Passive or Active Terminators

Daniel_F_Smith at Daniel_F_Smith at
Fri Apr 30 17:34:57 PDT 1999

* From the T10 Reflector (t10 at, posted by:
* Daniel_F_Smith at
Legal terminators are not the problem. If a terminator happens to start
sourcing current at 5 plus volts, due to a transient condition that pulls
it out of 2.85V regulation (or whatever the output amplifier happens to
do), then the bus sees voltage that's above a 3.3V device's clamping
circuitry.  If termination really and truly works as planned, there's no
problem.  John's original question had to do with leakage currents (and
clamping currents) above the Vdd of a 3.3V device.  How much clamping
current can we tolerate on a running bus without corruption?  I'd like to
spec my device with a known leakage current at 4.1V (or lower) and consider
anything above that a catastrophic condition which my device must protect
itself against!  If everybody on the bus is behaving themselves, then this
condition, where I clamp the bus, should not hurt bus activity. Besides
some weird transient problem, the only fault condition I can fathom is a
device actively negating the bus with a voltage higher than 3.7 volts. But
again, who gets hurt.  Everybody should be current limiting by now.  It
would be more practical if we didn't have to suffer for devices that insist
on going above 3.7 volts.  Terminators are designed to pull to 2.85 volts,
drivers are supposed to pull no higher than 3.7 volts, and multimode
devices pull to <3.63 volts.  Where's the problem?  Transient conditions
are the only unknown.  If it's hot plugging causing the transients, then
the bus may be corrupted. But why do 3.3V devices have to spec anything
above their Vdd?

* From the T10 Reflector (t10 at, posted by:
* "Bill Galloway" <BillG at>

Maybe I am missing something here.... I still do not understand why you
are worried about a legal terminator (active or passive). What voltage
were you going to clamp at?  Until I understand why you are worried
about the terminators I would say that the exposure to passively negated
legacy devices is zero.

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