Siemens SIMATIC S7-400H System Manual page 111

Calculating the maximum disable time for priority classes > 15 (T
The maximum disable time for priority classes > 15 is determined by four main factors:
● As shown in Figure 8-2, all the contents of data blocks modified since the last copy to the
● In the final update phase, all OBs are either delayed or disabled. To avoid any
● In link-up and update operations with master/standby changeover (see section Link-up
● The technological conditions in your process also decide how long an I/O update can be
1. Based on the bus parameters in STEP 7, for each DP master system define
2. Based on the technical data of the switched DP slaves, for each DP master system, define
3. Based on the technological defaults of your system, define
4. Based on your user program, define
S7-400H
System Manual, 09/2007, A5E00267695-03
standby CPU are transferred to the standby CPU again when the update is completed.
The number and structure of the DBs you write to in the high-priority classes is a decisive
factor in the duration of this operation, and so in the maximum disable time for priority
classes > 15. Relevant information is available in the remedies described below.
unnecessary extension of the maximum disable time for priority classes > 15 due to
unfavorable programming, you should always process the time-critical I/O components in
a selected cyclic interrupt. This is particularly relevant in fail-safe user programs. You can
configure this cyclic interrupt in your project and execute it automatically immediately after
the start of the maximum disable time for priority classes > 15, provided you have
assigned it a priority class > 15.
sequence (Page 99)), you also need to change over the active communication channel
on the switched DP slaves when the update is completed. This operation prolongs the
time within which valid values can neither be read nor output. How long this takes is
decided by your hardware configuration.
delayed. This is particularly important in time-monitored processes in fail-safe systems.
Note
For details, refer to the
Automation Systems, Fail-safe Signal Modules
internal execution times of fail-safe modules.
– T for the DP master system
TR
– DP changeover time (referred to below as T
– the maximum changeover time of the active communication channel (referred to below
as T ).
SLAVE_UM
– the maximum permissible time during is no update of your I/O modules (referred to
below as T ).
PTO
– The cycle time of the highest-priority or selected (see above) cyclic interrupt (T
– The execution time of your program in this cyclic interrupt (T
S7-400F and S7-400FH Automation Systems
)
P15
and
manuals. This applies in particular to the
)
DP_UM
)
PROG
Link-up and update
9.4 Time monitoring
S7-300
)
WA
111