Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

352 Practical Modern SCADA Protocols: DNP3, 60870.5 and Related Systems
13.2.4 The application layer
This consists of two sections, namely the fieldbus message specification (FMS) and the
lower layer interface (LLI). The application layer is defined in DIN 19245 Part 2.
13.2.5 The Profibus communication model
The part of the application process in a field device that is readable for communication
purposes is called the virtual field device (VFD). The VFD contains the communication
objects that may be manipulated by the services of the application layers. The objects of
a real device that are readable for the communication (variables, programs, data domains)
are called communication objects.
All communication objects of a Profibus station are entered into its local object
dictionary (source OD) and can be classified as either static or dynamic communication
objects.
Static communication objects are defined in the static object dictionary. They may be
predefined by the manufacturer of the device, or defined during the configuration of
the bus system. Static communication objects are used mainly for communication in the
field area.
Profibus recognizes the following static communication objects:
• Simple variable
• Array – sequence of simple variables of the same type
• Record – sequence of simple variables, not necessarily of the same type
• Domain – data range
• Event
Dynamic communication objects are entered into the dynamic part of the OD (list of
variable lists of program invocations). They may be predefined or defined, deleted or
changed by the application services in the operational phase.
Profibus supports the following dynamic communication objects:
• Program invocation
• Variable list (sequence of simple variables, arrays or records)