PLC Programming

Controlling a Traffic Signal Unit...
"WAIT" declaration
Now let us turn to the POU WAIT. This POU is supposed to become a timer with which we can
determine the length of the time period of each TRAFFICSIGNAL phase. Our POU receives as input
variable a variable TIME of the type TIME, and as output it produces a Boolean value which we want
to call OK and which should be TRUE when the desired time period is finished. We set this value with
FALSE by inserting at the end of the declaration (before the semicolon, however) " := FALSE ".
For our purposes we need the POU TP, a clock generator. This has two inputs (IN, PT) and two
outputs (Q, ET). TP does the following:
As long as IN is FALSE, ET is 0 and Q is FALSE. As soon as IN provides the value TRUE, the time is
calculated at the output ET in milliseconds. When ET reaches the value PT, then ET is no longer
counted. Meanwhile Q produces TRUE as long as ET is smaller than PT. As soon as the value PT
has been reached, then Q produces FALSE again. See the chapter on the standard library for short
descriptions of all POUs.
In order to use the POU TP in the POU WAIT we must create a local instance from TP. For this we
declare a local variable ZAB (for elapsed time) of the type TP (between the keywords VAR,
END_VAR).
The declaration part of WAIT thus looks like this:
Function Block WAIT, Declaration Part
"WAIT" body
In order to create the desired timer, the body of the POU must be programmed as follows:
Function Block WAIT, Instruction Part
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