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216VC62a HESG324442R13/E HESG324442R112/F Excitation module

CDPs may be perceived as a broadcast mechanism where the data contained in one CDP (a sending CDP) is transferred cyclically to another CDP (a receiving CDP), situated in another communication interface in the network. The broadcast mechanism provides for having several receiving CDPs configured to receive data from the same sending CDP (see Figure 2-17).

The message transfer services are implemented to enable stations on Advant Fieldbus 100 to send and receive message. Message transfer is not performed cyclically like process data transfer, but only when one (or more) of the attached communication interfaces have something to send. Message transfer does not influence process data transfer in any way. Process data transfer remains deterministic, this is possible since a certain amount of the bandwidth of the Advant Fieldbus 100 is reserved for message transfer, thus making sure that message transfer is possible, regardless of the bus load due to process data transfer.

Bus Master Function

 Both process data and message transfer requires an active bus master. The bus master function controls all transmissions on the Advant Fieldbus 100, while reception of data is controlled locally on the individual communication interface. In an Advant Fieldbus 100 network with one or more communication interfaces attached, one of these will be bus master while the other communication interfaces are active with supervising that the bus master operates correctly. Furthermore, the bus master responsibility is passed over to the next communication interface on the bus approximately every second, for more information see Section 2.10, Synchronization. All attached communication interfaces with bus administration function share the bus master responsibilities. As long as there are operational communication interfaces attached to the Advant Fieldbus 100 it is therefore made sure that process data will flow on the bus and that message transfer is possible.To secure a high availability on the bus, the bus administrators should have as low station address as possible. This is due to the algorithm to recover the bus operation if the bus master crashes: Each bus administrator supervises that it can hear master frames on the bus every millisecond. If it does not hear any master frames it starts to count the time that the bus is "silent". If the bus is silent for (8 + 2 * own station address) milliseconds the bus administrator will become bus master. This shows the importance of selecting station numbers on the bus as low as possible in order to minimize the time it takes for a recovery of the transmission on the bus after a bus master failure.