Arduino mega outputting 2.4 volts

I've got 2 arduinos connected together with one triggering an interrupt on the other one.

When the master device sets the pin high it reaches 2.4 volts, not enough to trigger the interrupt. Currently I have to touch the interrupt wire with a 5V power supply to get it to work.

I'm using a 20k pulldown resistor attached to pin 2 to keep the signal at 0V when not triggered. I've also tried no pulldown resistor and added a 500 ohm current limiting resistor in series with the interrupt line.

Anybody know what I can do to correct this problem.

Did you remember to connect the grounds?

Post a diagram of how it's wired.

Here is the circuit diagram.

I did remember to connect the grounds together.

Sounds like you forgot to set the pin as output ... pinMode(OutputPin, OUTPUT);

// Per.

I checked that, the pin was set as output. When I connect a scope it shows pulses but the pin goes from 0 to 2.4 volts.

I fixed the issue by using a transistor as a switch and connecting the output from the mega to the base. Strange enough now the pin operates at 5V just like it should.

Electronics is very confusing.