In industry, the use of computers applied to automatic control evolved from a single computer monitor anaSógicos some drivers interact with complex systems that multiple processors. These processes include single loop PID controllers and multiloop, operator stations, PLCs, smart transmitters, etc.. Integrated in one or more data networks in real time, also called process control networks.

Moreover, industrial plants have in many cases computer systems to meet their administrative and management needs. We call this system administrative network. This raises the idea to integrate not only digital equipment process control each other but also the integration of these with the administrative network. Presented here are the basics of networking, to better understand the problem of digital communications in relation to the two areas mentioned.

INDUSTRIAL NETWORK STRUCTURE

A computer network is formed by the interconnection of two or more devices via a transmission medium and can exchange information with each other. In the case of a fieldbus (fieldbus), ie a network that interconnects field-level implementation, the transmission medium is usually cables, but this is possible with fiber optics and telecommunications. The choice of transmission medium is often dependent on the interface and the required speed.

The network topology describes the way multiple devices in a network are interconnected. There are several topologies that differ according to three criteria: availability, redundancy and expandability. The arrangements are basic topologies of star, ring and bus.

- STAR STRUCTURE

All information is channeled through a central node such as a mainframe. Each device is served by its own connection. The data exchange between central and peripheral nicialmente from the periphery, is always handled via the central node. This topology has the advantage that if one of the lines is subject to interference, only the device connected to it is affected. Additionally, lines can be switched on or off during normal operation.

Figure 1.1 Example of a Star Topology

On the other hand, the device at the central node must act extremely reliable. If you fail or are overloaded due to excessive data transfers transactions, the whole system collapses. In addition, since the cables run to a central point, should have adequate systems link.

- RING STRUCTURE

The information is passed from device to device. There is no central control in the ring, instead of this, each device driver assumes the role of strictly defined intervals. Theoretically there is no limit to the number of allowed devices. The failure of a device is usually sufficient to disrupt the ring and stop all communications. To avoid this, bypass switches are incorporated to automatically switch when a device fails. This also allows devices to be added or removed without interrupting normal operation. A variation of the ring structure is known as token ring.

Figure 1.2 Example of Ring topology

- BUS STRUCTURE

All devices are connected to one line of data called bus, through which is passed on the information. A bus is said to have branches in a tree structure. Information arrives at the receiver without the aid of any other device, in effect, in contrast to a ring structure, the individual stations are passive. If you add a device to the bus, no additional interfaces are required on existing stations. Thus, the problem of a limited number of participants related to the star structure does not appear. The amount of wiring required is small and can add new devices without problem. A bus structure could allow cross-communication between any devices. Since everyone is connected to a common wire, the transmission must be strictly regulated.

Table 1.1 Comparison of network topologies