in Fig. 2(a). Each slot has the identical functionality except their toasting time. Initially, the knob is not pushed, but if one pushes the knob, the associated slot starts toasting for its toasting time: 20 seconds for the left slot, 40 seconds for the right slot. After the toasting time, each slot and its knobs pop up. Notice that even though one tries to push a knob when its associated slot is toasting, nothing happens. One can model it with FD-DEVS as shown in Fig. 2(b). Two slots are modeled as atomic FD-DEVS whose input event is "?push" and output event is "!pop", states are "Idle" (I) and "Toast" (T) with the initial state is "idle". When it is "Idle" and receives "?push" (because one pushes the knob), its state changes to "Toast". In other words, it stays at "Idle" forever unless it receives "?push" event. 20 (res. 40) seconds later the left (res. right) slot returns to "Idle". Atomic FD-DEVS Formal Definition where is a finite set of input events; is a finite set of output events; is a finite set of states; is the initial state; is the time advance function which defines the lifespan of a state where is the set of non-negative rational numbers plus infinity. is the external transition function which defines how an input event changes the schedule as well as a state of the system. The internal schedule of a state is updated by if , otherwise(i.e., ), the schedule is preserved. is the output and internal transition function where and denotes the silent event. The output and internal transition function defines how a state generates an output event, at the same time, how the state changes internally. Formal Representation of Ping-Pong Player The formal representation of the player in the ping-pong example shown in Fig. 1 can be given as follows. where ={?receive}; ={!send}; ={Send, Wait}; =Send for Player A, Wait for Player B; (Send)=0.1,(Wait)=; (Wait,?receive)=(Send,1), (Send,?receive)=(Send,0); (Send)=(!send, Wait), (Wait)=(, Wait). Formal Representation of One Slot Toaster The formal representation of the slot of Two-slot Toaster Fig. 2(a) and (b) can be given as follows. where ={?push}; ={!pop}; ={I, T}; =I; (T)=20 for the left slot, 40 for the right slot, (I)=; (I, ?push)=(T,1), (T,?push)=(T,0); (T)=(!pop, I), (I)=(, I). Formal Representation of Crosswalk Light Controller As mentioned above, FD-DEVS is an relaxation of SP-DEVS. That means, FD-DEVS is a supper class of