Siemens SINUMERIK 840D sl Function Manual page 170

Several Operator Panels on Several NCUs, Distributed Systems (B3)
Link communication
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
The byte addressed by link variable $A_DLB[0] is only written to N120 if the write job can
be transferred to the other NCUs in the link grouping in the same interpolation cycle. If this is
the case, the link variable is written and the system variable decremented at the same time.
A block change then takes place and N125 is processed in the same way.
If it is not possible for the write job to be executed immediately in N120, an attempt is made
to execute the write process in the next interpolation cycle.
Example
The following data is required for link communication:
Data format
BYTE
WORD
DWORD
REAL
170
cycle
Number Bytes required
2 2
1 2
3 12
1 8
24 (required size of the link variables memory)
Function Manual, 03/2009, 6FC5397-1BP10-4BA0
Extended Functions