Slave Prioritization/Multiple Dp Cycles; Fig. 6 Allocation Of Dp Slaves To Several Dp Cycles; Fig. 7 Multiple Dp Cycles In One Task Cycle - Beckhoff FC3102 Documentation

FC310x as master
Comparison of I/O at task start and I/O not at task start
An advantage when "I/O at task start" is selected is that the task and the DP cycle do not have to share the
available bandwidth, and that the DP cycle starts very regularly, any jitter being the TwinCAT jitter. If "I/O not
at task start" is selected, then it is more likely that a DP cycle will be omitted; the regularity of the start times
of the DP cycles depends additionally on the jitter in the task processing. The disadvantage of the setting "I/
O at task start" is that the dead time (system response time) increases.
3.3.2

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 [} 59] 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 [} 59] 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 can be
read in the ActualModulo variable which is described in the Master Diagnostics [} 20] chapter.
Fig. 6: 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 [} 40] or TwinCAT 2.9 [} 48]), 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 [} 12] chapter.
Fig. 7: Multiple DP Cycles in one Task Cycle
14 Version: 3.0 FC3101 and FC3102