How I can read a 5 V input state correctly with an Arduino?

Question

I want to read the output of a PLC with an Arduino. If the PLC sends a HIGH (5 V) on a OUTPIN, the Arduino should read this HIGH, else LOW. I am generally using this code for this:

pinMode(button1, INPUT_PULLUP);

This way I can only read if the input of Arduino becomes GND, otherwise the Arduino reads every time as HIGH. This is what I want.

But this time the input is only 5 V from the PLC. How to read the 5 V input correctly? If I don't use a pull-up, the Arduino reads absurd values. Like this:

pinMode(button1, INPUT);

How can I read a 5 V input state correctly with an Arduino? I don't want to use an additional resistor or something else.

Answer 1

The problem probably is in the way you test (as described in your first comment).

For testing, you need to switch the button1 pin between 0 V and 5 V, not between "open" and 5 V as you do now, because that won't work properly without a pull-down resistor.

Open (unconnected, floating) MCU input pins can easily change state and show any value. See, for instance, Why can a MCU input floating pin easily change state?