User Manual

PLC concepts made easy

4.3 Execution of the user program

Easy Book

Manual, 03/2014, A5E02486774-AF

You can configure the "startup after POWER ON" setting of the CPU complete with restart

method using STEP 7. This configuration item appears under the Device Configuration for

the CPU under Startup. At power up, the CPU performs a sequence of power-up diagnostic

checks and system initialization. During system initialization, the CPU deletes all non-

retentive bit memory and resets all non-retentive DB contents to initial values. The CPU then

enters the appropriate power-up mode. Certain errors will prevent the CPU from entering the

RUN mode. The CPU supports the following power-up modes: STOP mode, "Go to RUN

mode after warm restart", and "Go to previous mode after warm restart".

NOTICE

Warm restart mode configuration

The CPU can enter STOP mode due to repairable faults, such as failure of a replaceable

signal module, or temporary faults, such as power line disturbance or erratic power up

event.

If the CPU has been configured to "Warm restart mode prior to POWER OFF", it will not

return to RUN mode when the fault is repaired or removed until it receives a new command

from STEP 7 to go to RUN. Without a new command, the STOP mode is retained as the

mode prior to POWER OFF.

CPUs that are intended to operate independently of a STEP 7 connection should typically

be configured to "Warm restart - RUN" so that the CPU can be returned to RUN mode by a

power cycle following the removal of fault conditions.

The CPU does not provide a physical switch for changing the

operating mode. To change the operating mode of the CPU, STEP

provides the following tools:

•

"Stop" and "Run" buttons on the STEP 7 toolbar

•

CPU operator panel in the online tools

You can also include a STP instruction in your program to change the CPU to STOP mode.

This allows you to stop the execution of your program based on the program logic.

4.3

Execution of the user program

The CPU supports the following types of code blocks that allow you to create an efficient

structure for your user program:

● Organization blocks (OBs) define the structure of the program. Some OBs have

predefined behavior and start events, but you can also create OBs with custom start

events (Page 55).

● Functions (FCs) and function blocks (FBs) contain the program code that corresponds to

specific tasks or combinations of parameters. Each FC or FB provides a set of input and

output parameters for sharing data with the calling block. An FB also uses an associated

data block (called an instance DB) to maintain state of values between execution that can

be used by other blocks in the program.

● Data blocks (DBs) store data that can be used by the program blocks.