User Manual

Programming made easy

6.1 Easy to design your user program

Easy Book

Manual, 03/2014, A5E02486774-AF

A function block (FB) is like a subroutine with memory.

An FB is a code block whose calls

can be programmed with block parameters. The FB stores the input (IN), output (OUT), and

in/out (IN_OUT) parameters in 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 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 open an instance DB that stores the values of

the block parameters and the static local data for that call or "instance" of the FB. These

values are stored in the instance DB after the FB finishes.

You can assign start values to the parameters in the FB interface. These values are

transferred to the associated instance DB. If you do not assign parameters, the values

currently stored in the instance DB will be used. In some cases, you must assign

parameters.

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. By designing

the FB for generic control tasks, you can reuse the FB for multiple devices by selecting

different instance DBs for different calls of the FB.

The following figure shows an OB that calls one FB three times, using a different data block

for each call. This structure allows one generic FB to control several similar devices, such as

motors, by assigning a different instance data block for each call for the different devices.

Each instance DB stores the data (such as speed, ramp-up time, and total operating time)

for an individual device. In this example, FB 22 controls three separate devices, with DB 201

storing the operational data for the first device, DB 202 storing the operational data for the

second device, and DB 203 storing the operational data for the third device.