[SI-LIST] series resistor on SSTL-2

Hi all
I am not sure why there is a in line resistor (Rs) on SSTL-2 design and this 
in-line resistor was placed close to the DDR SDRAM DIMM.

I understand DDR protocol and still I cannot figure out the purpose of this 
in-line resistor (Rs). Is that something have to do with signal integrity ? If 
it is, why don't we place this resistor close to the driver side (North bridge) 
?

I am lost, can someone please do a brief explanation or send me a link so I can 
read more on it.

Thanks in advance

Jeremy


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