Function (Fc); Function Block (Fb) - Siemens SIMATIC S7 System Manual

99BProgramming concepts
6.3 Using blocks to structure your program
Configuring the operation of an OB
6.3.2

Function (FC)

A function (FC) is a code block that typically performs a specific operation on a set of input
values. The FC stores the results of this operation in memory locations. For example, use
FCs to perform standard and reusable operations (such as for mathematical calculations) or
technological functions (such as for individual controls using bit logic operations). An FC can
also be called several times at different points in a program. This reuse simplifies the
programming of frequently recurring tasks.
An FC does not have an associated instance data block (DB). The FC uses the local data
stack for the temporary data used to calculate the operation. The temporary data is not
saved. To store data permanently, assign the output value to a global memory location, such
as M memory or to a global DB.
6.3.3

Function block (FB)

A function block (FB) is a code block that uses an instance data block for its parameters and
static data. FBs have variable memory that is located in a data block (DB), or "instance" DB.
The instance DB provides a block of memory that is associated with that instance (or call) of
the FB and stores data after the FB finishes. You can associate different instance DBs with
different calls of the FB. The instance DBs allow you to use one generic FB to control
multiple devices. You structure your program by having one code block make a call to an FB
and an instance DB. The CPU then executes the program code in that FB, and stores the
block parameters and the static local data in the instance DB. When the execution of the FB
finishes, the CPU returns to the code block that called the FB. The instance DB retains the
values for that instance of the FB. These values are available to subsequent calls to the
function block either in the same scan cycle or other scan cycles.
Reusable code blocks with associated memory
You typically use an FB to control the operation for tasks or devices that do not finish their
operation within one scan cycle. To store the operating parameters so that they can be
quickly accessed from one scan to the next, each FB in your user program has one or more
instance DBs. When you call an FB, you also specify an instance DB that contains the block
parameters and the static local data for that call or "instance" of the FB. The instance DB
maintains these values after the FB finishes execution.
140
You can modify the operational
parameters for an OB. For example, you
can configure the time parameter for a
time-delay OB or for a cyclic OB.
S7-1200 Programmable controller
System Manual, 11/2011, A5E02486680-05