Distributed Control Systems (DCS) Parts

Distributed Control Systems (DCS) Parts

Communication Bus

The communication bus, i.e. the Nodebus, interconnects stations (Control Processors, Application Processors, Application Workstations, and so forth) in the system to form a process management and control node.

Depending on application requirements, the node can serve as a single, stand-alone entity, or it can be configured to be part of a more extensive communications network.

Operating in conjunction with the Nodebus interface electronics in each station, the Nodebus provides high-speed, redundant, peer-to-peer communications between the stations.

The high speed, coupled with the redundancy and peer-to-peer characteristics, provide performance and security superior to that

provided by communication media used in conventional computer-based systems. Station interfaces to the Nodebus are also redundant, further ensuring secure communications between the stations.

The Nodebus can be implemented in a basic, non-extended configuration or it can be extended through the use of Nodebus Extenders and Dual Nodebus Interface Extenders.

Nodebus Interface

The Nodebus Interface is a module which allows direct connection of a personal workstation (PW), with appropriate Nodebus connector card and software, to the Nodebus figure. In this configuration, the PW functions as a station on the node.

The Nodebus Interface allows connection of a station application workstation hosting an Ethernet configuration to Nodebus.

An Attachment Unit Interface (AUI) cable, connects the PW or an Ethernet hub configuration to the Nodebus via a Nodebus Interface.

A coaxial cable (ThinNet) connects an Ethernet daisy chain configuration to the Nodebus via a Nodebus Extender. The Nodebus Interface is non-redundant, and can be used in any of the Nodebus configurations described.

Dual Nodebus Interface

The Dual Nodebus Interface (DNBI) is a module which allows direct connection of stations to the appropriate Nodebus. Connection between the DNBI and station is made via an AUI cable.

For data transmission security, a separate (RS-423) control cable connects between the station and the DNBI to allow switching between the two redundant Nodebus cables.

Switching of the Nodebus cables is controlled by the station, which transmits commands to the DNBI via the control cable. Figure shows connection of a station to the Nodebus using a DNBI.

Dual Nodebus Interface Extender

The Dual Nodebus Interface Extender (DNBX) is functionally similar to the DNBI, but provides a greater cabling distance.

The principal transmission medium used is a coaxial Ethernet cable directly connected to the station end by a standard Ethernet transceiver. Figure shows remote connection of a station to the Nodebus using a DNBX.

Control Processor

The Control Processor performs regulatory, logic, timing, and sequential control together with connected:

  • Fieldbus Modules (FBMs)
  • Fieldbus Cluster I/O Cards (FBCs)

It also performs data acquisition (via the Fieldbus Modules), alarm detection and notification, and may optionally serve as an interface for one or more Panel Display Stations.

The non-fault-tolerant version of the Control Processor is a single-width processor module. The fault-tolerant version consists of two single-width processor modules.

Enhanced Reliability

The Control Processor offers optional fault- tolerance for enhanced reliability. The fault-tolerant control processor configuration consists of two parallel-operating modules with two separate connections to the Nodebus and to the Fieldbus.

The two control processor modules, married together as a fault-tolerant pair, are designed to provide continued operation of the unit in the event of virtually any hardware failure occurring within one module of the pair.

Both modules receive and process information simultaneously, and the modules themselves detect faults. One of the significant methods of fault detection is comparison of communication messages at the module external interfaces.

Upon detection of a fault, self-diagnostics are run by both modules to determine which module is defective. The non-defective module then assumes control without affecting normal system operations.

To further ensure reliable communications, the fault-tolerant control processor performs error detection and address verification tests in its Nodebus and Fieldbus interfaces.

For enhanced reliability during maintenance operations, the Control Processor is equipped with a recessed reset button. This feature provides for manually forcing a module power off and on (reboot) without removing the module from the enclosure.


The Control Processor uses three types of diagnostic tests to detect and/or isolate faults:

  • Power-up self-checks
  • Run-time and watchdog timer checks
  • Off-line diagnostics

Power-up self-checks are self-initiated when power is applied to the control processor. These checks perform sequential tests on the various control processor functional elements.

