Re: pulldown value
I think it is something like this:
1. Condition 1:
When either one of the inputs are high the output is supposed to be high. In this condition the forward resistance of the diode must be smaller than the pull down resistor to allow the output voltage to rise sufficiently high.
2. Condition 2:
When both the inputs are low the output is supposed to be low. Under this condition if the following stage is to be driven, it needs some minimum current to flow through the pulldown resistor to make the following stage work. If the resistor is very high, the drop across it due to the A=0, B=0 current may be high enough to turn the output high for the next stage. So the pull down resistor is to be low enough to improve this noise margin.
It is therefore a trade-off between proper o/p level and improvement in noise margin.