Siemens SIMATIC S5 Manual page 82

Introduction to STEP 5
The length of the OBs is calculated as follows:
Operation code max.
+ Header
+ BE operation
If the block is longer than 127 words (operation code), a BLD 255 operation with a program
counter sequence will result at the latest in the 128th codeword. Interposed BLD segmentation
operations are unaffected by this.
The following OBs are provided by the CPU 942F:
OB No. Function
OB must be user-programmed and is called by the operating system.
OB 1 Cyclic program scanning
Interrupt-driven program scanning
OB 2 Process interrupt A:
OB 13 Time interrupt: ( variable in each case: 100 msec. to 10 min. 55 sec. )
Handling of restart procedures
OB 21 Manual power-up
OB 22 Automatic power-up when power is restored
OB is already programmed. OB must be called by the user.
OB 251 PID control algorithm
1 This block does not appear in the "DIR PLC" display on the programmer.
2-8
127 words
5 words
1 word
Table 2-3. Overview of Organization Blocks
Interrupt generation by the 434 digital input module
1
S5-115F Manual
EWA 4NEB 811 6149-02