Pull-down resistor for inter-chip and sensor-to-chip communication

Question

I understand the concept of using a pull-up/pull-down resistor when implementing a button/switch with Arduino to avoid a floating state, and in fact I have implemented this quite often.

But I am not too sure if a pull-down resistor is necessary in chip-chip or chip-sensor communication.

I am connecting a coin acceptor to the Arduino (common ground). The coin acceptor's output pin gives a short pulse each time there is a coin inserted. So far I am connecting the output pin of the coin acceptor directly to an Arduino pin and it works without any problem. Is a pull-down resistor (on this line) usually required as precaution in this case?

Also I have the same question when connecting 2 pins of 2 separate Arduino's (also common ground) so that one Arduino can read pulses from the other.

Thanks in advance for any experience shared!

Dave

Answer 1

To your first questions: This depends of the coin acceptor. If the coin acceptor is switching between ground and Vdd then you do not need the pulldown. If the coin acceptor is switching Vdd and floating then you need it.

Anyway if you have some kind of "flying wires" between the arduino and the coin acceptor you should place a pulldown next to the arduino the get a clean signal. The flying wires can "catch" some EMF or act as a capacitor.... Both cases are not good for you.

Your second question: Basically no, but you should use it when you have long connections between them... Anyway in both cases, it won't hurt, so just place them precautiously.