Hi,

We may define and initialise a variable in SCL. I am wondering why one would then use SFC6 to find out if it is the first time the code has been called. When the code is called we may check for initial value of the variable indeed.

for example:

 VAR

  MyVar : INT := -1;

  ...

END_VAR

// Statement Section

IF MyVar = -1 THEN // STARTUP

  MyVar := 0;

  RETURN;

END_IF:

 best regards,

Zonouzi

Hi dear miami,

I told in a SCL function, not a specific function like OB1, and my bad, I should mentioned I mean by initialization, a functions called by OB100. May be my question is confusing, so I will elaborate more.

Instead of using the above technique to differentiate a function called in OB100 for the first time, why one would use the following:

SnErr := SFC6(TOP_SI:= TOP_SI, START_UP_SI:= START_UP_SI);

OB_Start := CHAR_TO_INT(BYTE_TO_CHAR(TOP_SI.NUM)) = 100;

IF OB_Start THEN

  // do something in first call

End_IF;

Best regards,

Zonouzi

Hi,

Know how protection ? I don't get what you mean.

I am just asking which technique is better and why, when you want to do something once and just in the first call of a subroutine called from OB100 ?

Best regards,

Zonouzi

• If you want to run your code on PLC from other vendors, your solution is good. The only thing to watch is to reset your variable (MyVar in your first post.) in a restart of the PLC.
• The SCF6 solution has the advantage, that it uses no global element (memory bits, global variable, counter, timer, etc)  to verify its status. Nothing to look for.
• But it only works in SIMATIC S7 and for users, who use SIMATIC not so often, it's not easy to understand. (This was the reason I answered your question why to do it like this with "know how protection")
• The OB_Main variable is only available in OB_Main, if you want it's information somewhere else, you have to copy it.
• The solution with the system memory bits has the problem, you have to select a system memory byte. If you forget it or use an other one, it will not work. 

Hi and thank you for your comments.

If you want to run your code on PLC from other vendors, your solution is good. The only thing to watch is to reset your variable (MyVar in your first post.) in a restart of the PLC.

 Very good point, you are right, I should make sure every time a restart occurs the code initialization should also happen. In some cases this may not be true.

The SCF6 solution has the advantage, that it uses no global element (memory bits, global variable, counter, timer, etc)  to verify its status. Nothing to look for.

 From your previous point, this solution will always works in S7.

But it only works in SIMATIC S7 and for users, who use SIMATIC not so often, it's not easy to understand. (This was the reason I answered your question why to do it like this with "know how protection")

 Very true.

The OB_Main variable is only available in OB_Main, if you want it's information somewhere else, you have to copy it.

 If you mean OB_Main variable, the variable passed to OB_Main for its first call, I think this will not be the case we are talking about, as OB100 is called before it, and I was looking for the difference for implementing a code segment for subroutines that are called from within OB100 and also subsequently in a cyclic OB like OB35. The code segment should differentiate between first call and subsequent calls of the subroutine.

The solution with the system memory bits has the problem, you have to select a system memory byte. If you forget it or use an other one, it will not work.

 Usually IDEs related to PLC have facilities to trace bit memory usage.

Anyway, thank you for the discussion, it made me clear the difference.

Best regards,

Zonouzi