The control program frequently polls the sensor
(requests a sample). The sensor usually responds with a message that contains the
most recently measured temperature. We distinguish controllable actions that the
program commands from observable actions that originate in the attached equipment.
All that the program can do in regard to observable actions is to wait for them
(and observe them). Polling is a controllable action and message arrival is an observable
action. Observable actions are prefixed with a question mark ? in Figure 3.1. A
reactive system is any system that includes observable actions. Sometimes reactive
32
Overview 33
Temperature
sensor
?Message
Reset,
Poll
Timer
StartTimer,
CancelTimer
?Timeout
Controller
?Command,
?Exit
Supervisor
Figure 3.1. Process controller, a reactive system.
systems are called event-driven systems. Reactive systems include embedded control
systems, operating systems, and most user interfaces. Reactive systems contrast
with closed systems, where all actions are controllable.
The supervisor program, acting on its own schedule, frequently commands the
control program to compute the calibration factor. The control program responds by
computing the calibration with the most recently obtained temperature sample. The
arrival of a calibrate command is another observable action. The control program
cannot predict which observable action will occur next, or when it will occur.
Pages:
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69