Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!cs.utexas.edu!uunet!portal!cup.portal.com!Chuck_SirVAX_Staatse
From: Chuck_SirVAX_Staatse@cup.portal.com
Newsgroups: sci.electronics
Subject: Bus Termination
Message-ID: <22144@cup.portal.com>
Date: 14 Sep 89 13:24:55 GMT
Organization: The Portal System (TM)
Lines: 32

Has anyone used a backplane termination technique combining both 
series and parallel termination?  Supposedly it reduces the swing 
between Vhigh and Vlow on the backplane which reduces noise. The  
circuit in question would look like this:

			   +5V 	
			    |	
			   _|_
			  |R1 |
			  |   |	
			   -|-
                            |
		            |		--------    		
                            |----------| R4	|---C   	
		--------    |		--------
     A---------| R2	|---B   	
		--------    |	
			   _|_
			  |R3 |
			  |   |	
			   -|-
                            |	
			  GND		

The idea is to make R2 and R4 about 1/5 the resistance of R1 or 
R3. Point A and C are bi-directional drivers. If point A is driven
to 0 volts the voltage divider R1,R2 causes point B to sit at 
about .8 volts ,conversly, if point A is driven to 5 volts 
the voltage divider R2,R3 causes point B to sit at around 4.2
volts.  If point C is CMOS with Vtp -1.2 and Vtn of 1.2 then
point C will switch properly.
  Anyone used this? Had problems with it?