"A Distributed Termination Scheme for GTL+ Backplane Bus Designs"

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Presented at the DesignCon98 Dr. Edward P. Sayre, P.E. Mr. Michael Baxter Dr. Jinhua Chen Mr. Nissim Cohen Tadiran Telecommunications, Ltd. Abstract An innovative distributed termination scheme has been developed for GTL+ bus designs which addresses reduction of reflections under partial module loading conditions and solves the problem of bus termination voltage generation in passive backplane 48 volt powered systems. It has been found that for specific but realistic logic card loadings conventional parallel termination results in small receiver noise margins due to surprisingly large reflections between the load terminations and the unpopulated portions of the bus. Additionally, in 48 volt powered systems where the high to low DC-DC regulation takes place on each logic module, the provision of termination voltages using a DC-DC converter on the backplane violates the requirement for an entirely "passive" backplane. The termination voltage can be more conveniently provided from one or more of the logic modules. For these reasons, NESA engineering staff has developed an unconventional distributed termination combining end point termination, with its desirable feature of reduced reflections in a fully loaded system, with additional on-card distributed terminations to reduce reflections for partial load configurations. This termination scheme is called "combined or distributed termination". This paper will compare the cases of conventional end point terminations with the distributed "combined termination" to demonstrate the advantages of this new termination scheme in terms of noise margin and reduced bus reflections under partial and full loading in a 100 MHz GTL bus system. Generation of on-card termination voltages simplifies the requirements for a passive 48 volt backplane system. ->This paper is available from NESA for $15.00 to cover management costs. We accept credit card payment for purchase of the papers. The requested paper will be emailed to you shortly after your order is received.

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