ohms-lawpullupresistors
I have two IC chips. On one chip I have one 10K ohm pull-up resistor and on another chip I have three 10K ohm pull-up resistors.
At first I was using four through hole pull-up resistors. Then I moved to an array of SMD resistor.
Could I simplify the design by attaching all four pins of the ICs to the same pull up resistor? Should this be a 40K ohm resistor? Is this OK or is it better to attach each pin to a different resistor?
Yes. Open drain outputs act like a switch to ground. The resistor acts as a pullup for when none of the outputs are active.
If you replace the open drain outputs in your circuit with buttons its easier to visualise:
simulate this circuit – Schematic created using CircuitLab
Resistance is resistance; there is no difference between using a 68ohm TH resistor, a 68ohm SMD resistor, or a 68ohm carbonized strand of cotton, assuming resistance is the only criterion you care about. It isn't, but you've already handled power dissipation by specifying a certain amount and finding an appropriate SMD resistor.
Best Answer
If these pullups are permanent (ie the only connection is from the input pin to the resistor) then there's no problem. If not, but all of the inputs should have simultaneously the same level, then it's OK too. I assume that's what you intended.
About the resistance, it theoretically should be lower, not higher, because when sharing the resistor the other input pins are acting as other resistors connecting that point to the ground, thus reducing the voltage at that input; but considering that today almost all chips are MOS with a very high input impedance, it makes no practical difference (for the number of pins you mentioned), so you can keep the same 10K ohm. If they were TTL, then maybe you should need to reduce it to make sure the voltage delivered is recognized as '1'.