Fieldbus

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... 

The component controllers used in the controller subsystem portion of the DCS can be of various types and include multiloop controllers, programmable logic controllers, personal computer controllers, singleloop controllers, and fieldbus controllers. The type of elec tronic con-troUer utihzed depends on the size and func tional characteristic of the process apphcation being controlled. See the earlier section on distributed control systems. 

Presently, fieldbus controllers are single-loop controllers with 8- and 16-bit microprocessors and are options to digital field-control devices. These controllers support the basic PID control algorithm... 

Currently, the trend in process control is away from centrahzed process control and toward an increased number of small distributed-control or PLC systems. This trend will put emphasis on the evolution of the fieldbus controller and continued growth of the PC-based controller. Also, as hardware and software improves, the functionality of the controller will increase, and the supporting hardware will be physically smaller. Hence, the traditional lines between the DCS and the PLC will become less distinct as systems will be capable of supporting either function set. 

FIDAP software package, 7 25 Fidelity, of transmission signals, 11 131 Field assays, enzyme immunoassay kits for, 14 144 Fieldbus, 20 672 impact of, 20 673... 

System installation in a permanent location may require a sample conditioning system featuring some degree of automation, such as automatic cleaning (the system illustrated above features such a system) and outlier sample collection and the need to interface to an existing control system process computer. The latter may require that the system operates with a standardized communications protocol, such as Modbus, for the chemical industry. Certain specialized industries use different protocols, such as the semiconductor industry, which uses SECS and SEC-11 protocols. A standardized approach designated the Universal Fieldbus is another method/protocol for process analyzers which is being supported by certain hardware manufacturers. 

The mass flow rate is calculated directly by multiplying the time difference or the phase shift with the calibration constant of the flowmeter thermal effects on the mass flow and density reading have to be included as well. This is commonly done with a microprocessor. The primary output from a CMF is mass flow. However, most electronic designs are also capable of providing temperature, density, and volumetric flow data. Further, totalizers provide mass or volume totals. Analog (4 to 20 mA) and digital output protocols are supported (e.g., PROFIBUS, FOUNDATION Fieldbus, HART, Modbus, scaled pulse, and others). 

Currently, remote I/O systems of several manufacturers are available for operation in zone 1 or Division 1. To a large extent, these systems are based upon the technology of intrinsic safety. The power supply only is designed according to another type of protection, e.g. flameproof enclosure - d A remote I/O suitable for hazardous areas may be an integration of input (I) and output (O) assembly units of automation systems and of Ex i-isolators of classic design. The installation may be made in a hazardous area and supersede the classic field distribution box (terminal box). Standardized fieldbus systems may be used for data transmission from/to the automation system. These remote I/O systems for hazardous areas are characterized as follows ... 

Typically, a remote I/O system is a modular assembly (see Fig. 6.210). In this figure, the power supply, the connection to the fieldbus, the internal bus (power and data line) are shown as well as input and output modules. Input and... 

A typical installation on location of a remote 1/O system is shown in Fig. 6.211. At the right side/top in the housing, the power supply with an integrated fieldbus interface can be seen. The module complies with flameproof enclosure - d and is designed in such a way that it can be replaced under live voltage and explosion hazard ( hot swap ). The auxiliary power lines and terminals comply with increased safety - e All I/O modules comply... 

One single fieldbus instead of numerous individual cables... 

A new technology (fieldbus) asks for instruction and experience in practice... 

During international fieldbus standardization at the start of the 1990s a physical layer had been specified as IEC 61158-2 with an option for intrinsic safety. In the meantime, two standardized fieldbus systems comply with said standard Profibus PA and Foundation Fieldbus HI. These are the most important characteristics ... 

On the basis of these technical boundary conditions, such a fieldbus is suited (independently of the bus protocol applied here) for fully digitized sensors and actuators. 

Meanwhile, an increasing number of transmitters and positioners etc. is available either with a Profibus PA or a Foundation Fieldbus HI interface (or with both of them). 

The schematic circuit diagram of an intrinsically safe fieldbus according to IEC 61158-2 (physical layer) is given in Fig. 6.212. The field devices are certified as intrinsically safe apparatus. Four-wire transmitters with an external power supply are explosion protected by an additional type of protection,... 

Figure 6.212 Schematic circuit diagram of an intrinsically safe fieldbus according to I EC 61158-2 (physical layer).

Deviating from conventional installations (in many cases characterized by several field devices connected to a common terminal box/junction box), the fieldbus cable with its intrinsically safe circuit passes from transmitter to transmitter (Fig. 6.213). Each transmitter is fitted with a branch box (T box) to connect it to the fieldbus (Fig. 6.214). At the physical end of the bus segment, a terminating impedance shall be fitted. 

Advantages and disadvantages of fieldbus systems are given as follows ... 

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