Slave Prioritization/Multiple Dp Cycles - Beckhoff EL6731 Documentation

Parameterization and commissioning
5.1.9

Slave Prioritization/Multiple DP Cycles

Distribution of the DP slaves over a number of DP cycles (prioritization of the slaves)
Slaves can be prioritized so that the DP cycle time can be kept as short as possible in systems in which a
few slaves must be polled very rapidly, whereas a larger cycle time would be adequate for other slaves. It is
possible to specify for each slave in what multiple of cycles (Divider under Data-Exch Poll-Rate on the
slave's Features [} 97] tab) it will be polled. Distributing the polling is then helpful, as, for instance, in the case
illustrated below where there are four slaves, each of which is only to be addressed in every second cycle. It
is possible to make settings so that two slaves will be polled in one cycle and the other two slaves in the
other cycle, so that the overall DP cycle time is kept as constant as possible. For this purpose, the Features
[} 97] tab for the slave offers the Modulo setting under Data-Exch Poll-Rate. In the illustrated case, slaves 3
and 5 have Modulo 0 while slaves 4 and 6 are given Modulo 1. The current modulo value can be read in the
ActualModulo variable which is described in the Master Diagnostics [} 118] section.
Fig. 122: Diagram showing allocation of DP slaves to several DP cycles
Multiple DP Cycles in one Task Cycle
If the task cycle time is more than twice as long as the DP cycle time, it is possible for a number of DP cycles
to be carried out within a single task cycle, in order to acquire the most up-to-date input data possible. Based
on the Number of DP cycles per task cycle factor, which can be set on the "FC310x" tab of the master (for
TwinCAT 2.8/2.9), a timer with the cycle time (task cycle time/number DP cycles per task cycle) is started
when the first DP cycle starts, through which further DP cycles can then be started. It is, however, necessary
to check that the last DP cycle is completed in good time (before the next task start), since otherwise one DP
cycle will fail (or possibly more than one, depending on the ratio expressed in the number of DP cycles per
task cycle), as described in the Synchronization [} 41] section.
Fig. 123: Diagram showing several DP cycles in a task cycle
Multiple data samples within one task cycle
The two functionalities just described can now be combined in order, for instance, to give one or more slaves
in a 2 ms cycle new data every 1 ms, or to obtain new data from the slave in order to achieve better
regulation quality. In this case, settings are made under Additional Data_Exchange Samples on the slave's
Features [} 97] tab instead of under Data-Exch Poll-Rate (as described above).
In the sample shown below, first set the factor Number of DP cycles per task cycle in the "FC310x" tab
(for TwinCAT 2.8/2.9) of the master to 2. So that the task can send 2 different values to the slave, or is able
to receive 2 different values from the slave, the appropriate slave is to be entered into the System Manager
EL6731 Version: 2.8 129