Practical use case: A Boolean signal that changes value every 500ms

Recently I was asked for such an example from customers so I decided to make a post out of it. Interestingly they struggle implementing this simple logic.

As you might imagine I will use our powerful SCL script for this. Once you have the proper tools it gets really easy.

Here is a picture of what we need to achieve:

To implement this behavior I will use two built-in FBs.

1. A pulse timer (TP_TIME)

2. A falling edge trigger (F_TRIG)

The provisioned time of the timer should match the pulse width. And since it is a pulse timer its output will stay raised (true) until the time elapses. When the timer output is lowered (false) we can invert the value of the output signal and reset the timer. To detect this event I will rely on the falling edge trigger.

Here is how the code looks:

FUNCTION_BLOCK "MAIN"

VERSION:1.0

VAR_OUTPUT

out : BOOL; // output

END_VAR

VAR

timer : TP_TIME; // pulse timer

trigger : F_TRIG; // FE trigger

END_VAR

BEGIN

#timer(IN:=true, PT:=T#500ms); // start the timer

#trigger(CLK:=#timer.Q); // detect FE on timer output

if (#trigger.Q) then // FE detected ?

#out := not #out; // invert the output

#timer(IN:=false); // reset timer

end_if;

END_FUNCTION_BLOCK

Really simple, isn't it?

To test the code just paste it in the SCL Editor, connect a signal to the output and save the changes.

I hope this will serve as an example to everyone who needs similar logic implemented in Process Simulate.

Enjoy!