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RJ45 connector. Other networking components such as hubs, switches or repeaters can also be used. However, to establish the greatest amount of “determinism” a switch is recommended.

The use of ETHERNET as a fieldbus allows continuous data transmission between the plant floor and the office. Connection of the ETHERNET TCP/IP fieldbus controller to the Internet even enables industrial processing data for all types of applications to be called up world-wide. This makes site independent monitoring, visualization, remote maintenance and control of processes possible.

WAGO Kontakttechnik GmbH is a member of IAONA Europe, an organization with the purpose of establishing ETHERNET in automation technology.

5.2 Network Architecture – Principles and Regulations

A simple ETHERNET network is designed on the basis of one PC with a network interface card (NI), one crossover connection cable, one ETHERNET fieldbus node and one 24 V DC power supply for the coupler voltage source.

Each fieldbus node consists of an ETHERNET TCP/IP fieldbus coupler/controller, I/O modules, and an end module. The individual bus modules are connected to the ETHERNET TCP/IP fieldbus coupler/controller via an internal bus when they are snapped onto the carrier rail.

It is possible to connect 64 I/O modules to an ETHERNET TCP/IP fieldbus coupler/controller.

Note

Use of the WAGO 750-628 Bus Extension Coupler Module and 750-627 Extension End Module enables support of up to 255 I/O modules on the 750-841 controller.

Sensors and actuators are connected to the digital or analog I/O modules on the field side. These are used for process signal acquisition or signal output to the process, respectively.

The fieldbus coupler/controller automatically detects all connected I/O modules, creating a local process image of them. This can consist of a mixed arrangement of analog and digital modules.

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DC 24V connection

sensor and actuator

connection

fieldbus connection

Fieldbus communication between master application and fieldbus controller takes place using either the MODBUS TCP protocol or Ethernet/IP.

5.2.1 Transmission Media

General ETHERNET transmission standards

For transmitting data the ETHERNET standard supports numerous technologies with various parameters (e.g., transmission speed, medium, segment length and type of transmission).

Tab. 5-1: ETHERNET Transmission Standards

Beyond that there are still further transmission standards, for example: 100Base-T4 (Fast ETHERNET over twisted conductors), 100Base-FX (Fast ETHERNET over fiber-optic cables) or P802.11 (Wireless LAN) for a wireless transmission.

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The media types are shown with their IEEE shorthand identifiers. The IEEE identifiers include three pieces of information.

The first item, for example, “10”, stands for the media.

The third part of the identifier provides a rough indication of segment type or length. For thick coaxial cable, the “5” indicates a 500 meter maximum length allowed for individual thick coaxial segments. For thin coaxial cable, the “2” is rounded up from the 185 meter maximum length for individual thin coaxial segments. The “T” and “F” stand for ‘twisted pair’ and ‘fiber optic’, and simply indicate the cable type.

10Base-T, 100BaseTX

Either the 10BaseT standard or 100BaseTX can be used for the WAGO ETHERNET fieldbus node.

The network architecture is very easy and inexpensive to assemble with S- UTP cable as transmission medium or with cables of STP type.

Both types of cable can be obtained from any computer dealer.

S-UTP cable (screened unshielded twisted pair) is single-shielded cable of Category 5 with overall shield surrounding all twisted unshielded conductor pairs and an impedance of 100 ohm.

STP cable (shielded twisted pair) is cable of Category 5 with stranded and individually shielded conductor pairs; no overall shield is provided.

Wiring of the fieldbus nodes

A crossover cable is required for direct connection of a fieldbus node to the network card of the PC.

If several fieldbus nodes are to be connected to a network card, the fieldbus nodes can be connected via an ETHERNET switch or hub with straight through/parallel cables.

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An ETHERNET switch is a device that allows all connected devices to transmit and receive data with each other. The switch can also be viewed as a “data traffic cop” where the hub “polices” the data coming in and going out of the individual ports, so the data will only be transmitted to the required node. WAGO recommends using a switch rather then a hub, this will allow for a more deterministic architecture.

Attention

The cable length between the node and the hub cannot be longer than 100 m (328 ft.) without adding signal conditioning systems (i.e., repeaters). Various possibilities are described in the ETHERNET standard for networks covering larger distances.

5.2.2 Network Topologies

In the case of 10Base-T, or 100BaseTX several stations (nodes) are connected using a star topology according to the 10Base-T ETHERNET Standard.

Therefore, this manual only deals with the star topology, and the tree topology for larger networks in more detail.

Star Topology

A star topology consists of a network in which all nodes are connected to a central point via individual cables.

A star topology offers the advantage of allowing the extension of an existing network. Stations can be added or removed without network interruption. Moreover, in the event of a defective cable, only the network segment and the node connected to this segment is impaired. This considerably increases the fail-safe of the entire network.

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Tree Topology

The tree topology combines characteristics of linear bus and star topologies. It consists of groups of star-configured workstations connected to a linear bus backbone cable. Tree topologies allow for the expansion of an existing network, and enables schools, etc. to configure a network to meet their needs.

5-4-3 Rule

A consideration in setting up a tree topology using ETHERNET protocol is the 5-4-3 rule. One aspect of the ETHERNET protocol requires that a signal sent out on the network cable must reach every part of the network within a specified length of time. Each concentrator or repeater that a signal goes through adds a small amount of time. This leads to the rule that between any two nodes on the network there can only be a maximum of 5 segments connected through 4 repeators/concentrators. In addition, only 3 of the segments may be populated (trunk) segments if they are made of coaxial cable. A populated segment is one that has one or more nodes attached to it. In Figure 5-5, the 5-4-3 rule is adhered to. The furthest two nodes on the network have 4 segments and 3 repeators/concentrators between them.

This rule does not apply to other network protocols or ETHERNET networks where all fiber optic cabling or a combination of a backbone with UTP cabling is used. If there is a combination of fiber optic backbone and UTP cabling, the rule is simply translated to 7-6-5 rule.

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