I feel like I know electronics fairly well but I have always had a hard time determining needed resistance between connected IC's. Is there a general rule of thumb or chart somewhere I don't know about? Could someone explain how to figure the resistance needed between a 4027 and a 4001 when the supply voltage is 12v?

 

Zero ohms works very well for all possible levels of Vcc (roughly 3 volts to 18 volts for 4000 CMOS). Why do you think there should be some resistance between the output of one and the input of the other?

 

A rule is "Do Not Use Resistors where they are not needed." Why do you think a resistor is needed? What purpose would a resistor perform in this application? CMOS devices such as 4027s and 4001s are voltage driven devices. Maybe if you provided a specific circuit it would be easier to provide a useful answer answer to your question.

beenthere hit it right zero ohms. They are hard to find though.

 

The reason no resistors are needed is because the input resistance of a CMOS gate is typically high. Thus, very little current is drawn. The "CMOS Cookbook" (an excellent book, BTW) states the fanout can be 50 (or more) -- the actual fanout is limited by the line capacitances rather than the gate input impedances.

 

What about a 555 to a 4xxx logic?
I put 100 ohms between the two.

 

The only thing necessary is that the signal from the 555 go to Vcc so the CMOS sees a high. The CMOS, as in post #4, does not impose a load on the 555 output.