I am new to structure text and I don't understand how it work. please advise on the code below.

PROGRAM PLC_PRG

VAR

M1: INT;

M2: INT:=1;

END_VAR

FOR m1:=1 TO 5 BY 1 DO

m2:=m2+1;

END_FOR;

this program should run 5 times and m2 should return value 6.

but it didn't return value 6, and it is still running and adding on

value to m2. how can it be?....

for.pro [23.29 KiB]

It is running 5 times for each scan of the program.

Since the program gets continually scanned, it will run another 5 times on the 2nd scan, 3rd scan, etc.

Think of the main program in a PLC as being contained inside a "DO Forever" loop.

If you want the FOR loop to execute only once, you need to add some sort of condition, perhaps an IF statement around it

Declair a flag

xRunFor: BOOL := TRUE;

Then in the code...

IF xRunFor THEN

... FOR Loop

xRunFor := FALSE;

END_IF;

Thanks for your explanation..