Beckhoff network variables - Settings

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.

• NOTICE
• 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.127: Sample for communication via network variables

Data representation on different platforms Please note that simple and complex data (WORD, ARRAYs, REAL, STRING, user-defined structures) are represented internally in a different manner on different platforms! x86 platforms use byte-alignment, others (ARM) 2-byte or 4-byte alignment. This means that if a complex structure is created in an x86/PC PLC project and an ARM PLC project, they can each have a different effective size and a different internal structure. (see Fig. “Data representation e.g. x86 Systems vs. ARM Systems”) In the sample, the structure in the CX (and hence the network variable to be created there) is larger than in the PC; also the word and real variables do not match each other because a variable can begin at any byte position in the PC, but only at every even-numbered one in the CX. Consequences Recommendation for structures that are identical on both end devices - firstly, all 4-byte variables (must lie at an address that is divisible by 4) - then all 2-byte variables (must lie at an address that is divisible by 2) - then all 1-byte variables Further recommendations - if STRING(x) is used, the "EndOfString" zero is also interpreted as a character, otherwise x+1 must be divisible by 4 - the above rules also apply to sub-structures. Please refer to the notes in the Structure section of the Infosys. Consequences Use of Bus Terminal Controllers (BICxxxx, BXxxxx) Since the representation of floating point numbers (REAL) on Bus Terminal Controllers (BCxxxx, BXxxxx) differs from that in the x86, these cannot be transmitted. "SINT", for sample, can be used for signed values.

Fig.128: Data representation e.g. x86 Systems vs. ARM Systems

Settings in the System Manager

Appearance of the variables Depending on the platform used (PC or EL66xx), the publisher/subscriber will appear differently. A publisher/subscriber can be created On a PC network interface, see Fig. Publisher settings - RT Ethernet on an EL66xx, see Beckhoff network variables - Settings

The following settings options are available in the Beckhoff System Manager TwinCAT 2.10 build 1328:

Publisher, Box

Fig.129: Publisher RT Ethernet settings

RT Ethernet settings:

• MAC-Broadcast: Sent to all network devices, destination MAC FF:FF:FF:FF:FF:FF.
• Multicast: A destination MAC address becomes a multicast address if the first bit in the first byte of the MAC (the so-called group bit) is set. With the Beckhoff ID "00 01 05" the default target address "01 01 05 04 00 00" is formed, as shown in Fig. Publisher RT Ethernet settings.
  The MAC range 01:00:5E:00:00:00 to 01:00:5E:FF:FF:FF is intended for general multicast application, with the first 3 bytes specified by the IEEE and the last 3 bytes derived from the lower part of the IP address of the destination PC. The resulting destination MAC therefore never physically exists in the network. Instead, the destination network card detects Ethernet frames formed in this way as multicast frames sent to it, although the Ethernet port itself can have another, unique MAC address. Please refer to the relevant literature for further rules relating to the formation of multicast MAC/IP addresses.
• Unicast: Either direct entry of the destination MAC or via the AMS Net ID of the destination device, e.g. 123.456.123.456.1.1, in which case this route must be entered in the local AMS router (right-click on the TwinCAT icon in the taskbar --> Properties --> AMS router)

Use of broadcast and multicast Network variables sent as broadcast or multicast at MAC or IP level can generate high network load (depending on the cycle time), since they are multiplied into the whole connected network. This may cause simple network devices such as printers to crash. With short cycle times all network traffic may become blocked. We strongly recommend using unicast addressing, taking into account variable identification, as described above.

Advanced Settings:

• Data exchange: The task cycle time * divider is the rhythm at which this network variable is sent. (not for EL66xx).
• VLAN support: In conjunction with manageable switches the Ethernet frame parameterized here can be assigned a fixed route via VLAN tagging (Virtual Local Area Network).

Fig.130: Publisher settings - UDP/IP

UDP/IP settings - the addressing technique of the IP network layer with IP addresses is used. UDP is a connection-less protocol without feedback.

• Broadcast: Sent to all device with destination IP (v4) 255.255.255.255
• Multicast: The destination IP must be specified, see notes on MAC multicast
• Unicast: Specify the target device (e.g.: 192.168.0.1), making sure that it can be reached through the subnet mask

Use of broadcast and multicast Network variables sent as broadcast or multicast at MAC or IP level can generate high network load (depending on the cycle time), since they are multiplied into the whole connected network. This may cause simple network devices such as printers to crash. With short cycle times all network traffic may become blocked. We strongly recommend using unicast addressing, taking into account variable identification, as described above.

Advanced Settings:

• "ARP Retry Interval": In order to ascertain the presence of the recipient, the publisher sends an ARP request to the target device at these intervals. If the network administration of the recipient is operational, it sends an ARP reply. This is only meaningful with unicast.
  In the event of an error bit 3 is set (0x0004) in the diagnostic FrameState variable.
  Note: ARP handling (ARP = Address Resolution Protocol: allocation of hardware/MAC addresses to network addresses [IP]) is managed by the operating system (Windows).
• "Disable Subscriber Monitoring": deactivates the procedure described above.
• "Target Address changeable": In this case the destination IP can be changed dynamically.

Publisher, Variable

Fig.131: Publisher variable settings

Settings:

• "Variable ID": Identification number with which the variable is sent. Can be changed online via PLC where appropriate.
• "Data exchange": see above (not for EL66xx).
• "On change only": NWV is only sent if the value changes (not for EL66xx).

Subscriber, Box

Fig.132: Subscriber settings

Settings:

• "Receiving Options": Only permits NWVs from a certain publisher for this subscriber
• "Multicast Configuration": ditto.

Process data:

•   "VarId": If activated, the variable ID can be modified online

Subscriber, variable

Fig.133: Subscriber variable settings

Settings:

• "Variable ID": Only permits NWVs with a certain ID for this subscriber. Can be changed dynamically via PLC where appropriate.
• "Ignore Data Type Hash": Hash calculation is currently not supported

Process data:

• "Quality": See explanatory notes above.
• "CycleIndex": This index is incremented with each successful transfer, IF this is done by the opposite side, i.e. the publisher. If the publisher is an EL66x, the user must increment CycleIdx there.
• "VarData": Transferred data.