Fig. 10 Normal Dp Cycle; Fig. 11 First Occurrence Of A Faulty Dp Cycle; Fig. 12 Following Dp Cycles - Beckhoff FC3102 Documentation

Fig. 10: Normal DP cycle
first occurrence of a faulty DP cycle (slave 3 does not answer)
Fig. 11: First occurrence of a faulty DP cycle
subsequent DP cycles (slave 3 no longer in the polling list)
Fig. 12: Following DP cycles
It can still happen that the slave answers incorrectly (e.g. because, as a result of a local event on the slave,
the DP connection has been removed). In this case, the telegram is not repeated, but the system continues
by sending the next telegram. The DpState [} 25] is set to a value other than 0, the slave is removed from
the polling list and is no longer addressed in the following DP cycle (which means that the time at which the
following telegram is sent changes), until the DP connection can be established again.
Reactions in the master
The master's reactions can be set differently for each slave (see the tab for the slave's Features [} 59]).
Effect on the DP connection (NoAnswer reaction) if the slave either does not answer or does not
answer correctly
This specifies whether the DP connection to the slave should be removed immediately in the absence of a
correct reception telegram, or only after the DP watchdog time has elapsed (see the slave's PROFIBUS
[} 58]tab).
1. If the DP connection is to be removed immediately (Leave Data Exch, default setting) the slave is re-
moved from the polling list and is no longer addressed in the following DP cycles until the DP connec-
tion is established once again. In order to re-establish the DP connection to the slave, at least 7 tele-
grams are sent, and the process generally requires at least 10-20 ms.
2. If the DP connection is only to be removed when the slave has not answered (or not answered cor-
rectly) within the DP watchdog time (Stay in Data-Exch (for WD-Time)), a further attempt is made in
the next polling cycle to address the slave, but if the slave does not answer, a repeat is not sent.
The "Stay in Data-Exch (for WD-Time))" (2.) setting makes sense if the PROFIBUS cycle is to continue to
operate at the most regular possible period even if a slave fails, and if the failure of a slave for one or more
cycles can be tolerated (e.g. in the DP/MC (Equidistant) [} 25] operation mode). In this case the DP
watchdog time for the slave should be set according to the tolerable outage time of the slave, and the Max
Retry limit (DX) (TwinCAT 2.8: see PROFIBUS [} 42] tab of the master, TwinCAT 2.9: see Bus Parameters
[} 49] dialog) should be set to 0.
Normal DP cycle (12 Mbit/s, 5 slaves, on average 20 bytes I, 20 bytes O per slave) in mode "Stay in
Data Exch (for WD time)"
FC3101 and FC3102 Version: 3.0
FC310x as master
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