Tag Archives: difference between fb and fc siemens

Difference between FB and FC’s Siemens PLC

Siemens PLC Difference between FB and FC’s

 characteristic  FC  FB
 Can be called as a subroutine  Yes  Yes
 Can assign parameters to Input/Output/InOut  Yes  Yes
 Temporary variables can be used  Yes  Yes
 Can use static variables (can be kept)  No  Yes
 Requires instance data block (per call)  No  Yes
 Parameters passed as addresses to internal use  Yes  No
 Parameters are passed to the internal use via the instance data block  No  Yes
 Can call FB or FC  Yes  Yes
 Can be called by FB or FC  Yes  Yes
 Can use FB as multiple background calls  No  Yes
 Can not write all parameters when called  No  Yes

Articles 7 and 8 in the above table are very important.
FB has memory (except Temp variable), FC has no memory (cannot remember the result of the last scan cycle),

To emphasize that the FC parameter is passed to the block to pass the address, remembering the address instead of a data.

FB parameter input and output block is the data, remember that the data is not the address, FB has a private storage space, the parameter access block is transferred through the DB data block, copy into / copy out

See the following example to understand:
FC block content.

FB block content, exactly the same as FC

Call FC block

Call FB block