Subdivisión de redes de telecomunicaciones

Chapter 1 Introduction to Fieldbus Systems
In this chapter we will present an introduction to the Distributed Computer Controlled Systems (DCCS) communications that are used to connect various industrial systems, or what are known as FieldBuses. First we will present the definition of the term Fieldbus. Second we will give brief notes on the history of communication in industrial system and itsdevelopment. Then we will follow up to the evaluation stage of the fieldbus systems. After that we will demonstrate the relationship between the fieldbus architecture and the reference OSI network model. In addition we will numerate many types of Fieldbuses that exist in the market for the time being. Doing this, we will describe each type and its standardization information, and the CERNrecommendation on them. Before we finalize, we will make a comparison table to compare between the most famous five Fieldbuses types existing on the market. At the end we will talk about the future trend of the fieldbus technology.

1.1. The Definition of a Fieldbus.
First we will define the fieldbus term and then we will go through its history in brief in the next sections. The augmented term Fieldbusis consisting of two terms, Field and Bus [Fieldbus Introduction]. To start, the meaning of Field, as defined in industrial world, is a geographical or contextual limited area. From the industry point of view the Field is an abstraction of the plant levels. As for the term Bus is a well-known word in computer science as a set of common line that electrically (or even optically) connects variousunits (circuits) in order to transfer the data among them.

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The origin of the fieldbus was to replace any point-to-point links between the field devices (Field Devices are simply the Sensors and Actuators of the plant) and their controllers (like PLC's, CNC's …etc.) by a digital single link on which all the information is transmitted serially and multiplexed in time. We will see that thefieldbus transfers, in most cases, this information in small-sized packets in serial manner. Choosing the serial transmission has many merits in comparison with other kinds of transmission like parallel transmission. For instance, the sequential or serial transmission reduces the total required number of the connecting lines over greater distances than that of the point-to-point or even paralleltransmissions. A set of rules must be defined in order to accomplish data transfer between the units along the bus. This set of rules is called Communication Protocol or just the Protocol. This is unlike the case of the ordinary point-to-point transmission where any two connected entities send and receive data from each other whenever the data is available. The protocol is responsible for twoimportant rules on the bus, the mechanism that any unit can acquire or seize the bus (from the network terminology this means the way of Medium Access), and the synchronization between those multi-units on the bus. The medium access protocol choosing is a vital (crucial) step in designing the DCCS. This is because of the odd nature of the bursty traffic of such control systems. So the existed LANprotocols such as the token ring or the CSMA1 are not appropriate for the control applications. For the token ring case, the more nodes you add, the longer the time each node will wait till it can transmit its data. Needless to say that the CSMA protocols, which are contention based protocols, will add randomness to the overall Response Time2 of the node data (we will explain in chapter 4 the responsetime in more detail). This came from the way that the CSMA allow only one node to transmit data if and only if no other node is seizing the medium. The randomness occurs when a

Carrier Sense Multiple Access. Response time is the elapsed time interval between sending the data from producer node, and the receiving of that data at the consumer node.
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collision3 happens as the nodes...