PLC Programming

Controlling a Traffic Signal Unit...
"TRAFFICSIGNAL" declaration
Let us now turn to the POU TRAFFICSIGNAL. In the declaration editor you declare as input variable
(between the keywords VAR_INPUT and END_VAR) a variable named STATUS of the type INT.
STATUS will have four possible conditions, that is one for the TRAFFICSIGNAL phases green, yellow,
yellow/red andred.
Correspondingly our TRAFFICSIGNAL has three outputs, that is RED, YELLOW and GREEN. You
should declare these three variables. Then the declaration part of our function block TRAFFICSIGNAL
will look like this:
Function block TRAFFICSIGNAL, declaration part
"TRAFFICSIGNAL" body
Now we determine the values of the output variables depending on the input STATUS of the POU. To
do this go into the body of the POU. Click on the field to the left beside the first network (the gray field
with the number 0001). You have now selected the first network. Choose the menu item 'Insert' 'Box'.
In the first network a box is inserted with the operator AND and two inputs:
Click on the text AND, so that it appears selected and change the text into EQ. Select then for each of
the two inputs the three question marks and overwrite them with "STATUS" respecively "1".
Click now on a place behind the EQ Box. Now the output of the EQ operation is selected. Choose
'Insert' 'Assign'. Change the three question marks to GREEN. You now have created a network
with the following structure:
STATUS is compared with 1, the result is assigned to GREEN. This network thus switches to GREEN
if the preset state value is 1.
3-2 CoDeSys V2.3