Siemens SIMATIC S7-400H Manual page 112

Linking and Synchronizing
7. The generation of new OB request for all OBs (in other words, also for those
having a priority class > 15) is inhibited so that no new interrupts are stored
and, consequently, no request errors occur.
Not until the end of the update are the queued interrupts requested again and
processed. The time stamp of the interrupts that are generated after a delay
cannot be evaluated.
The user program is not executed any more and there are no more I/O
updates.
8. Generation of the start event for the watchdog interrupt OB with special
handling if its priority class > 15 and execution of this OB, as necessary.
Note
The watchdog interrupt OB with special treatment is of particular significance if it
must respond within a particular time to modules or program segments. This is
typically the case for fail-safe systems. For more details see the manuals S7-400F
and S7-400FH Programmable Controllers and S7-300 Programmable Controllers;
Fail-Safe Signal Modules.
9. Transfer the outputs and the entire data block contents that have been
modified. Transfer the timers, counters, memory markers and inputs. Transfer
the diagnostics buffer
During this data synchronization, the clock pulse for watchdog interrupts, delay
interrupts and S7 timers is stopped. This results in any synchronism that might
have existed between watchdog interrupts and time-of-day interrupts being lost.
10.Lift all restrictions. Delayed interrupts and communication functions will be
caught up. All OBs continue to be executed.
Equidistance from the previous calls can no longer be guaranteed for delayed
watchdog interrupt OBs.
Notice
Process interrupts and diagnostics interrupts are stored by the I/O. If such
interrupts were set by modules of the remote input/output station they will be
caught up when the block is lifted. If they were set by modules of the central I/O
they can then only all be caught up if the particular interrupt request did not occur
again during the block.
If the PG/ES requested a master/standby switch-over then on conclusion of the
update the previous standby CPU becomes the master and the previous master
CPU switches to STOP mode. Otherwise the two CPUs go to RUN (Redundant
mode) and execute the user program in synchronism.
6-10
Automation System S7-400H Fault-tolerant Systems
A5E00068197-07