Siemens SINUMERIK 840D sl Function Manual page 170

B3: Several operator panels connected to several NCUs, distributed systems - only 840D sl
2.8 Link communication
● Reciprocal overwriting of the same data item by multiple channels of a single NCU or
● Reading a data item before it has been updated by a channel of its own NCU or of a
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.
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 grouping normally see the new value after two
interpolation cycles. However, the limited bandwidth can lead to delays in transferring write
jobs to the other NCUs in the link grouping (message delays). Possible situations include:
● Writing a large number of link variables in an interpolation cycle
● Writing link variables and requesting an axis container rotation in the same interpolation
● Writing link variables and transferring an alarm in the same interpolation cycle
Synchronizing a write job
If certain applications require the new value of a link variable to be transferred to the other
NCUs in the link grouping in precisely two interpolation cycles, writing may be carried out by
means of a synchronized action, and the $A_LINK_TRANS_RATE system variable written in
accordance with the number of write elements available. The system variable contains the
value specifying how many write jobs can still be transferred to the other NCUs in the link
grouping in the current interpolation cycle.
Programming example
N120 WHEN $A_LINK_TRANS_RATE > 0 DO $A_DLB[0] = 9
N125 WHEN $A_LINK_TRANS_RATE > 0 DO $A_DLB[1] = 7
170
different NCUs
different NCU
NOTICE
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.
cycle
Function Manual, 09/2009, 6FC5397-1BP20-0BA0
Extended Functions