Fast input-output control

Let’s make outputs 1 and 2 blink with 250 ms interval, and let’s make outputs 3 and 4 blink with 500 ms interval.

If you have Hardella IDE opened, you can either open code sample in the IDE, or you can create a new project with this code (File > New > Project > Blinking leds).

As project opens you can see the following:

1. PRU Configuration – it describes which programs are assigned to each PRU core, it specifies PRU cycle length for each core, and it sets debounce intervals. In this particular case PRU Configuration is named BlinkningLeds:

   

   Debounce is disabled (see no debounce). As you can see, PRU0 executes BLINK_3_4 with 500 ms interval. That is PRU0 core will execute program BLINK_3_4 at most once every 500 ms.

2. Here’s the program for PRU0. In this case it is named BLINK_3_4:

   

   It should not be that hard, however let’s discuss that in more details. The program writes the value of state into fast out 3 (out3), then it assigns the value of NOT state to fast out 4 (see out4). The value of state variable is altered when enable is TRUE only.

   How can enable change its value? As you can see, enable variable has CoDeSys data exchange configured (see data exchange). That means enable would be passed from the main PLC cycle, however BLINK_3_4 would be executed at PRU cores.

3. Here’s a program for PRU1. It is named BLINK_1_2:

   

   The contents is exactly the same as in BLINK_3_4, except out1 and out2 are used.

As you build the project, Hardella generates MemoryTransfer CoDeSys programs that will have variables to control enable. Here’s an example of the relevant program for PRU0:

PROGRAM BlinkningLeds_Pru0MemoryTransfer
VAR_INPUT
  BLINK_3_4_enable : BOOL;
END_VAR
...

That means in order to pass enable:=TRUE to PRU0 program, it is required to execute the following code in the main PLC program:

BlinkningLeds_Pru1MemoryTransfer(BLINK_3_4_enable := TRUE)