Beckhoff CU2508 Documentation page 57

All network variables must be declared in the System Manager before they can be used.
The following intervention options are then available during operation:
• Sending of a configured network variable can be blocked dynamically
• The destination IP or destination MAC can be changed dynamically
• The variable ID "variable ID" can be changed dynamically
• The NWV content can be changed, but not the size (bit size)
Diagnostic variables on the publisher and subscriber side provide information about the connection quality.
If network variables are used, the temporal boundary conditions for the network topology used must be taken
into account: In switched mode (MAC addressing) communication cycles of approx. 10 ms and below can be
achieved, in routed mode (IP addressing) a few 100 ms may be achievable in some cases.
Diagnostic variable "quality"
If the processing tasks operate with different cycle times or the user changes the DataExchangeDi-
vider, this must be taken into account in the analysis of the diagnostic variables. In conjunction with
a fast Subscriber (e.g. 10 ms), a slow Publisher (e.g. 100 ms) leads to poor connection quality (as
reported by the diagnostic variable "Quality").
Dynamic temporary blocking of sending a Publisher must also be taken into account. In this case
the Subscriber registers poor quality.
Diagnostic variable "CycleIndex"
Please note the following information in order to decide whether you have to serve the variable Cy-
cleIndex.
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 regarding 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. 78: Interrelationship between quality and delayed network variable
Real-time Ethernet port multiplier Version: 2.6
Commissioning
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