Block Link-Up And Update; Example Of A Time-Critical Process - Siemens SIMATIC S7-400H Manual

6.2.4

Block Link-up and Update

Link-up and update is associated with a scan-cycle time extension. Within this
there is a margin of time in which no I/O updating is performed (see Section 6.3
"Time Monitoring"). This must be particularly observed if using distributed I/O and a
master/standby switch-over takes place after the update (i.e. in the event of a
configuration modification whilst in operation).
Caution
!
Only perform link-up and update in non-critical process states.
To specify the start time of the link-up and update yourself, the function SFC 90
"H_CTRL" is available. You will find a detailed description of this SFC in the
manual on System Software for S7-300/400, System and Standard Functions).
Notice
If the process tolerates a longer scan-cycle time extension at any point of time,
there is no need to call the 90 "H_CTRL" SFC.
The CPU self-test is not performed during link-up and updating. In the case of a
fail-safe system, therefore, make sure that you do not delay the update over too
long a period. For more details refer to the S7-400F and S7-400FH Programmable
Controllers manual.

Example of a time-critical process

A slide block with a 50 mm long cam moves on an axis at a constant velocity
v = 10 km/h = 2.78 m/s = 2.78 mm/ms. A push button is located on the axis. The
push button is thus activated by the cam during a period of ∆t = 18 ms.
In order for the activation of the push button to be recognized by the CPU the
blocking time for priority classes > 15 (see below for definition) must be clearly
below 18 ms.
Since in STEP 7 you are able to set the maximum blocking time for priority
classes > 15 only to 0 ms or a value of between 100 and 60000 ms, you need to
remedy the situation with one of the following measures:
• Move the start of link-up and update to a time at which the process state is not
critical. To do this, use SFC 90 "H_CTRL" (see above).
• Use a substantially longer cam and / or clearly reduce the velocity of the slide
block before it reaches the push button.
Automation System S7-400H Fault-tolerant Systems
A5E00068197-07
Linking and Synchronizing
6-13