Instructions
The attached download "STEP7_V11_Average" includes a STEP 7 project with the function block FB1. You can use this block to calculate a current arithmetic average value from any number of values read in. Fig. 01 shows the call and parameters of the FB1 in OB1.

Fig. 01

Input parameters of FB1 "Average"

• "Reset" with the data type: Bool
  The default value is False. If the "Reset" input receives the value True, the block is reset and the "Average" receives the value Zero.
   
• "Value" with the data type: Real
  This parameter is for reading in the input values from which the current average value is calculated.
   
• "By_Value_Change" with the data type: Bool
  Defines the mode for calculating.

• True: the block calculates the new average value only if the "Value" parameter changes.
• False: the average value is recalculated each cycle.

Output parameters of FB1 "Average"

• "Average" with the data type: Real. Returns the arithmetic average value calculated.

Program description
In the case of input values read in consecutively, the current arithmetic average is recalculated for each sum when the value of the "Value" parameter changes. The division denominator for calculating the average value increments by the value "1" for each input value read in.
 

No.	Value	Average value
1	X1)	X1 / 1
2	X2)	(X1+X2) / 2
3	X3)	(X1+X2+X3) / 3
...	...	...
n	Xn	(X1+X2+X3+ ... Xn) / n

Table 01

Table 02 shows an example of five values read in one after the other. The average value is calculated upon input of the fifth value "14.0":

• Average value = (5.0+3.0+1.0+7.0+14.0) / 5 = 6

The average value is also calculated when the value at the "Value" input is zero. Cyclic interrupts or a call in OB1 are suitable for the FB1's runtime level.

The attached download contains a STEP 7 project with the blocks described above (FB1, instance DB1 and OB1).

STEP7_V11_Average.zip ( 1500 KB )

Copy the "STEP7_V11_Average.zip" file into a separate directory and then start the file with a double-click. The STEP 7 V11 project is now unpacked automatically with all the associated subdirectories. You can then use STEP 7 V11 + SP2 to open and process the extracted project.

Creation environment
The screens and downloads in this FAQ response were created with STEP 7 V11 + SP2. The table below lists the components that have been used to create this entry and verify the functions described.
 

Test environment	Version
PC platform	Intel(R) Core(TM)2 Duo CPU P8700 2.53 GHz
PC operating system	MS Windows XP SP3
STEP 7 (TIA Portal)	V11 + SP2
CPU 1214C (6ES7214-1AE30-0XB0)	Firmware V2.1

Table 03