Supplementary information
18.13 Link-up and update sequence
CPU 410 Process Automation/CPU 410 SMART
352 System Manual, 05/2017, A5E31622160-AC
Disabling of link-up and update
Link-up and update entails a cycle time extension. This includes a period during which no I/O
updates are performed; see Chapter Time monitoring (Page 120). You must pay special
attention to this if you are using distributed I/O and a master/standby changeover occurs
after the update (thus, when the configuration is modified during operation).
Always perform link-up and update operations when the process is not in a critical state.
You can set specific start times for link-up and update operations at SFC 90 "H_CTRL". For
detailed information on this SFC, refer to Manual
System Software for S7-300/400, System
and Standard Functions
.
Note
If the process tolerates cycle time extensions at any time, you do not need to call SFC
90
The CPU does not perform a self
-test during link-up and updating. If you use a fail-safe user
program, you should avoid any exc
essive delay for the update operation. For more details,
S7-400F and S7-400FH Automation Systems
.
Example of a time-critical process
A slide block with a 50 mm cam moves on an axis at a constant velocity
v = 10 km/h = 2.78 m/s = 2.78 mm/ms. A switch is located on the axis. So the switch is
actuated by the cam for the duration of ∆t = 18 ms.
For the CPU to detect the actuation of the switch, the inhibit time for priority classes > 15
(see below for definition) must be significantly below 18 ms.
With respect to maximum inhibit times for operations of priority class > 15, STEP 7 only
supports settings of 0 ms or between 100 and 60000 ms, so you need to work around this by
taking one of the following measures:
● Shift the start time of link-up and updating to a time at which the process state is non-
critical . Use SFC 90 "H_CTRL" to set this time (see above).
● Use a considerably longer cam and/or substantially reduce the approach velocity of the
slide block to the switch.
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