Sharing the same serial TTL input between two Arduinos

Using Arduino LEDs and Multiplexing
1

Hello,
I need to input the same TTL level signal to two Arduinos. I wonder if I could just wire it direct to each board or if I need to isolate the boards either with a two serial resistors(2 to 5K?), one to each input, or, if I have to fan out the signal using two buffers like 7407?
Thanks
Jean-Pierre

2

I don't see a huge problem with just connecting to both, as long as you don't set them both to output at some stage.

3

Make sure you connect the grounds of both arduino's together.