Beckhoff EL6731 Documentation page 117

Parameterization and commissioning
The reaction of the master if the DP connection to the slave is removed
This specifies whether removing the DP connection to a slave has no other effects (No Reaction, the default
setting), or whether the master should enter the STOP state, thus removing the DP connections to all the
slaves.
Effect on the state of the master (Clear mode), if the DP connection to the slave is removed
Clear mode (see Fault Settings [} 89] dialog) can be used to specify that the master should switch to or
remain in "Clear" state, as long as at least one MC slave (setting: "Only MC slaves") or any slave (setting:
"All slaves") does not respond correctly (i.e. has a DpState [} 121] not equal 0).
The Reaction of the Master setting (see the slave's Features [} 97] tab), which was described in the
previous section, has priority over the Auto-Clear mode, so that when an appropriately set slave fails, the
Master enters the STOP state.
Failure of the master
Monitoring in the PLC/IO task
In the presence of a persistent bus fault, the DP cycle can be extended up to 100 ms, even at 12 Mbaud. In
order to monitor the DP master, there is a status variable CycleCounter, and this can be linked in the PLC
(see the Master Diagnosis [} 118] section). This variable is incremented by the master after each DP cycle,
so that failure of the master can be detected in by monitoring this variable in the PLC.
Monitoring in the slave
In order to monitor failure of the master and data transmission on the PROFIBUS, a watchdog (see the
box's Profibus [} 96] tab) can be activated (default setting: watchdog activated with 200 ms). The Watchdog
must be set to at least twice the maximum Estimated Cycle Time and Cycle Time (see "FC310x or
"EL6731" tab (for TwinCAT of the master)).
Failure of the PLC/IO task (FC310x) or EtherCAT interruption (EL6731)
A distinction is made between the cases PLC stop, reaching of a breakpoint and task stop [EL6731:
EtherCAT interruption] (IO task, NC task is only stopped on system stop). In the case of a PLC stop, the
output data is set to 0 by the PLC, whereas when a breakpoint is reached the data initially remains
unchanged.
In the master, the task is monitored with a monitoring time (according to the Task Watchdog setting times
the task cycle time, see Fault Settings [} 89] dialog). If no new data transfer takes place within this monitoring
time, the master switches to "Clear" state according to the setting Reaction on PLC Stop or Reaction on
Task Stop (see Fault Settings [} 89] dialog; outputs are set to 0 or safe state (Fail_Safe = 1 in the GSD file,
default setting) or remains in "Operate" state (outputs retain the last value). The "Operate" setting is valuable
when the outputs should not be cleared when a breakpoint is reached in the PLC. However, if the PLC stops,
the outputs will still be set to 0 (by the PLC), even if the master remains in the "Operate" state. It should,
however, be noted that the outputs will only be zeroed if the previous DP cycle is completed in time (see the
Synchronization [} 41] section). It should therefore only be set during the commissioning phase.
Host failure [only FC310x]
In order to monitor the host for crashes (e.g. Blue Screen in the case of a PC), a watchdog time can be set
(see Fault Settings [} 89] dialog). If this watchdog timer elapses, the master enters the OFFLINE state, so
that the DP connections to all the slaves are removed, and the master logs off from the PROFIBUS, ceasing
to carry out bus accesses.
Start-up behavior
The DP connections to all the slaves are established when the TwinCAT system starts up. Until the highest
priority task that is involved has not been started, the master still does not send any Data_Exchange
telegrams even after the DP connection has been established, and sends only diagnostic telegrams. As
EL6731 Version: 2.8 117