BECKHOFF BC3100 - Manual (Page 22)

PROFIBUS-DP basics
22 BC3100
data is switched through to the outputs. The user can terminate Sync
operation by issuing an Unsync control command.
Similarly, a Freeze control command sends the addressed DP slaves into
Freeze mode. In this operating mode, the inputs of all the DP slaves are
frozen in their current state. The input data is not updated again until the
DP master sends the next Freeze control command to the relevant
devices. You terminate Freeze operation by issuing an Unfreeze control
command.
System behavior To ensure that the devices are largely exchangeable, the system behavior
for the PROFIBUS DP has also been standardized. It depends principally
on the operating mode of the DPM1, which can be governed either locally
or from the planning device via the bus. The following principal modes are
distinguished:
Modes
Stop No data communication is taking place between the DPM1 and
the DP slaves.
Clear The DPM1 reads the input information of the DP slaves and keeps
the outputs of the DP slaves in a safe state.
Operate The DPM1 is in the data transfer phase. In a cyclic
communication, the inputs of the DP slaves are read and the
output information is transferred to the DP slaves.
The DPM1 uses a multicast command to broadcast its local status
cyclically at regular intervals to all its subordinate DP slaves (the interval
can be configured). The system’s response to an error (such as the failure
of a DP slave) which occurs during the data transfer phase of the DPM1 is
determined by the operating parameter "Auto Clear“. If this parameter has
been set to "True“, then, as soon as any one DP slave ceases to be ready
to transfer user data, the DPM1 will switch the outputs of all its subordinate
DP slaves to a stable state and then enter Clear mode. If the parameter is
set to "False“, then the DPM1 will remain in Operate mode even in an error
situation and the user can govern the system response himself.
Data traffic between DPM1
and the DP slaves The DPM1 automatically deals with data traffic between itself and its
subordinate DP slaves in a fixed and continually repeating order. During
the planning phase of the bus system, the user specifies which DP slaves
belong to which DPM1, which DP slaves should be included in the cyclic
transfer of user data, and which should be excluded from it.
The data traffic between the DPM1 and the DP slaves can be subdivided
into three phases: parametrization, configuration and data transfer. Before
it admits a DP slave to the data transfer phase, the DPM1 checks – in the
phases parametrization and configuration – whether the intended
configuration from the original plan agrees with the actual device
configuration. This check covers the device type, the format and length
information and the number of inputs and outputs, all of which must agree.
This gives the user reliable protection against parameter errors. In addition
to transferring user data, which the DPM1 carries out automatically, it is
also possible, at the user’s request, to transmit new parameters to the DP
slaves.
Protective mechanisms In the field of decentralized peripherals, security considerations make it
imperative that systems should be equipped with highly effective protective
functions to prevent incorrect parametrization or a failure of the
communications equipment. On both the DP master and the DP slaves,
PROFIBUS DP uses monitoring mechanisms which are implemented as