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TRICONEX 2481 base card

Bandwidth fragmentation All load calculations that are shown in the previous chapters concerns calculations of the average busload. There is however one additional factor that must be considered when configuring the Advant Fieldbus 100, that is bandwidth fragmentation. It can not be taken for granted that the bus can use all of the 70% (2000 m) or 50% (8500/15000 m) bandwidth. Bandwidth fragmentation means that the 1 ms time slots can not house a CDP even though the CDP load would below the maximum 70% (2000 m) or 50% (8500/15000 m). This is illustrated with the example below:Bandwidth fragmentation appears on 2000 m, 8500 m and 15000 m bus lengths but is more obvious the longer bus length that is used since the transfer times for each CDP increases with the bus length. Due to bandwidth fragmentation the guaranteed achievable CDP busload is 33.4% for 8500 meter and 29.4% for 15000 meter. One way to try to circumvent the bandwidth fragmentation is to change the sizes of the CDPs and the cycle times.

No 1ms CDPs are allowed on a bus configured for 8500 or 15000m. • The number of CDPs configured in each Communication Interface must be less than the maximum the module can handle, which means 3999 for CI520, CI526 and CI522, 200 for CI626, 100 for CI625 and 50 for PM810.

Transmission Constraints 

Due to the organization of the scan table (and the entire operating principles of the communication interface firmware) some constraints are imposed on the Advant Fieldbus 100 transmissions. In order to guarantee that message transfer is possible, at least 25% of the time slots on 2000 meter and 50% on 8500 and 15000 meters are reserved for message transfer. The rest of the time slots may be used freely for CDP communication. The practical maximum bus load value depends on the configuration. This is dependent on the combination of very fast CDPs (for example 1-2 ms) with very slow ones (that is 2048 and 4096 ms). As soon as one CDP with cycle time 2048 ms is used the time slot table is made wider but lower, see Figure 2-22. For 4096 ms the width is even more increased and height decreased. The following rules must be observed when configuring the Advant Fieldbus 100: • The bus load (calculated as in Section 3.7.1, Bus Load Calculation on 2000 meters) caused by CDPs having a cycle time of 1 ms must be less than or equal to 50% in total. • The maximum bus load on 2000m bus length (calculated as in Section 3.7.1, Bus Load Calculation on 2000 meters) is 70%. For 8500m and 15000m the maximum bus load is 50% (calculated according to Section 3.7.2, Bus Load Calculation on 8500 meters and Section 3.7.3, Bus Load Calculation on 15000 meters. • The maximum bus load could be further limited (below 70% and 50%) by bandwidth fragmentation for the current CDP configuration, see Chapter 3.7.4.