Deciphering datasheet descriptions: "normalized series 54/74 load"

Question

I was reading something when I came across a project using the SN74180 parity checker IC. I begin looking at the datasheet when I came across this description:

The SN54180/SN74180 are fully compatible with other TTL or DTL circuits. Input buffers are provided so that each data input represents only one normalized series 54/74 load. A full fan-out to 10 normalized series 54/74 loads is available from each of the outputs at a low logic level. A fan-out to 20 normalized loads is provided at a high logic level to facilitate the connection of unused inputs to used inputs. Typical power dissipation is 170 mW.

What is meant by a "normalized series load" and why would you need an input buffer? 

It seems like this description is alluding to the fact that this device supports up to 10 fan-out loads for each output at logic LOW, and 20 at logic HIGH.

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

It would seem that the fact that it can support 20 fan-out at logic HIGH would be moot since the 10 fan-out at LOW would be a bottle-neck anyway. Surely I am misunderstanding this characteristic.

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

Answer 1

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.

Answer 2

It's not a normalized series load, it's a normalized 7400-series load. They're talking about representative values for the current of a 7400-series input pin.

The fan-out can be different for different logic levels because TTL inputs consume different current at high and low levels, and the high-side and low-side output drivers are different. The datasheet shows that for the SN74180, high-level input current is less than 40 microamps, while low-level input current is less than 1.6 milliamps. The high-level output current is no more than 800 microamps, while the low-level output current can be up to 16 milliamps.

What I'm not clear on is how this helps with tying off unused inputs. I can see how connected unused inputs to used inputs would be easier from a board layout standpoint, but I don't know why the high-level current load would matter more.