Siemens SIMATIC S7-400H Manual page 54

Link-up and Update
4. Transfer of all the data block contents that have been modified the since
link-up.
5. Communication jobs from which the CPU itself derives jobs for other modules
(e.g. I/O) are given a negative acknowledgement (see list below).
6. Initial calls (in other words, calls resulting in manipulation of the work memory,
refer also to the system software for S7-300/400 system and standard
functions ) of communication functions receive a negative acknowledgement.
All the remaining communication functions are delayed and caught up on once
the update is complete.
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 for
S7-400F and S7-400FH Programmable Controllers, Fail-Safe Systems 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 diagnostic 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.
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S7-400H Programmable Controller Fault-Tolerant Systems
A5E00068197-04