Red and green indicators at the front of the control processor module reflect the successful (or non-successful) completion of the various phases of the control processor startup sequence.

The run-time and watchdog timer checks provide continuous monitoring of control processor functions during normal system operations. The operator is informed of a malfunction by means of printed or displayed system messages.

Off-line diagnostics are temporarily loaded into the system for the purpose of performing comprehensive tests and checks on various system stations and devices.

Using the off-line diagnostics, a suspected fault in the control processor can be isolated and/or confirmed.

Engineering Interface

The engineering interface, i.e. Application Processor, is microprocessor-based application processor/file server stations. They perform two basic functions:

As application processor (computer) stations, they perform computation intensive functions.

As file server stations, they process file requests from tasks within themselves or from other stations. Bulk storage devices used with the Application Processors include floppy disk drives, hard disk drives, streaming tape drives, and CD-ROMs.

The Application Processors operate in concert with other system stations (such as communication processors, workstation processors, and control processors), which provide the necessary means for data input/output and operator interfacing.

A smaller system can utilize a single Application Processor, while a larger system can incorporate several Application Processors, each configured to perform specific functions. Some functions can be performed by individual Application Processors, while others can be shared by two or more Application Processors in the same network.

For all models of the Application Processor, applications range from minimal functions, such as the storage of memory images, alarm events, and historical data, to larger-scale applications such as database management and program development.

> Application Processor Functions

The following sections describe the major functions performed by the Application Processors.

System and Network Management Functions

The Application Processors perform system management functions, which include collecting system performance statistics, data reconciliation, performing station reloads, providing message broadcasting, handling all station alarms and messages, and maintaining consistent time and date in all system stations.

The Application Processor also performs network management functions, which comprise that portion of system management functions which deal with the network.

Database Management

Database management involves the storage, manipulation, and retrieval of files containing data received and/or produced by the system.

The Application Processors utilize the industry-standard Relational Data Base Management System.

File Requests

Each Application Processor contains a file manager, which manages all file requests associated with bulk memory attached to the Application Processor.

Each Application Processor also supports a remote file system that allows tasks in one station to share files in another.

Historical Data

The Application Processors can be configured to contain the Historian function, which maintains a history of application messages and continuous and discrete I/O values.

These values may represent any parameters such as measurements, setpoints, outputs, and status switches from stations that have been configured to collect data and send it to a Historian. In addition, the Historian computes and stores a history of averages, maximums, minimums, and other derived values.

This information is maintained for display, reporting, and access by application programs. An archiving facility saves the data on removable media, where applicable.

The Application Processors can be configured to maintain a history of errors, alarm conditions, and selected operator actions.

The occurrence of errors, alarms, and events in other stations can be stored (for later review and analysis) by sending a message defining the event to the Historian in one or more Application Processors.

Graphic Display Support

The Application Processor supports graphic displays by storing and retrieving display formats, by providing access to objects stored on the Application Processor, and by storing tasks which execute in a workstation processor.

Application Processors not only provide storage of information and file management for displays, but also execute programs that perform display and trend service.

Production Control Software

Production control software represents a large range of packages that require varied Application Processor resources.

The following is a list of packages provided:

  • DBMS
  • Historian
  • Spreadsheet
  • Physical Properties Library
  • Mathematics Library

The operation and performance of the production control software are determined by the particular Application Processor configuration.


Configuration refers to the process of entering or selecting parameters to define what a software package does, or to define the environment for a software package.

The Application Processors support configuration functions by providing bulk storage for configuration parameters and by executing some of the configuration processes.

Application Development Facilities

Application development tools are provided to build programs for all system stations. These include tools to document, enter, translate, link, test, and maintain programs written in several programming languages.

The Application Processor supports program development for all stations (workstation processors, control processors, and so forth).

Assembly language, FORTRAN, and C programs can be written on the Application Processor using standard operating system tools.

An optional package is available including text editors, debuggers, linkers, revision control, and compilers, plus execution statistics functions.

User Application Program Execution

The Application Processors also execute user application programs. These may be application packages such as special optimizations, test data collections, special data reductions, or other packages that you may have already developed.

The allocation of resources reserved for user application varies with each Application Processor.


