6 Organization Blocks
6.1 Overview
OBs (Organization blocks) are the interface between the operating system of the CPU
and the user program. For the main program OB 1 is used. There are reserved numbers
corresponding to the call event of the other OBs. Organization blocks are executed corre-
sponding to their priority. OBs are used to execute specific program sections:
n On start-up of the CPU
n On cyclic or clocked execution
n On errors
n On hardware interrupts occur
6.2 Main
6.2.1 OB1 - Main - Program Cycle
The operating system of the CPU executes OB 1 cyclically. After STARTUP to RUN the
cyclical processing of the OB 1 is started. OB 1 has the lowest priority (priority 1) of each
cycle time monitored OB. Within the OB 1 functions and function blocks can be called.
When OB 1 has been executed, the operating system sends global data. Before
restarting OB 1, the operating system writes the process-image output table to the output
modules, updates the process-image input table and receives any global data for the
CPU.
Cycle time is the time required for processing the OB 1. It also includes the scan time for
higher priority classes which interrupt the main program respectively communication pro-
cesses of the operating system. This comprises system control of the cyclic program
scanning, process image update and refresh of the time functions.
By means of the Siemens SIMATIC manager the recent cycle time of an online connected
CPU may be shown. With PLC > Module Information > Scan cycle time the min., max.
and recent cycle time can be displayed.
The CPU offers a scan cycle watchdog for the max. cycle time. The default value for the
max. cycle time is 150ms as scan cycle monitoring time. This value can be reconfigured
or restarted by means of the SFC 43 (RE_TRIGR) at every position of your program. If
the main program takes longer to scan than the specified scan cycle monitoring time, the
OB 80 (Timeout) is called by the CPU. If OB 80 has not been programmed, the CPU goes
to STOP. Besides the monitoring of the max. cycle time the observance of the min cycle
time can be guaranteed. Here the restart of a new cycle (writing of process image of the
outputs) is delayed by the CPU as long as the min. cycle time is reached.
The CPU's operating system forwards start information to OB 1, as it does to every OB, in
the first 20 bytes of temporary local data. The start information can be accessed by
means of the system function SFC 6 RD_SINFO. Note that direct reading of the start
information for an OB is possible only in that OB because that information consists of
temporary local data.
The following table describes the start information of the OB 1 with default names of the
variables and its data types:
Description
Function
Cycle time
Scan cycle monitoring
time
Access to local data
Local data
VIPA SPEED7
Organization Blocks
Main > OB1 - Main - Program Cycle
HB00 | OPL_SP7 | Operation list | en | 18-30 71
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