The RCX, which is used to control robotic arm, has a major limitation: It has
only three outputs. Obviously, for an arm, you need at least four: height of the
arm and the forearm, rotation of the base, and open/close the hand.
I call "Multiplexors" devices that are meant to control two or
more motors using a smaller number of output of the RCX.
figure illustrates a 1-to-2 multiplexor:
These devices are quite important given the very limited
number of outputs on the RCX (only three). Most realistic application you'll
want to realize is likely to have more than that.
In the following, I describe
two methods to implement multiplexors: