Industrial Data Communications, Fifth Edition

Lawrence Thompson, Timothy Shaw

• Éditeur: International Society of Automation
• Année: 2015
• ISBN:
  9781941546093
• Sujet: Technik & Maschinenbau
• Sous-sujet: Maschinenbau

This edition will help you gain a solid understanding of legacy and advanced communications systems in automation and control environments as well as the advances in communication technology. You will learn about cybersecurity methods and standards—including ANSI/ISA/IEC 62443—and how to implement communications systems in a safe and secure manner. Read in-depth descriptions of critical subjects including: •Standards including EIA/TIA-232/485, IEEE 802.3, IEEE 802.11, and IEEE 802.15•Protocols such as Modbus, Data Highway Plus, Ethernet, and TCP/IP•SCADA, DCS, and fieldbus systems•Ethernet and router technologies •Wireless communicationAs automation becomes more thoroughly networked with advances in speed, connectivity, and security; this fifth edition of an ISA best seller is still designed to give technical professionals with little or no background in data communications the knowledge they need to succeed. Additionally, even those with nominal knowledge will find information to enhance troubleshooting and to understand both legacy systems and the more advanced systems now being installed throughout automated facilities. As before, the text emphasizes the practical aspects of commonly used systems rather than design criteria. It contains a complete description of the relevant terminology, standards, and protocols including EIA/TIA-232/485, IEEE 802.3, IEEE 802.11, and IEEE 802.15. New material in this edition includes information on updated Ethernet and router technologies; a more detailed description of VPNs; and expanded information on cybersecurity (including ANSI/ISA/IEC 62443). A complete glossary and index allows the book to be used as a handy reference. SCADA, DCS, and fieldbus systems are all explained, as well as operating system considerations from a communications perspective. This is a book for newcomers to automation data communications, as well as a reference for those who are currently working in the field.

1

Communication Concepts

This first chapter deals with the fundamentals of data communications. We primarily probe and discuss the factors that affect all communications from a “big picture” perspective, so that when the details are presented in later chapters, you will know which niche is being filled. A portion of this chapter—the discussion of data organization sometimes called a coding (ASCII is an example)—is almost a technical history in itself (more on this subject is provided in appendix B).

Elements

Communication has particular elements. In any communication there must be a source (in data communications this is often called the transmitter) and one or more destinations (typically called the receivers). The purpose of communication is to transmit data from the source to the destination(s). The data is transmitted through a medium of one kind or another, varying according to the technology used. When we speak, we use audio waves to project sound data through the medium of air. In data communications, the media we use are: electrical conductors (usually referred to as copper connections), light pipes (often referred to as fiber-optic cabling), and electromagnetic transmission (usually referred to as wireless or radio).

This book is all about how we move and organize data. (Data itself is useless unless organized, at which time it becomes information.) Data communications topologies are organized as one-to-one (point-to-point), one-to-many (multi-drop), or many-to-many (networked). Figure 1-1 illustrates these three forms of data communications organization.

Note that while data communications terms can be couched in technical symbology, the concepts behind them are relatively simple. Members of a network may have defined relationships: they may have no ranking (be peers, all have the same communications value) or they may be ranked (such as master/slave). Point-to-point means just that, from one point to another, or directly from source to destination (whether workstation-to-server or peer-to-peer). Multi-drop topology more closely resembles a network than it does point-to-point topology. In general, multi-drop involves a master station of some kind with slave stations, as opposed to peer stations. (According to the following definition of a network, a multi-drop system can also be classified a network.) For now, we will define a network simply as three or more stations (whether peers, master/slave, or some other ranking) connected by a common medium through which they may share data. Later in this book we will tighten our definition, dividing networks into wide area or local area, and so on.

Figure 1-1. Forms of Data Communications Organization

Modes

In data communications, there are three modes of transmission: simplex, half-duplex, and duplex (see figure 1-2). These terms may be used to describe all types of communications circuitry or modes of transmission, whether they are point-to-point, multi-drop, or networked. It is important to understand these three terms because almost all descriptive language pertaining to data communications uses them.

Figure 1-2. Modes of Data Communications

A communications channel may consist of a communications circuit, which can be either a hardware configuration (consisting of hardware components and wiring) or a “virtual” circuit (a channel that consists of software programming for communication channels that are not physically connected). “Virtual circuit” refers more to the process of communication than to the hardware configuration.

NOTES

1. The differences between a “mode” and a “circuit” are rather arbitrary. However, the reader needs to be aware that not all channels are hardware, and even though a channel is physically capable of operating in a certain mode, it does not mean that the channel is being utilized in that mode. As an example, a duplex channel could be operated in half-duplex mode.
   
   
2. In many cases, the literature still uses the term “full-duplex” when referring to duplex mode.

You should be aware that constraints on the mode the communication channel is capable of using may be due to hardware or software. For example, if the hardware is duplex, the software may constrain the hardware to half-duplex. However, if the hardware cannot support a mode, no amount of software will cause it to support that mode, although it may appear to do so to human observation. An example is a half-duplex system that appears (due to the speed and message attributes) to be duplex to the human user.

The three data communications modes are as follows:

• Simplex or Unidirectional Mode. In this mode, communication occurs only in one direction—never in the opposite direction; in figure 1-2 it is from Station A to Station B. The circuit that provided this mode of operation was originally called simplex (in the 1960’s telephone industry), but this led to confusion with more current telephony terminology. “Unidirectional” is a more appropriate name for this mode of transmission and using the name for this circuit would be much more descriptive; however, old habits (and names) are hard to change, therefore we will use the term simplex in this text (even though we would prefer to use unidirectional) so the reader will not be confused when referencing technical data.
  
  
• Half-Duplex Mode. In this mode, communication may travel in either direction, from A to B or from B to A, but not at the same time. Half-duplex communication functions much like meaningful human conversation does, that is, one speaker at a time and one (or more) listener(s).
  
  
• Duplex Mode. In duplex mode, communication can travel in both directions simultaneously: from A to B and from B to A at the same time.

Serial and Parallel Transmission

Figure 1-3. Serial and Parallel Transmission Concepts

Serial transmission (see figure 1-3) uses one channel (one medium of transmission) and every bit (binary digit, defined below) follows one after the other, much like a group of people marching in single file. Because there is only one channel, the user has to send bits one after the other at a much higher speed in order to achieve the same throughput as parallel transmission.

NOTE A nibble is 4 bits (a small byte, a byte is 8 bits).

In parallel transmission (regardless of the media employed), the signal must traverse more than one transmission channel, much like a group of people marching in four or more columns abreast. For a given message, four parallel channels can transmit four times as much data as a serial channel running at the same data rate (bits per second). However, parallel transmission running any appreciable distance (based on data rate—the faster the data rate, the more the effects for a given distance) encounters two serious problems: First, the logistics of having parallel media is sure to increase equipment costs. Second, ensuring the data’s simultaneous reception over some distance (based on data rate—the higher the data rate, the shorter the distance) is technically quite difficult, along with ensuring that cross-talk (a signal from one transmission line being coupled—electrostatically or electromagnetically—onto another) is kept low. Cross-talk increases with signaling rate, so attempting to obtain a faster data rate by using additional parallel conductors becomes increasingly difficult.

Figure 1-4 illustrates what the signals would look like in serial and parallel transmission....