Technical introduction

For PRP, a so-called "Redundancy Control Trailer" (RCT) is attached to each Ethernet frame by the TwinCAT PRP Adapter and the Ethernet frame is sent twice via both NICs. When receiving Ethernet frames the RCT is evaluated, double received Ethernet frames from the individual networks are detected. The RCT is removed from the Ethernet frame received first and transferred to the higher-level applications, and the Ethernet frame received twice later is evaluated and then discarded. In addition, the so-called "Supervision Frame" is sent as a multicast and as a sign of life via both NICs at a configurable interval, and is received and evaluated accordingly by other DANs.

For the administration and diagnosis of the redundant communication, information about all known nodes of the network is stored as "Nodes" in a so-called "Nodes Table" on the basis of the received Ethernet frames. This "Node Table" and further information are provided by TwinCAT PRP Adapter and are suitable for a very detailed diagnosis of the network and the detection of errors in the network. (See PRP Adapter Diagnosis)

For the operating system, the network interface card configured for LAN-A in the TwinCAT PRP Adapter is used as a single virtual interface to the redundant networks. All Ethernet frames sent via this interface in the operating system are sent to both networks using PRP, and all received Ethernet frames are forwarded to the operating system via this interface. Thereby the TwinCAT PRP Adapter already removes the RCT and processes other PRP specific Ethernet frames and does not forward them. In the same way, all Ethernet frames for the functions configured in TwinCAT are passed on by both the TwinCAT Realtime Ethernet Adapter and the TwinCAT PRP Adapter to the corresponding drivers and not to the operating system. The network interface card for LAN-B is disabled for the operating system and displayed as if there were no link.