The LM2901 is a so called open-collector component. This means that it can pull to ground (low), but positive potential (high) has to be provided externally, typically by a resistor.
Advantages of this type of circuit are:
- can be used to level-shift by using a different voltage for the output circuit
- can be used to vreate a so called wired-or (Google) to combine several outputs into one logic signal
The value of the pull-up resistor is entirely up to you. Probably the two most important issues to consider are:
- power requirements
- output current in high state
In the low state of the output the resistor takes the full voltage from Vcc to Gnd and will consequently dissipate power. You want to have a large resistance to dissipate very little power.
The resistor forms a low pass filter together with all the capacitances that are connected to the output (wire traces, input circuits etc.). You want the output to switch fast, therefore you make the resistance small to minimize the RC time constant.
These requirements are contradictory: a large resistance will dissipate little power, but slow down the circuit. And vice versa. Therefore you have to find a suitable compromise, taking into account factors as e.g. the expected mean on-time of the output transistor (aka duty ciycle of the output signal). If the output is on for a comparatively short time and off for a long time, you may be able to tolerate higher dissipation during that short time much easier than if the output is low for a long time.
Output current in high state:
The load current (from other inputs connected to the output) will drop some voltage across the load resistor. This voltage drop has to be small enough to still ensure a logic high level. With CMOS gates the input current is (almost) negligible and therefore not an issue. If you drive a load that needs considerable current (e.g. a bipolar inpu circuit) you have to take into account the input current of this circuit as it will typically not be negligible.