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

M. Susan Tweeton (mstweeto@collins.rockwell.com)
Thu, 25 Jun 1998 11:18:34 -0500

There was a question about bi-directional buses a few days ago, but this
one poses some interesting problems.

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.

ANY IDEAS?

Thank you in advance for your comments.

Susan Tweeton
SI/Design Engineer
Rockwell-Collins
Cedar Rapids, IA
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