6
Overview of PROFIBUS
Section 1-1
1-1-6 Protection Mechanisms
Monitoring Time PROFIBUS-DP provides effective protection functions against parameteriza-
tion errors or failure of the transmission equipment. Time monitoring is pro-
vided both at the master and the slave devices. The monitoring interval is
specified when the system is configured.
Monitoring at the Master The PROFIBUS-DP Master monitors data transmission of the slaves with the
Data-Control-Timer. A separate control timer is used for each slave. This
timer expires if response data is not correctly transmitted by the slave within
the monitoring interval. The user is informed when this happens. If the auto-
matic error reaction (Auto-CLEAR) has been enabled, the PROFIBUS-DP
master exits its OPERATE state, switches the outputs of all assigned slaves
to the fail-safe status and changes to the CLEAR state.
Monitoring at the Slave Slave devices use a watchdog to detect failures of the master or the bus. If
data communication with the master does not occur within the set watchdog
time interval, a slave automatically switches its outputs to the fail-safe mode.
Also, access protection is provided for the inputs and outputs of the slaves
operating in multi-master systems. Only authorized masters can access their
slaves.
1-1-7 Network States
PROFIBUS-DP distinguishes four different network states:
OFF-LINE • Communication with all PROFIBUS-DP participants (masters and slaves)
is stopped. The Master ceases to access the PROFIBUS network.
STOP • Communication between the master and its slaves is stopped. Only com-
munication between the master and other masters is still possible.
CLEAR • The master tries to set parameters, check the configuration, and perform
data exchange with its associated slaves. Data exchange involves read-
ing the inputs of the PROFIBUS-DP slaves and writing zeros to the out-
puts of the slaves.
OPERATE • The master exchanges data with its assigned slaves, inputs are read and
outputs are written. Also, the master cyclically sends its local status to all
its assigned PROFIBUS-DP slaves (using a broadcast message).
Auto-CLEAR
Fail-safe State
If an error occurs during the data exchange phase of the master, the ‘Auto-
CLEAR’ function determines the subsequent actions. If this function has been
disabled, the master remains in the OPERATE state. If the function has been
enabled, the master automatically changes the network to the CLEAR state,
in which the outputs of the assigned PROFIBUS-DP slaves are switched to
zero, i.e. the ‘fail-safe’ state. The master continues to read the inputs of the
slaves.
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