PLC Programming

3 - We Write a Little Program
At first it is checked whether Q has already been set at TRUE (as though the counting had already
been executed), in this case we change nothing with the occupation of ZAB, but we call the function
block ZAB without input (in order to check whether the time period is already over).
Otherwise we set the variable IN in ZAB at FALSE, and therefore at the same time ET at 0 and Q at
FALSE. In this way all variables are set at the desired initial condition. Now we assign the necessary
time from the variable TIME into the variable PT, and call ZAB with IN:=TRUE. In the function block
ZAB the variable ET is now calculated until it reaches the value TIME, then Q is set at FALSE.
The negated value of Q is saved in OK after each execution of WAIT. As soon as Q is FALSE, then
OK produces TRUE.
The timer is finished at this point. Now it is time to combine our two function blocks WAIT and
SEQUENCE in the main program PLC_PRG.
"SEQUENCE" first expansion level
First we declare the variables we need. They are: an input variable START of the type BOOL, two
output variables TRAFFICSIGNAL1 and TRAFFICSIGNAL2 of the type INT and one of the type WAIT
(DELAY as delay). The program SEQUENCE now looks like shown here:
Program SEQUENCE, First Expansion Level, Declaration Part
Create a SFC diagram
The beginning diagram of a POU in SFC always consists of an action "Init" of a following transition
"Trans0" and a jump back to Init. We have to expand that.
Before we program the individual action and transitions let us first determine the structure of the
diagrams. We need one step for each TRAFFICSIGNAL phase. Insert it by marking Trans0 and choosing
'Insert' 'Step transition (after)'. Repeat this procedure three more times.
If you click directly on the name of a transition or a step, then this is marked and you can change it.
Name the first transition after Init "START", and all other transitions "DELAY.OK".
The first transition switches through when START is TRUE and all others switch through when DELAY
in OK produces TRUE, i.e. when the set time period is finished.
CoDeSys V2.3 3-5