Beckhoff EL6601 Documentation page 106

Commissioning
Basic principles of Beckhoff network variables
• Quality:
Time in [100 µs] by which arrival of the NWV at the Publisher was delayed.
Relative arrival location:
Input process image of the TwinCAT system
Relative arrival time:
Time at which the next cycle is loaded into the input image
Note:
The reason for determining the delay so precisely is that the NWVs are managed directly by the IO
driver, independent of the cycle. Nevertheless, the data of an NWV that is delayed by a few percent of
the cycle time will not be taken into account until the input process image is read during the next task
cycle.
Note EL6601/EL6614:
Even with the EL66xx the NWV arrival time is defined as the time when the data are available in the
input process image of the RT device, not the time of arrival at the EL66xx or in the input image of the
EtherCAT device.
Fig. 126: Interrelationship between quality and delayed network variable
• 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. Sample
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. Sample for communication via network vari-
ables, PC2/Subscriber2 not only receives 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.
106 Version: 4.2 EL6601, EL6614