wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

Profibus 32-9
32.5 Cyclic Data Exchange: MS0—Relation
The central control (DPM1) performs the following functions:
• Reads input information from the slaves cyclically
• Writes output information to the slaves cyclically
The bus cycle time should be shorter than the program cycle time of the central automation system,
which is approximately 10.ms for many applications.
32.5.1 Device Model
Every field-device is composed of modules, which are located in slots. Every module can have one or
several channels. These modules may be real physical modules or only virtual organization units. The
configuration and number and version of modules may be fixed or variable. The possible modules, order,
and parameters are described in the Generic Station Description (GSD) file, which is provided by the
manufacturer of the device and used by the configuration tool of the controller to support the engineer
in the planning of the Profibus system.
32.5.2 Initialization and Supervision of the Relation
Data communication between the DPM1 and its assigned slaves is automatically handled by the DPM1
in a defined, recurring sequence. The user defines the assignment of the slave(s) to the DPM1 when configuring
the bus system. The user also defines which slaves are to be included or excluded in the cyclic
user data communication.
Data communication between the DPM1 and the slaves is divided into three phases: parameterization,
configuration, and data transfer. Before the master includes a DP slave in the data transfer phase,
a check is run during the parameterization and configuration phase to ensure that the configured setpoint
configuration matches the actual device configuration. During this check, the device type, format,
and length information and the number of inputs and outputs must also correspond. This provides the
user with reliable protection against parameterization errors. In addition to user data transfer, which is
automatically executed by the DPM1, the user can also request that new parameterization data are sent
to the slaves.
For safety reasons, it is necessary to ensure that DP has effective protective functions against incorrect
parameterization or failure of transmission equipment. For this purpose, the DP master and the
slaves are fitted with monitoring mechanisms in the form of time monitors. The monitoring interval is
defined during configuration.
The DPM1 uses a Data_Control_Timer to monitor the data communication of the slaves. A separate
timer is used for each slave. The time monitor is tripped if no correct user data transfer is executed
within the monitoring interval. In this case, the user is notified. If the automatic error handling
(Auto_Clear = True) is enabled, the DPM1 exits the operate state, switches the outputs of the assigned
slaves to the fail-safe state, and shifts to the clear mode.
The slave uses the watchdog control to detect errors of the master or of the transmission. If no data
communication with the master occurs within the watchdog control interval, the slave automatically
switches its outputs to the fail-safe state.
In addition, access protection is required for the inputs and outputs of the slaves operating in multimaster
systems. This ensures that only the authorized master has direct access. For all other masters, the
slaves provide an image of their inputs and that can be read without access rights.
© 2011 by Taylor and Francis Group, LLC