Image Processing Reference
In-Depth Information
Star
Ring
Line, bus
Tree
Meshed network
Daisy chain
FIGURE .
Topological network structures typically used in fieldbus systems.
the fibers (as in byteflight). Unless the coupler is a fully fledged node (i.e., addressable by the fieldbus
protocol), this star topology is logically equivalent to a line structure.
Another simple topological form is the “ring.” Here, each node has two network interfaces (an
input and a separate, independent output), and the nodes are arranged one after another in the form
of a chain. In its entirety the ring can be viewed as one large shift register, and it is usually also
operated in this way (the most prominent example is INTERBUS). As there is no need to address the
nodes explicitly, it is a very fast method to exchange data, and also a very deterministic one with low
jitter(thisiswhySERCOSusesthistopologyfortheinterconnectionofdrives).Avariantofthering
topology, actually a daisy-chain structure, was introduced with Industrial Ethernet. Here, each node
contains a small switch, and the nodes are not connected in a star topology, but cascaded like a string
of pearls. his layout is used, e.g., in PROFINET.
The “line,” often referred to simply as “bus” (although this term is not unique), is the most suc-
cessful and most commonly used network topology in the fieldbus world. It was the logical and most
efficient replacement for the former starlike point-to-point cabling in that one single line should con-
nect all network nodes (just as in the original concepts of Ethernet with the famous yellow cable and
other types of coax cables). In many cases, the line topology is based on the RS interface. his is
an inexpensive, fully differential, multi-point interface standard using a shielded twisted pair cable
with Ω characteristic impedance. Maximum cable length is m, the maximum achievable data
rate is Mbit/s. he maximum number of nodes per segment depends on the electrical characteris-
tics of the driver circuits and was originally limited to (termed unit loads). Enhanced transceivers
with higher input impedance meanwhile allow up to nodes per segment. Beyond this, repeaters
arenecessarytoregeneratethedatasignals.Someieldbussystems(likeCANandallCAN-based
systems) use RS transceivers in a modified way (i.e., by essentially applying the data signal not
to the actual data input but the output enable used to switch the output to a high-impedance state)
to generate asymmetric bit patterns supporting a special form of medium access. A crucial aspect in
practice is the proper electrical termination of the bus line to avoid signal reflections disturbing the
data transfer. Wrong or missing termination is the most frequent problem leading to communication
failures. A variant of the line topology used for P-NET therefore requires that the ends of the cable
be connected, to form a closed loop. Nevertheless, electrically this structure is still a bus.
The “tree structure” is a composite network structure and characterized by one or more substations
being dependent on a root node. Each substation can in turn be a root node for a lower-level segment.
In many cases, the actual connections between the stations are regular point-to-point connections
or lines. In automation technology, tree structures are the usual way to build hierarchical, relatively
complex networks. he root nodes usually have routing capabilities, so that the data traffic can at least
