Siemens S7 SFB4 problem

[cjd1965]

Hi
I have used a lot of these in a multi instance function block for alarm debouncing. However if the customer does not wish to use a debounce, my intention was they just set 0 as a preset. However I have found that if a value of 0 is used the timer output does not turn on. All the presets come from a MP370 WinCC Flex.

What is a preferred way of turning on the 'timer done' bit in this situation?

I am considering checking if 0 is entered and overwriting it with 0.1 in the PLC or modifying the HMI to use scripts to do the same thing

Cheers

 

[Jan R]

Hello,

At the place you don't use the time you can fill it in the DB at minimum time ?? ( =1 ms) or define it in MP370 as minimum/maximum time (1 - 999ms)

 

[L D[AR2P#0.0]]

An alternative would be use your owm FB which acts as an interface to SFB4. The code attached will use SFB4 if the time is non zero, else the output follows the input. You would replace all calls to SFB4 with a call to FB4. You could also check the time values are sensible if you wanted to.

FUNCTION_BLOCK FB 4
TITLE =
VERSION : 0.1

VAR_INPUT
  IN : BOOL ; 
  PT : TIME ; 
END_VAR
VAR_OUTPUT
  Q : BOOL ; 
  ET : TIME ; 
END_VAR
VAR
  sfbTimer : SFB4; 
END_VAR
BEGIN
NETWORK
TITLE =use sfb4 but if PT=0 then output=input
	  CALL #sfbTimer (
		   IN					   := #IN,
		   PT					   := #PT,
		   Q						:= #Q,
		   ET					   := #ET);
	  L	 #PT; 
	  L	 0; 
	  <>D   ; 
	  BEC   ; 
	  A	 #IN; 
	  =	 #Q; 
END_FUNCTION_BLOCK

 

[cjd1965]

Thanks for the replies. I cannot do much in the HMI because they enter the data in seconds and i use linear scaling to convert to mSecs for the SFB4 preset.

I think that i will use the suggestion of L D[AR2,P#0.0] and will play with tomorrow on the hardware.

Thanks again.