Siemens SINUMERIK 840D sl Function Manual page 166

Several Operator Panels on Several NCUs, Distributed Systems (B3)
2.8 Cross-NCU user communication, link variables
Number of write elements
The write elements available for writing to link variables are limited. Their number is defined
by the machine data:
MD28160 $MC_MM_NUM_LINKVAR_ELEMENTS.
If no more write elements are available for an intended write process, alarm 14763 is issued.
The set number of write elements only limits the number of write processes that can be
written in one block.
Dynamic response during write
Writing the link variables is immediately completed for the local NCU in the current
interpolation cycle (in the sequence of commands). If the user does not exceed the number
of possible write processes (can be checked in system variable $A_LINK_TRANS_RATE) in
the current interpolation cycle, all other NCUs will have access to the written information 2
interpolation cycles later. If link variables are used exclusively to coordinate the channels of
a multi-channel NCU, they can be written in the same interpolation cycle.
Note
The user (machine manufacturer) must ensure that the time is consistent for larger data
blocks that are logically associated with one another. The transmission is word by word. The
data quantity which can be transferred in the same interpolation cycle is specified in system
variable $A_LINK_TRANS_RATE (see "System variables of the link memory"). A
transmission can be protected by marking variables of the link memory as semaphores.
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Function Manual, 01/2008, 6FC5397-1BP10-3BA0
Extended Functions