Digital communication

Radio waves have the lowest energy (lowest frequency). We use them for radio and television communication, digital imaging, remote controls, and wireless linkages for laptop computers. Radio waves are also used in NMR spectroscopy and in magnetic resonance imaging (MRI) (Chapter 14). 

Because of its severe approximations, in using the Huckel method (1932) one ignores most of the real problems of molecular orbital theory. This is not because Huckel, a first-rate mathematician, did not see them clearly they were simply beyond the power of primitive mechanical calculators of his day. Huckel theory provided the foundation and stimulus for a generation s research, most notably in organic chemistry. Then, about 1960, digital computers became widely available to the scientific community. 

During the early 1970s, the necessary telecommunications technology became available with packet switching. ARPANet, the first operational packet-switched digital communications network, was implemented by the U.S. Department of Defense. Commercial systems (eg. Telenet, TYMNET, and GENet) became available shortiy thereafter. 

In order to meet their initial requirements, several manufacturers have developed digital communications capabilities for communicating with smart transmitters. These can be used either in addition to or in lieu of the 4-20 milliamp signal. Although most manufacturers release enough information on their communications features to permit another manufacturer to provide compatible instruments (and in some cases provide an open communication standard), the communications cap ihty provided by a manufacturer may be proprietaiy. 

Microprocessor technology permitted these technical issues to be addressed in a cost-effec tive manner. In the mid-1970s, a process control architecture referred to as a distributed control system (DCS) was introduced and almost instantly became a commercial success. A DCS consists of some number of microprocessor-based nodes that are interconnec ted by a digital communications network, often called a data highway. The key features of this architecture are as follows ... 

As the data within the DCS are digital in nature, interfaces to upper level computers are technically easier to implement. Unfortunately, the proprietary nature of the communications networks within commercial DCS products complicate the implementation of such interfaces. Truly open DCS architectures, at least as the term open is used in the mainstream of computing, are not yet available. 

Remote control units. These units are used to control unit processes. Basic control functions such as the PID algorithm are implemented here. Depending on other hardware components used, data acquisition capability may be required to perform digital control. They may be configured to supply process set points to single-loop controllers. Radio telemetiy may be installed to communicate with MUX units located at great distances. 

More microprocessor-based process equipment, such as smart instruments and single-loop controllers, with digital communications capability are now becoming available and are used extensively in process plants. A fieldbus, which is a low-cost protocol, is necessary to perform efficient communication between the DCS and these devices. So-called mini-MAP architec ture was developed to satisfy process control and instrumentation requirements while incorporating existing ISA standards. It is intended to improve access time while... 

Digital Field Communications An increasing number of valve-mounted devices are available that support digital communications in addition to, or in place of, the traditional 4—20 mA current signal. These control-valve devices have increased functionality, resulting in reduced setup time, improved control, combined functionality of traditionally separate devices, and control-valve diagnostic capabihty. Digital communications also allow the control system to become completely distributed where, for example, the process PID controller could reside in the valve positioner or in the process transmitter. 

Distributed Control System (DCS) A system that divides process control functions into specific areas interconnected by communications (normally data highways) to form a single entity. It is characterized by digital controllers, typically administered by central operation interfaces and intermittent scanning of the data highway. 

A typical relay is shown in Figure 12.29(b). These relays can also be made digital to be connected to a central control system for close monitoring and control of a process. Now they can have much wider application, such as better communication and information feedback facilities, to optimize a process and maximize productivity. For more details refer to Section 13.2.. ) or contact the manufacturers. 

The foregoing demonstrates that modern digital devices offer highly reliable, state-of-die-art microprocessor technology that controls turbomachinery strings, monitors operating data, stores data for subsequent evaluation, communicates to odier systems, and can expand with future technologies. 

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