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The communication system is the only mean of the system platform for
the exchange of data between application modules, so-called architecture objects (AO). It
must support both, the exchange of information between tightly-coupled AO's, e.g. located
on the same processor-board as well as between AO's distributed over different hardware
units connected through a bus-system. A standardised protocol is defined to ensure uniform
data formats and a fixed set of messages. The protocol architecture is derived from the
OSI base reference model. It comprises however only two layers: a Message Transport System
(MTS) equivalent to the OSI layers 1 to 4 and an Application Services System (ASS)
equivalent to the layers 5 to 7.
The MTS offers connection-oriented services for a transparent
transport of arbitrary messages between AO's. It can be adapted to use any kind of
existing mechanisms for information exchange, e.g. operating system services like message
queues and shared memory, as well as LAN-protocols like TCP/IP.
The ASS has the task to handle the application protocol. This
includes connection management, the assembly/disassembly of messages and data conversion
between different data representations like Little and Big Endian. Additional
functionality covers synchronous and asynchronous services as well as the segmentation of
large data into smaller message units.
The OSACA application protocol is realised on a client/server-basis
using object-oriented principles. In a server AO every functionality that shall be
accessible from external is mapped to a communication object (CO). From a client's point
of view a server is a set of communication objects which can be accessed by sending and
receiving messages using the communication system.
There is a fixed set of classes for communication objects, which are
managed by the so-called Communication Object Manager (COM). Through the separation of
application and communication services the application programmer can fully focus on
solving his control specific problem.
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