Beckhoff CU2508 Documentation page 58

Commissioning
• Variable ID
The variable ID (16 bit) is used for global identification of the individual process data. Therefore, an ID
in the Publisher or Subscriber group may only be used once within a TwinCAT device, see Fig.
Example for communication via network variables: Publishers 1 and 2 on PC1 must have different IDs
(10 and 8), although the same ID (8) may be used in Publisher 2 and Subscriber 1.
Selecting the variable ID
In order to achieve unambiguous allocation we recommend using different IDs for each data com-
munication between connected PCs.
Reason: In Fig. Example for communication via network variables, PC2/subscriber2 not only re-
ceives the designed ID=8-variable from PC1/Publisher2, but, since it is sent as a broadcast (!), it
also receives the NWV from PC3/Publisher1. Differentiation is then no longer possible in PC2.
• Cycle Index
The 16-bit cycle index is a counter sent by the Publisher together with the data. It is generally
incremented with each transmission and can therefore be used as an indicator for flawless transfer. It
can be read on the subscriber side as CycleIndex. Its appearance depends on the Publisher platform:
◦ Publisher on a PC: The variable CycleIndex is not visible and is automatically and cyclically
incremented by the System Manager
◦ Publisher on an EL66xx: The variable CycleIndex is visible and must be incremented/served by
the user such that it is not equal 0 on the subscriber side.
Fig. 79: Example for communication via network variables
58 Version: 2.6 Real-time Ethernet port multiplier