In this lift door circuit , there are two set of inputs A , B and A' , B' . A and B are the inputs given inside the lift , i.e with inputs A and B lift moves from one floor to the other .  Inputs A' and B' are given outside the lift , i.e with these inputs lift is called to a particular floor .  If the person inside the lift wants to go to a particular floor and at the same time lift gets a call from that particular floor , then the lift door opens at that floor . In all the remaining cases , the lift doors remains closed .

NOTE : The inputs A , B and A' , B' are in GREY CODE .