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Active Termination of a High-Speed, Multi-Drop, Bidirectional Digital Transmission System

IP.com Disclosure Number: IPCOM000101683D
Original Publication Date: 1990-Aug-01
Included in the Prior Art Database: 2005-Mar-16
Document File: 3 page(s) / 63K

Publishing Venue

IBM

Related People

Feng, D: AUTHOR [+3]

Abstract

The conventional method of controlling reflections on a high-speed, transmission system is to use, at the end points of the transmission system, a resistive (passive) network that matches its DC resistance to the characteristic impedance of the transmission line. A different approach to control reflection is to incorporate up level and down level clamps (active termination) in each transceiver that is connected to the transmission line. With the clamping approach the end points of the transmission line are left un-terminated.

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Active Termination of a High-Speed, Multi-Drop, Bidirectional Digital Transmission System

       The conventional method of controlling reflections on a
high-speed, transmission system is to use, at the end points of the
transmission system, a resistive (passive) network that matches its
DC resistance to the characteristic impedance of the transmission
line.  A different approach to control reflection is to incorporate
up level and down level clamps (active termination) in each
transceiver that is connected to the transmission line.  With the
clamping approach the end points of the transmission line are left
un-terminated.

      Fig. 1 depicts an actively terminated transmission line.  Fig.
2 depicts the design of the clamp circuit.

      A Most Positive Up Level (MPUL) and a Least Positive Down Level
(LPDL) is established for the transmission system.  Any driver can
drive to any receiver.  Because there is no termination at the end
points of the transmission line, the switching amplitude of a signal
will double at the  far end and reflect back toward the driver along
the transmission line.  The clamp circuit starts to conduct (draw
current off the line - reduce the voltage) when the MPUL is exceeded
or (supply  current to the line - increases the voltage) when the
down level exceeds the LPDL.

      With an actively terminated transmission line, the following
benefits are obtained.
1.   Because the active termination is distrusted along the
tra...