PLC Programming

Chapter 3 - We Write a Little Program
3 We Write a Little Program
3.1 Controlling a Traffic Signal Unit...
Create POU
Let us now start to write a small example program. It is for a simple traffic signal unit which is
supposed to control two traffic signals at an intersection. The red/green phases of both traffic signals
alternate and, in order to avoid accidents, we will insert yellow or yellow/red transitional phases. The
latter will be longer than the former.
In this example you will see how time dependent programs can be shown with the language resources
of the IEC1131-3 standard, how one can edit the different languages of the standard with the help of
CoDeSys, and how one can easily connect them while becoming familiar with the simulation of
CoDeSys.
Starting always is easy: Start CoDeSys and choose 'File' 'New'.
In the dialog box which appears, the first POU has already been given the default name PLC_PRG.
Keep this name, and the type of POU should definitely be a program. Each project needs a program
with this name. In this case we choose as the language of this POU the Continuous Function Chart
Editor (CFC)
Now create three more objects with the command 'Project' 'Object Add' with the menu bar or with the
context menu (press right mouse button in the Object Organizer). A program in the language
Sequential Function Chart (SFC) named SEQUENCE, a function block in the language Function Block
Diagram (FBD) named TRAFFICSIGNAL, along with a POU WAIT, also of the type function block,
which we want to program as an Instruction List (IL).
What does TRAFFICSIGNAL do
In the POU TRAFFICSIGNAL we will assign the individual trafficsignal phases to the lights, i.e. we will
make sure that the red light is lit red in the red phase and in the yellow/red phase, the yellow light in
the yellow and yellow/red phases, etc.
What does WAIT do
In WAIT we will program a simple timer which as input will receive the length of the phase in
milliseconds, and as output will produce TRUE as soon as the time period is finished.
What does SEQUENCE do
In SEQUENCE all is combined so that the right light lights up at the right time for the desired time
period.
What does PLC_PRG do
In PLC_PRG the input start signal is connected to the traffic lights' sequence and the "color
instructions" for each lamp are provided as outputs.
TRAFFICSIGNAL simulation
Now test your program in simulation mode. Compile ('Project' 'Build') and load ('Online' 'Login') it.
Start the program by 'Online' 'Start', then set variable ON to TRUE, e.g. by a double-click on the
entry "ON" in the input box of the CFC editor. This will mark the variable as prepared to be set to
. Then press or command 'Online' 'Write values', to set the value. Now variable
START in ABLAUF (which we had set to TRUE manually in the first extension level of the program)
gets this value by variable ON, which is used in PLC_PRG. This will make run the traffic light cycles.
PLC_PRG has changed to a monitoring window. Click twice on the plus sign in the declaration editor,
the variable display drops down, and you can see the values of the individual variables.
CoDeSys V2.3 3-1