Deciphering datasheet descriptions: "normalized series 54/74 load"
What is meant by a "normalized series load"


A digital input requires a certain amount of current in order to pull the input stage high or low enough to have a voltage within the logic thresholds. This amount of current is designated "1 normalized series load", and it varies per family.


  and why would you need an input buffer?


Certain logic devices, e.g. muxes, have complex structures that consist of more than 1 load. The input buffer hides these extra loads.


  How can this number be different for each type of logic level?


Both TTL inputs and TTL push-pull outputs are asymmetrical, but not balanced to each other. This results in a discrepancy in the load handling.


  What is meant by ".. facilitate the connection of unused inputs to used inputs"?


This one I'm not sure about. Floating TTL inputs are seen as high regardless, so I'm not sure how this would come into play.