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

33-6 Industrial Communication Systems
Periodic communication is used for the exchange of cyclic process data that support the services
of the peripheral message specification (PMS) through a process data channel. PMS is a subset of the
manufacturing message specification (MMS). This enables direct access to the cyclically transmitted
process data and is able to transmit process-relevant data quickly and efficiently. From the application
point of view, it acts as a memory interface.
Instead, asynchronous communication is used to transfer configuration files provided by the network
management channel (peripheral network management, PNM) through a parameter channel. The data
transmitted in the parameter channel have low dynamics and occur relatively infrequently (e.g., updating
text in a display) and are used for manufacturer-independent configuration, maintenance, and startup of
the INTERBUS system.
Network management is used, for example, to start/stop INTERBUS cycles, to execute a system
reset, and fault management. Furthermore, logical connections between devices can be established and
aborted via the parameter channel.
INTERBUS must then transmit both parameter data and time-critical process data simultaneously,
avoiding any interference between the two data flows. To this aim, the data format of the summation
frame has been extended by including a specific time slot inside the time assigned to each node. Whereas
process data are fully exchanged at every cycle, parameter data telegrams, which are up to 246 bytes long
and are not featured by real-time constraints, will be exchanged in several consecutive bus cycles by
inserting a different part of the data in the time slot provided for the addressed devices. The peripherals
communication protocol (PCP) performs this task: It inserts a part of the telegram in each summation
frame and recombines it at its destination (see Figure 33.6). The parameter channels are activated, if
necessary, and do not affect the transfer of I/O data. The longer transmission time for parameter data
that is segmented into several bus cycles is sufficient for the low time requirements that are placed on the
transmission of parameter information.
INTERBUS uses a master/slave procedure for data transmission. The parameter channel follows the
client/server paradigm. It is possible to transmit parameter data between two slaves (peer-to-peer communication).
This means that both slaves can adopt both the client and server function. With this function,
layer 2 data is not exchanged directly between the two slaves but is implemented by the physical
master/slave structure, that is, the data are first transmitted from the client to the master and then
forwarded to the server from the master. The server response data are also transmitted via the master.
However, this diversion is invisible for slave applications.
The task of a server is described using the model of a virtual field device (VFD).
The VFD model unambiguously represents that part of a real application process, which is visible
and accessible through the communication. A real device contains process objects. Process objects
n – 5
n – 4
n – 3
n – 2
......
Data
Data
Service
Cycle n – 1
Header
Loopback
word
Node 1
Node 2
….
Node k
FCS
Summation frame (bus cycle)
FIGURE 33.6
Transmission of acyclic parameter data with a segmentation and recombination mechanism.
© 2011 by Taylor and Francis Group, LLC