OK,
so for a current project I need 4 arduinos to talk to eachother. They don't need to say much: actually just a single digital signal. In this case it isn't feasible to connect them via a pc (for various reasons) so I was wondering if it is possible to connect the digital out of 1 arduino to the digital in of another to make it act as an input....sounds simple, but I'm afraid it could have burnt consequences.
If it helps to be specific:
I have 4 arduinos and they each need to know if any of the digital inputs on the other boards have been triggered. Since I can't in this case have them communicate through a pc. I was thinking to set up a 5th arduino as a server. Ideally, each of the 4 would have a digital out connected to digital in on the "serverArduino" it would check if any were pressed, and if so would send a signal through digital out to the digital ins of the 4 others so they would know.
If someone could let me know if this is possible and what the circuit could look like, I would appreciate it.
Nick
Sorry, not specific enough 
Do the Arduinos need to know which one has been triggered? Or just that any of the others has?
If the latter, you can use a diode on the output of each Arduino to simulate an open-collector/open-drain output, set the output low when it's "active", and use it like an old-fashioned "wired-AND" (a google search should turn up lots of explanations of the theory and examples of the practice). Then you won't need that "central" controller at all.
If you need to know which one is triggered, just connect the output of each Arduino to inputs on the other 3. Again, no need for a "central" controller. A single Arduino output should be able to drive inputs on several others.
Make sure that the Arduinos all have their grounds connected together, so they all have the same notion of what's "digital high" and "digital low". If they're more than a few feet apart, you might have problems with the wires acting as antennas and picking up electrical noise. In that case, you may need to use something like RS-232 or RS-485 drivers and receivers to get clean signals between boards.
ran