Beckhoff EL6751 Documentation page 97

Parameterization and commissioning
Guard time and life time factor
If the master requests the guard messages in a strict cycle, the slave can detect the failure of the master. In
this case, if the slave fails to receive a message request from the master within the set Node Life Time (a
guarding error), it assumes that the master has failed (the watchdog function). It then puts its outputs into the
error state, sends an emergency telegram, and returns to the pre-operational state. After a guarding time-out
the procedure can be re-started by transmitting a guarding telegram again.
The node life time is calculated from the guard time (object 0x100C) and life time factor (object 0x100D)
parameters:
Life time = guard time x life time factor
If either of these two parameters is "0" (the default setting), the master will not be monitored (no life
guarding).
Heartbeat: Node Monitoring without Remote Frame
In the heart beat procedure, each node transmits its status message cyclically on its own initiative. There is
therefore no need to use remote frames, and the bus is less heavily loaded than under the guarding
procedure.
The master also regularly transmits its heartbeat telegram, so that the slaves are also able to detect failure of
the master.
Heartbeat procedure
Fig. 113: Schematic diagram: "Heartbeat procedure"
Protocol
The toggle bit is not used in the heart beat procedure. The nodes send their status cyclically (s). See
Guarding [} 96].
EL6751 Version: 3.5 97