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

RAM and load memory
During the link-up, the system transfers the user program blocks (OBs, FCs, FBs, DBs, SDBs)
from load memory of the master to RAM on the standby CPU. Exception: If the load memory
modules are FLASH cards, the system only transfers the blocks from work memory.
9.3.4

Disabling 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 106). Make allowances for this feature
in particular when using distributed I/Os and on master/standby changeover after updating
(that is, when modifying the configuration in Run).
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 generally tolerates cycle time extensions, you do not need to call SFC 90
"H_CTRL".
The CPU does not perform a self-test during link-up and updating. In a fail-safe system, you
should therefore avoid any excess delay times 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.
In order to enable the CPU to detect the actuation of the switch, the disable time for priority
classes > 15 (see below for definition) must be clearly 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 a considerably longer cam and/or substantially reduce the approach velocity of the
S7-400H
System Manual, 09/2007, A5E00267695-03
CAUTION
) manual.
S7-400F and S7-400FH Programmable Controllers
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.
Link-up and update
9.3 Link-up and update
105