Re: [SI-LIST] : Terminations scheme for bi-directional bus

Scott McMorrow ([email protected])
Thu, 25 Jun 1998 15:11:25 -0700


Great questions! As your questions imply, the termination scheme
is dependent upon the functionality of the signal on the bus.



Dennis Tomlinson wrote:

>> > I have 8 boards connected to a backplane. At least four, but as many as
>> > all 8 boards CAN be inserted at one time. Each board has a transceiver
>> > connected to one bi-directional bus. Any of the transceivers can be the
>> > driver. The transceiver nets to the connectors are about 3 inches long
>> > and the backplane traces between connectors are about 1.25 inches. The
>> > transceiver is a 74ACTQ245. All boards have a Z0 of about 60 ohms.
>> >
>> > What termination techique should I try? I've tried using 10 or 20 ohm
>> > series resistors near each transceiver, but the driver signal has a ring
>> > back that crosses the 0.8 volt threshold on the falling edge. Because
>> > any number of receivers can be on the bus at one time, I can't adjust
>> > the termination resistance to accommodate all loading options.
> Hi folks,
> I'd like to step in here and ask a few questions if I may:
> 1. Is pulse monotonicity a requirement?
> 2. Is first incidence switching necessary, or
> 3. Is there enough slack in the timing budget so that
> second or third incidence is "good enough?"
> 4. With 10 to 20 Ohms series terminators, is
> over/undershoot a problem? Partially loaded? Fully loaded?
> 5 And finally, is your system malfunctioning because
> of SI problems with these signals?
> Cheers,
> Dennis

Scott McMorrow
Principal Engineer

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