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no_tr
tr
no_tr
N
0 s,
T
tr
all
5 s, { no_tr,tr }
X
0 s,
all
all
Fig. 5. PLC-Automaton for the lter FES
react as fast as possible to the input values. However in the state with output
T
the inscriptions are 5
s
and
fno; no trg
. This species that in this state the
automaton should ignore all inputs
for 5 seconds. This realises
the lter requirement of FES discussed in Section 4. On the other hand, the
automaton should react to an input Error as fast as possible by switching to the
state with output
no
and
no tr
X
. As fast as possible means here within 2
" PLC seconds.
6
Programs: Structured Text
For PLCs several dedicated programming notations have been devised. The one
resembling most clearly other imperative programming languages is ST ( Struc-
tured Text )[19,26].
Fig. 6 shows an ST program PRG FES implementing the PLC-Automaton of
Fig. 5. In ST timers are declared as variables of a special type TP . The program
PRG FES contains a declaration of such a variable called timer . The statement
timer ( IN := TRUE
;
PT := t # 5
:
0s )
switches the timer on and sets it to 5 seconds. This statement is executed when
entering state 1. The condition NOT timer.Q is true as soon as the timer has
expired. This condition has to be met in order to react to the input no tr by
switching to state 0. The statement
timer ( IN := FALSE ; PT := t # 0 : 0s )
switches the timer o. It is executed when leaving state 1.
7
Semantic Link
We have now discussed three levels of abstraction for real-time systems, each
one with its own notation: requirements, design specications, and programs.
How can these notations be linked together in a correct way ? To this end, we
instantiate the logic-based approach outlined in Section 2.
 
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