Reading Arduino A's output pin with Arduino B's input pin

[pmw]

Hello. I am very new to electronics and hardware. My background is in computer science and software engineering. I am getting my feet wet with Arduino boards.

I've successfully created a 4-bit binary counter (with four LEDs and two pushbuttons) with the Arduino inventor's kit. For wiring the pushbutton, I followed the CIRC-07 card that came with the kit. It uses a 10K resistor as a pull-up resistor. It works great. Call this my Unit Under Test (UUT). I want to verify the UUT's functionality using a separate Arduino board. Call this other board the tester. I am using the tester with LabVIEW through the LIFA (LabVIEW Interface for Arduino). LIFA+LabVIEW work well: I am able to read digital pins in real time.

The problem is that I am not able to connect my UUT's output pins to the tester's input pins. When I do, I get floating behavior. The value read quickly (many times a second) alternates between 0 and 1.

To help diagnose the problem, I used a voltmeter to make sure the problem isn't code/LIFA-related. With the voltmeter, when I measure the voltage between the UUT's LED output pin and UUT's ground, it is what I expect: when the LED is lit, I read 5V, and when it's off I read 0V. When I, however, switch from the UUT's ground to the tester's ground, I read 0V no matter whether the LED is lit or not.

I am sure I am making some newbie mistake. Any suggestions for me? Thank you.

[adafruit_support_bill]

When I, however, switch from the UUT's ground to the tester's ground, I read 0V no matter whether the LED is lit or not.

You need to connect the grounds together. The ground is the reference against which voltages are measured. If you want to share signals between boards, they must also share a ground.

[pmw]

Thank you, that was the trick.