[ros-users] Creating a hardware device

I am interested to know how one goes about creating an interface for
a device, such as an arm or hand. In particular, the question is what is
the lowest level component in the ROS architecture which one would implement
first? Presumably, once that low level element was in place, one could then
go on to create services or actions that make use of that component.
Could someone point me to (or write?) a summary of what this stack looks
like for something like the PR2 or Shadow arms?