Write Elements; Dynamic Response During Write - Siemens SINUMERIK 840D sl Function Manual

Read
A preprocessing stop is initiated when a link variable is read.
Checks
The following checks are performed for the link variables and link variables memory:
● Observance of the value range limits
● Access to format limit
● Observance of defined memory area in link variables memory
The user/machine manufacturer is solely responsible for preventing the following errors:
● Accessing with incorrect data format
● Accessing the wrong address (index i)
● Reciprocal overwriting of the same data item by multiple channels of a single NCU or
different NCUs
● Reading a data item before it has been updated by a channel of its own NCU or of a different
NCU
Note
Data consistency
The user/machine manufacturer is solely responsible for ensuring data consistency within
the link variables memory, both on a local-NCU basis and across NCUs.
3.2.2.3

Write elements

In the case of write access to the link variables memory (e.g. $A_DLB[4] = 21), what is known
as a link variables write element is required for managing the write process within the system.
The maximum number of write elements that are available for each interpolation cycle is set
by means of the following machine data:
MD28160 $MC_MM_NUM_LINKVAR_ELEMENTS
The maximum number of write elements thus restricts the number of link variables that can be
written during each interpolation cycle.
3.2.2.4

Dynamic response during write

The link variables are written with main-run synchronism. The new value may be read by the
other channels in its own NCU no later than the next interpolation cycle. It can be read in the
next part program block in its own channel.
The channels of the other NCUs in the link group normally see the new value after two
interpolation cycles. However, the limited bandwidth can lead to delays in transferring write
Extended Functions
Function Manual, 10/2015, 6FC5397-1BP40-5BA3
B3: Distributed systems - 840D sl only
3.2 NCU link
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