Disabling Of Link-Up And Update - Siemens SIMATIC S7-400H System Manual

Link-up and update
12.3 Link-up and update sequence
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 changeover after updating
(that is, when modifying the configuration 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 the
Standard Functions
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
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-
● Use a considerably longer cam and/or substantially reduce the approach velocity of the
154
CAUTION
manual.
S7-400F and S7-400FH Automation Systems
critical . Use SFC 90 "H_CTRL" to set this time (see above).
slide block to the switch.
System Software for S7-300/400, System and
manual.
System Manual, 03/2012, A5E00267695-11
S7-400H