Hi,
Generally there can be a series resistor for current limiting.
It lkmits the current accirding Ohm's law. I = V_R / R
And there can be pull_up and pull_down resistors.
Two aspects to calculate the resistance value:
* it needs to provide valid voltage levels when the signal is released. Here again Ohms law applies. Besides the input signal levels (V_IH, V_IL) input current (I_in), leakage currents (wiring, traces, connectors, dirt...), pull_up_ voltage ... need to be calculated in.
* it needs to provide the voltage levels within a given time. Here additionally the capacitance of the signals play a role (and the coupling to signals nearby). Tau = R x C may be used as raw estimation. In detail you need to calculate all in one big formula.
Also you need to consider erroneous (high voltage) signals nearby may play a role, like ESD.
Often the desingner just choose the value according his experience ... and the application.
For low power applications you may choose high ohmic resistors (but mind crow bar current in the input stages, if the signal is not very close to the rails.)
Fir high safety apllications with high noise immunity one may choose lower ohmic resitors. The useful range may be ftom some hundreds of Ohms up to Megaohms.
Klaus