Link-up and update
12.3 Link-up and update sequence
S7-400H
154 System Manual, 03/2012, A5E00267695-11
12.3.4 Disabling of link-up and update
Link-up and update entails a cycle time extension. This includes a period during which the
I/O is not updated; see section Time monitoring (Page 155). Make allowances for this feature
in particular when using distributed I/Os and on master/reserve change
over after updating
(that is, when modifying the configuration during operation).
CAUTION
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 the
System Software for S7-300/400, System and
Standard Functions
manual.
NOTICE
If the process tolerates cycle time extensions at any time, you do not need to call SFC 90
"H_CTRL".
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 excessive delay for the update operation. For more
details, refer to the
S7-400F and S7-400FH Automation Systems
manual.
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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