Siemens SIMATIC S7-400H System Manual page 150

Link-up and update
12.3 Link-up and update sequence
8. Generating the start event for the cyclic interrupt OB with special handling.
Note
The cyclic interrupt OB with special handling is particularly important in situations where
you need to address certain modules or program parts within a specific time. This is a
typical scenario in fail-safe systems. For details, refer to the
Automation Systems
manuals.
To prevent an extension of the special cyclic interrupt, the cyclic alarm OB with special
handling must be assigned top priority.
9. Transfer of outputs and of all data block contents modified again. Transfer of timers,
10. Cancel all restrictions. Delayed interrupts and communication functions are executed. All
Note
Process interrupts and diagnostic interrupts are stored by the I/O devices. Such interrupt
requests issued by distributed I/O modules are executed when the block is re-enabled.
Any such requests by central I/O modules can only be executed provided the same
interrupt request did not occur repeatedly while the status was disabled.
If the PG/ES requested a master/standby changeover, the previous standby CPU assumes
master mode and the previous master CPU goes into STOP when the update is completed.
Both CPUs will otherwise go into RUN (redundant system mode) and execute the user
program in synchronism.
When there is a master/standby changeover, in the first cycle after the update OB 1 is
assigned a separate identifier (see
Functions
configuration, refer to section Switch to CPU with modified configuration or expanded
memory configuration (Page 152).
150
and
counters, bit memories, and inputs. Transfer of the diagnostic buffer.
During this data synchronization, the system interrupts the clock pulse for cyclic
interrupts, time-delay interrupts and S7 timers. This results in the loss of any synchronism
between cyclic and time-of-day interrupts.
OBs are executed again.
A constant bus cycle time compared with previous calls can no longer be guaranteed for
delayed cyclic interrupt OBs.
Reference Manual). For information on other aspects resulting from modifying the
S7-300 Automation Systems, Fail-safe Signal Modules
System Software for S7-300/400, System and Standard
S7-400F and S7-400FH
System Manual, 03/2012, A5E00267695-11
S7-400H