Chapter 5. Ethernet Global DataGFK-2224Q January 2017 75UnicastTransitioning from a multicast or broadcast exchange to unicast production causes samples to be sent to asingle node. Thus the exchange will now only be visible to a single remote node and processed only if thatnode contains a consumed exchange with matching Producer ID and Exchange ID.5.4 Ethernet Global Data TimingThe Ethernet Interface and PLC CPU share internal memory for Ethernet Global Data operations.CPU ETHERNETINTERFACESHAREDMEMORYINTERNALMEMORYNETWORKFigure 37: Memory Sharing between PLC and Ethernet InterfaceIn the producing PLC, the CPU updates its shared internal memory with a data sample when requested by itsEthernet Interface. The update affects the length of the PLC sweep only for that particular exchange; it has littleeffect on the PLC average sweep time. When the Ethernet Interface’s producer period expires, it produces thedata sample from shared internal memory onto the network.In a consuming PACSystems PLC, shared internal memory is updated as soon as the Ethernet Interface gets adata sample from the network. There is no user-configurable consumer period. The CPU updates its referencetables from shared internal memory at the end of the sweep after it is notified by the Ethernet Interface thatfresh data has arrived for a specific exchange. The data is made available to the application on the next PLCsweep after it is received. Some other types of Ethernet Interfaces implement a consumption period timer.EGD SynchronizationEthernet Global Data attempts to provide the most up-to-date process data, consistent with the configuredschedule.The Ethernet interface maintains a timer for each produced exchange. When the timer for the exchangeexpires, the Ethernet interface requests that the data for the exchange be transferred from reference memoryduring the output scan portion of the CPU sweep. At the output portion of the sweep, the CPU puts the datainto the shared memory. Once the data has been transferred by the CPU sweep, the Ethernet interfaceimmediately formulates a sample and transfers the sample on the network. (If updated data is not available atthe next production timer expiration, the Ethernet interface produces a sample containing the previous data tothe network.)As soon as a sample for a consumed exchange is received, it is transferred to the CPU during the next inputscan portion of the CPU sweep.The result of this scheduling method for Ethernet Global Data is a variability of up to one producer CPU sweeptime in the interval between samples produced on the network. This variability in the time between samples ispresent to assure that the most up-to-date data is being transferred.In general, it is not useful or necessary to configure the production period to be less than the CPU sweep time. Ifthe producer period for an exchange is set lower than the CPU sweep time, the Ethernet interface will send a“stale” sample (a sample containing the same data as previously sent) at the configured interval. When thefresh CPU data becomes available at the end of the sweep, the Ethernet interface will immediately sendanother sample with the fresh data. The timer of the produced exchange is not reset when this sample is sent.This can result in more samples in the network than would be expected from the configured period.
