying.
The worst-case for assertion is a fully charged and loaded bus and for negation
is a fully discharged and loaded bus. The characteristic impedance of a fully
loaded bus may be as low as 57% of the cable impedance or about 50 ohms. A
driver in the middle of the bus "sees" two transmission lines in parallel or 25
To achieve the thresholds of VIH = 2 V and VIL = 1 V on the first transition,
there must be sufficient current available to drive the load(s) to these
levels. For negation,
VO(t=0+) = (IOH(t=0+) + IOL(t=0-)) x 25 + VO(t=0-)
2 = (IOH + 0.048) x 25 + 0.25
IOH = 0.022 A
It is difficult, at best, to simulate t=0- conditions in a test circuit. The
"kick" needed from the active-negation may be verified by driving a 90 ohm
ground referenced resistor to 2 V. This requires different test circuits for
low-to-high and high-to-low driver output conformance testing.
Since these are ac output current requirements, you may make a case for some
sort of frequency limitations. However, I suspect it will be difficult (but not
impossible) to come to an agreement on the requirement and how to test it.
Texas Instruments, Inc.
4307725 at mcimail.com
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