Automating Manufacturing Systems - Process Control and ...

plc states - 12.4
power up
service
needed
reset button
idle
coin inserted
coin return
inserting
coins
no cups
OR no coffee
OR jam sensor
cup removed
coin return
right amount
entered
make
coffee
button pushed
user
choose
Figure 12.3
State Diagram for a Coffee Machine
12.1.1 State Diagram Example
Consider the traffic lights in Figure 12.4. The normal sequences for traffic lights
are a green light in one direction for a long period of time, typically 10 or more seconds.
This is followed by a brief yellow light, typically 4 seconds. This is then followed by a
similar light pattern in the other direction. It is understood that a green or yellow light in
one direction implies a red light in the other direction. Pedestrian buttons are provided so
that when pedestrians are present a cross walk light can be turned on and the duration of
the green light increased.
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