B3: Distributed systems - 840D sl only
2.2 NCU link
Extended Functions
Function Manual, 03/2013, 6FC5397-1BP40-3BA1
89
Write
A link variable is written with main-run synchronism.
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.
2.2.2.3 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.
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