A look at Fig. 4 reveals that the original hope arising from
the fieldbus standardization disaster, i.e., that Ethernet could be
the basis for a unique industrial communication solution, was
futile. In fact, the situation has not changed too much compared
with the heterogeneity of the traditional fieldbus systems. Interoperability
between different industrial Ethernet solutions is
not possible in a direct way. Nevertheless, as many industrial
Ethernet solutions were developed by the same companies who already have fieldbus systems on the market, care was taken
to allow for backward compatibility and cooperation between
the “old” fieldbuses and “new” Ethernet installations. This
was again done to increase market acceptance and to provide
a migration path for the steady replacement of the fieldbus
systems by industrial Ethernet. A direct interconnection by
means of bridges between different media types is not possible
without losing timing guarantees. Still, higher layer protocols
(particularly application-layer protocols) and at least data objects
are compatible and allow for an interconnection on a high
level. From this compatibility viewpoint, four different classes
of networks exist