Topological Network Structures used in Fieldbus Systems

Topology is an important property of fieldbus. It is employed in the selection and definition of the best and suitable physical layout of the fieldbus network. The physical layer of the fieldbus has to meet somewhat demanding requirements like immunity to electromagnetic disturbances, robustness, intrinsic safety for hazardous areas, etc.

The most important topologies for wired automation networks include:

• Star
• Ring
• Line (Bus)
• Tree
• Meshed network
• Daisy chain

These topological network structures are illustrated below:

Star Topology

The star topology was the standard wiring in automation before the introduction of fieldbus. The PLC was the center, attached to the distributed I/O elements with dedicated lines. The clear cabling overhead was one of the main reasons to develop serial bus systems. When Ethernet was adopted for automation purposes, the star topology returned. Presently, the central element is the Ethernet switch, and all Ethernet nodes are connected by means of a structured cabling i.e. a dedicated link to each network node. Another application of the star topology is for fieldbus systems using optical fibers as transmission medium. In this application, the center is an active star coupler linking the fibers. 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.

Ring Topology

In this topology, 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. As a whole, the ring can be viewed as one large shift register, and it is normally operated this way (a good application example is the INTERBUS). Since there is no need to address the nodes explicitly, it is a very fast method for exchanging data and is also deterministic with few variations ( hence the reason why SERCOS utilizes this topology for the interconnection of drives).

Daisy-chain Topology

This is a variant of ring topology, which was introduced with industrial Ethernet. Here, each node contains a small switch, and the nodes are not connected in a star topology, but in a cascaded form. This layout is employed in PROFINET.

Related: What is Fieldbus in Industrial Communication Networks?

Line Topology

The line also called the bus is the most successful and most commonly used network in fieldbus world. It was the logical and most efficient replacement for the former star-like point-to-point cabling in that one single line should connect all the network nodes. In various cases, the line topology is based on the RS 485 interface. This is an inexpensive, fully differential, multipoint interface standard using a shielded twisted pair cable with 120 ohm characteristic impedance. The maximum cable length is 1200 m, and the maximum achievable data rate is 10 mbps. The maximum number of nodes per segment depends on the electrical characteristic of the driver circuits and was originally limited to 32 nodes (termed to as unit loads). The enhanced transceivers with higher input impedance allow up to 256 nodes per segment. Beyond this repeaters are required to regenerate the data signals.

A number of fieldbus systems like CAN and all CAN-based systems use RS 485 transceivers in a modified way, (that is, 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.

An important aspect in practice is the proper electrical termination of the bus line to avoid signal reflections disturbing the data transfer. Incorrect or missing termination is the often the common problem leading to the communication failures. A variant of the line topology used for the P-NET therefore requires the ends of the cable to be connected so as to form a closed loop, although electrically, this structure is still a bus.

Related: How RS-232, RS-422 and RS-485 Networks are applied in Instrumentation

Tree Structure

This is a composite network structure which is 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 instances, the actual connections between the stations are regular point-to-point connections or lines. In automation topology, the tree structures are the usual way to build hierarchical, relatively sophisticated networks. The root nodes have routing capabilities so that the data traffic can at least partly be confined to individual areas of the network. Nevertheless, this requires caution in planning phase of the network when defining the communication relations between end nodes. Nodes exchanging lots of information should be kept in common subnets to avoid congestion on the backbone links. Tree structures are typically used in building automation networks like LonWorks, BACnet or European installation Bus (EIB).

Meshed Networks

Mesh networks where multiple paths through the network exists play only minor role in fieldbus systems because they require appropriate routing strategies to keep messages from circling in the network hence causing congestion. LonWorks and P-NET offer the possibility of building meshed networks.

Related: Basic Features of Foundation Fieldbus (FF)

Free Topologies

A number of fieldbus systems allow free topologies where the nodes can be connected in a multipoint manner without restrictions. Basically, this is a variant of the line topology because the nodes are electrically interconnected. However, contrary, to the standard line topology, there are no stringent limitations concerning the line length, the length of stubs, or branches that would cause severe signal degradation. This makes cabling very convenient for the person doing the installation nevertheless it poses a substantial challenge for the signal processing in the nodes and their transceivers.

Examples of fieldbus systems employing free topologies are LonWorks (where the free topology transceiver is widely used because of its robustness even in cases where conventional RS 485 transceivers would be sufficient) or ASi which was designed for utmost simplicity and ease of handling in harsh industrial environments.

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