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
PLC ONE 