Data highway

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 ... 

Data highway. This is the communication link between com-... 

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. 

If feasible, raised panel computer-room type floors should not be used. Where raised floors must be used to accommodate wiring and cables, they should be constructed entirely of noncombustible materials and provided with floor panels easily removable by hand. Power distribution in the subfloor space should be in conduit. Redundant data highway cables should be run as remotely from each other as possible and not mixed with power cables. 

The distributed control system (DCS) hardware areas are often referred to as "process computer rooms." I/O Rooms contain the incoming and outgoing wiring, cables and data highway links, and often small transformers and other related electrical equipment. Often, additional space is needed for a master process engineering computer terminal/work station for process control system changes and for critical safety instrumented systems (SIS) for interlocks and emergency shutdowns. 

A tremendous cost savings is realized by the multiplexing of signals due to the elimination of many multi-conductor cables and thousands of field terminations. Also, because the PCMs are remote from the central control room, they can be installed in the equipment modules. This means that each equipment module can be virtually stand-alone in terms of electrical and control systems, thus allowing almost complete prefabrication in the fabrication yard the only remaining field work is connecting to the data highways. 

An important feature of the distributed control system chosen is that the PCMs can communicate over a microwave link, thus allowing the well pad manifold modules to be included in the GC control system the hardwire data highway is limited to a distance of 5000 ft. (1524m) from the Host. The well chokes and manifold controls will remain on the SCAOA system, but the well pad will require continuous process control of the test separator, water injection flow control, and artificial lift gas flow control. 

Assuming we were to undertake the development of another operation the size of Prudhoe Bay with similar logistical problems, one of the first considerations would be, could not the SCADA and DCS functions be combined into s single system We believe today the answer to this is yes. In 1979/80, when our current systems were being configured, this was not possible. No proven DCS system had been, operated with a microwave link in its data highway. 

Since that time, however, several of the more established control systems vendors have moved into the DCS area and indications are that they are prepared to develop the microwave radio data highway. 

If the DCCS is required to replace an existing control system (i.e. if the plant is being retrofitted) then the microcontrollers will normally be located in the central control room (CCR). In this case, the distribution features of the DCCS will not be fully utilised and the significance of the digital data highway (DH) will also be reduced. 

The data highway (which is the main artery of communication) connects the various DCCS components to the host and operator stations. Figure 7.105 demonstrates the distributed architecture of a typical DCCS. 

This consists principally of the host which is used to supervise several process areas and may well have six or more data highways linked to it. Hosts are powerful computers with their own operator stations and other peripherals. They are an integral element of modern control systems and only very small processes may be controlled and monitored effectively without a host. However, if not configured properly, a host computer can be overloaded quite easily and will then fail to operate satisfactorily. One serious loading problem for a host is the fast scanning rate required for DDC loops (usually once per second and up to three times per... 

Communication within a DCCS is implemented by means of the system bus or data highway along which data are exchanged between elements of the system at the same or at different hierarchical levels. The bus represents the basic system interface and should provide the following features 50 ... 

The system architecture is a major factor in determining the capacity/constraints of a DCCS and this will be substantially affected by the type of data highway employed and the way in which it is configured. Various DH configurations are illustrated in Fig. 7.107. 

A more recent development is to employ Local Area Networks (LANs) where the LAN is a serial communication link used as a data highway in the DCCS for the integration of intelligent terminals at the higher hierarchical levels. There are two principal types of LAN applicable to DCCS, viz. ... 

Fio. 7.107. Some typical data highway configurations (a) multidrop DH (b) partially clustered/multidrop DH (c) star configuration (d) ring DH... 

Fig. 7.108. Typical DCCS video displays (Kent P4000 system) (a) overview of data highway (b) plant overview as mimic diagram (c) alarm summary (d) unit (group) display (e) historic and current trends...

FIG. 8-65 A DCS using a broadband (high-bandwidth) data highway and field bus connected to a single remote control unit that operates smart devices and single-loop controllers. 

Microprocessor-based equipment, such as smart instruments and single-loop controllers with digital communications capability, are now used extensively in process plants. A field bus, which is a low-cost protocol, is necessary to perform efficient communication between the DCS and devices that may be obtained from different vendors. Figure 8-65 illustrates a LAN-based DCS with field buses and smart devices connected to a data highway. 

Larger, commercial installations will also be equipped with a Supervisory Control And Data Acquisition system (SCADA). The SCADA system is connected via a data highway with the PLC(s) used in the system. 

Instrumentation philosophy (local/remote control, hardwired/data highway, failure mode(s), analog/digital, emergency alarms, etc.). 

Coaxial cables used for data highways must be tested using sine-wave reflective testing techniques. Circuits involving intrinsically safe (IS) instmmentation must be tested (e.g., loop impedance, inductance, L/R [Inductance/Resistance] ratio) in accordance with the manufacturer s instmctions and approved by the customer. 

Network Cabling Data Highways Power Supplies Cabling to I/O racks Field Cabling... 

---