Page 241 - Instrumentation Reference Book 3E

Fieldbus 225
I User application 1

7. Application

IS0 model

Communication medium
Figure 13.31 lSO/OSl seven-layer model

Manufacturer’s Application & vendor specific
application Not in the standard
Application Provides a standard (primitive)
layer interface to the manufacturer’s application

Communication protocol Link Arranges data transfer between
(e.g., IEC fieldbus) layer adjacent and remote devices
I Physical Converts data to a form suitable

for transmission and receives data
layer
from the medium
I

Communication medium
Figure 13.32 Three-layer implementation of the fieldbus concept.

single stacdard becomes a clear leader it may be Interoperability allows devices from different
necessary for suppliers to provide interfaces into manufacturers to communicate with one another
whatever protocol their customers use. without the need for special drivers.
One of the key issues is that of interchange- Interoperability will allow a manufacturer to
ability and interoperability. Interchangeability compete in the market on the basis of features
allows one manufacturer’s devices to be substituted exclusive to their product and the added value
for those from another manufacturer on the bus content of the device. rather than simpiy the cost
without the need for recoiifiguration of the of the device. However, the users are concerned
system. This has considerable benefits for a user that this will complicate the situation and that if a
wishing to keep the cost of standard components special feature is present, it may be used at some
to a minimum and to avoid having to retrain their stage, possibly as a work round, and leave them
maintenance staff to any significant extent. There subject to a monopoly of supplier. On the other
are two keys to achieving this: the user needs hand. in a networked system there is always a
to select powerful substitute instruments with possibility to perform the missing function in
so-called instantiable function blocks that pro- another device. Moreover, communications is
vide the functionality of the previous device. Sec- only one aspect of interchangeability. If the
ond. the user must from the beginning select a length of the flowtube or temperature range is
system with an open host that only requires the different, one device cannot replace the other
standard device description and capabilities files anyway .
to interact with a device. If the host requires For small manufacturers, membership fees for
proprietary files, the user’s choice will be limited. the various organizations, the cost of development