Easy question here; I'm confused about how shields work. Specifically what happens when you plug in a shield that uses pin 13 for something, then you want to actually plug in an LED into pin 13 of the shield and turn it on using the Arduino. Does attaching a shield to your Arduino mean that you cannot use the Arduino for anything else? I would assume not, seeing as I see shields with headers on them, but how can I use the Arduino with a shield for more than just the shield electronics?
My opinion: The best reply possible here is, "Use Shields with caution".
You need to plan around what each one does and what ports/functions it uses. You will quickly realize that using more than 2 shields at once could become problematic. Especially if they are of the complicated variety... like an Ethernet shield.
I don't believe that Shield designers consider themselves bound to avoid pins used by other shields... if it does work out that two randomly selected shields play well together... then consider yourself lucky.
All shields use some of the 19 available IO pins on the Arduino and you can use any one shield without problems.
Often two or more can be used at the same time but then it depends on the actual pins used by each shield and you have to look at the data sheet for each shield to see if they are compatable.
As for pin 13, you can often use it even if a shield also uses it, but that depends on exactly what the shield does with the pin, once again you have to consult the shield's data sheet.
Interesting, this is what I was thinking. Its just that when one looks at just about any shield they all have the same number of breakaway headers on top of the shield that the Arduino gave them. Seems to me like it would be smarter / more helpful for people to at least mark on the shield's PCB which pins are used by that shield and which are not.
Ah well, I will be cautious of this but this seems like a good thing for the Arduino team to take a closer look at and maybe create a standard for.
There's a thread here about the different pins used by shields
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1284962949/5#5
with a table (as yet unverified) showing the pins used by a lot of shields.