Sommerville-Software-Engineering-10ed

618 Chapter 21 ■ Real-time software engineering
Timeout
Card
inserted
into reader
Reading
do: get CC
details
Initializing
do: initialize
display
Waiting
do: display
welcome
Card removed
Validating
do: validate
credit card
Card OK
Hose out of holster
Ready
Hose in
holster
Nozzle
trigger on
Delivering
do:
deliver fuel
update display
Timeout
Invalid card
Nozzle trigger off
Resetting
do: display CC
error
Stopped
Nozzle trigger on
Payment ack.
Paying
do: debit
CC account
Hose in
holster
Figure 21.5 State
machine model of a
petrol (gas) pump
The names chosen in the state machine diagram are descriptive. The associated
information indicates actions taken by the system actuators or information that is
displayed. Notice that this system never terminates but idles in a waiting state when
the pump is not operating.
The fuel delivery system is designed to allow unattended operation, with the following
sequence of actions:
1. The buyer inserts a credit card into a card reader built into the pump. This causes
a transition to a Reading state where the card details are read and the buyer is
then asked to remove the card.
2. Removal of the card triggers a transition to a Validating state where the card is
validated.
3. If the card is valid, the system initializes the pump and, when the fuel hose is
removed from its holster, transitions to the Delivering state, where is ready to
deliver fuel. Activating the trigger on the nozzle causes fuel to be pumped; this
stops when the trigger is released (for simplicity, I have ignored the pressure
switch that is designed to stop fuel spillage).