PLC Programming

Controlling a Traffic Signal Unit...
'Login' to get into simulation mode. Start program with 'Online' 'Start'. Open POU ABLAUF by a
double-click on entry "ABLAUF" in the Object Organizer. The program is started now, but to get it run,
variable START must be TRUE. Later this will be set by PLC_PRG but at the moment we have to set
it manually within the POU. To do that, perform a double-click on the line in the declaration part,
where START is defined (START=FALSE). This will set the option "" behind the variable in
turquoise color. Now select command 'Online' 'Write values' to set this value. Thereupon START will
be displayed in blue color in the sequence diagram and the processing of the steps will be indicated
by a blue mark of the currently active step.
When you have finished this intermediate test use command 'Online' 'Logout' to leave the simulation
mode and to continue programming.
"SEQUENCE" second expansion level
In order to ensure that our diagram has at least one alternative branch, and so that we can turn off our
traffic light unit at night, we now include in our program a counter which, after a certain number of
TRAFFICSIGNAL cycles, turns the unit off.
At first we need a new variable COUNTER of the type INT. Declare this as usual in the declaration
part of SEQUENCE, and initialize it in Init with 0.
Action Init, Second Version
Now select the transition after Switch1 and insert a step and then a transition. Select the resulting
transition and insert an alternative branch to its left. After the left transition insert a step and a
transition. After the resulting new transition insert a jump after Switch1.
Name the new parts as follows: the upper of the two new steps should be called "Count" and the lower
"Off". The transitions are called (from top to bottom and from left to right) EXIT, TRUE and
DELAY.OK. The new part should look like the part marked with the black border in the following
image:
3-8 CoDeSys V2.3