The Application Processors utilize three types of diagnostic tests to detect and/or isolate faults:

  • Power-up self-checks
  • Run-time and watchdog timer checks
  • Off-line diagnostics
  • Power-up self-checks are initiated when power is applied

Application Processor. These checks perform sequential tests on the various Application Processor functional elements.

Any malfunction detected during the power-up self-checks is reported by means of messages printed or displayed on a directly connected printer or terminal.

The run-time and watchdog timer checks provide continuous monitoring of Application Processor functions during normal system operations.

For any processor model, you are informed of a malfunction by means of printed or displayed system messages.

Off-line diagnostics are temporarily loaded into the system for the purpose of performing comprehensive tests and checks on various system stations and devices.

Using the off-line diagnostics, a suspected fault in the Application Processor can be isolated and/or confirmed.

Workstation Components

The workstation components provide user interface to all System CRT display functions. A selection of workstation components is available for command and data entry, along with CRT pointer manipulation and control.

These components interact with software resident in versions of the system Workstation Processors (WPs) and Application Workstation Processors (AWs).

Many of these components (displays and keyboards) are “common” and allow interchangeability and simplicity in mixed technology configurations.

Workstation components include:

  • Alphanumeric Keyboard
  • Annunciator and Annunciator/Numeric Keyboards
  • Workstation Display (with/without Touchscreen)
  • Mouse
  • Trackball
  • Industrial Pointing Device
  • Workstation Processor or Application Workstation Processor
  • Personal Workstation
  • Modular Industrial Console

Selection of the touch screen, mouse, trackball or industrial pointing device is required for picking display objects on the CRT. The touch screen has sufficient resolution for all functions normally associated with a process operator.

Only the mouse or trackball provides the picking resolution necessary for engineer-related functions (for example, building graphic displays).

The touch screen associated with Workstation Display and the annunciator type keyboards connects to a Graphics Controller Input Output (GCIO) interface unit located beneath the workstation display.

The GCIO interfaces to the Workstation Processor and/or Application Workstation that provide secure, high-speed, bidirectional data flow.

The alphanumeric keyboard and trackball connect together in a functional grouping via a serial communications link to the processors.

Personal Workstations (PW) utilize separate serial communication links for alphanumeric keyboard and mouse/trackball. These buses allow a variety of component connections.

Alphanumeric Keyboard

The alphanumeric keyboard is used any time text is entered into the system.

It consists of the full set of alphanumeric keys plus punctuation and special symbol keys laid out in the standard format, and a numeric data entry pad (with cursor control).

Annunciator Keyboard

The Annunciator Keyboard is an array of LED/switch pairs. It also contains a horn silence switch and a lamp-test switch.

Each LED, under control of the processor software, may be ON, OFF, or FLASHING as determined by the process conditions. The LEDs, when used in conjunction with the unit’s audible annunciator, form an effective means of calling a user’s attention to specific areas of the system.

The switch associated with each LED can be used to invoke any pre-configured displays or operator responses.

Workstation Display with/without Touchscreen

The workstation display is an analog cathode ray tube (CRT) color monitor supporting ultra-high resolution applications. The monitor is suitable for mounting onto a Modular Industrial Workstation or on a

desktop. The monitor can include a touchscreen optional feature. Figure below shows the monitor with a tilt and swivel base mounted on the GCIO interface unit. The GCIO interface supports the touchscreen, annunciator and annunciator/ numeric keyboard, and audible horn options.

The optional touch screen is bonded to the front surface of the CRT monitor. The user selects display objects by touching them on the screen. The touch screen senses the action and sends a data signal to the workstation processor’s software indicating the position of the selection.


The trackball is a stationary component that contains a rotatable sphere. The trackball can be located on a table top. Rotation of the sphere causes CRT pointer movement analogous to the mouse action. Buttons are also provided for user selections/manipulations.

Modular Industrial Console

Modular Industrial Consoles provide flexible mounting arrangements of components. They allow users to configure centralized or distributed control centers tailored to the functional requirements of each interaction point in the plant.

The modular console furniture described herein may incorporate a mix of equipment – console displays, input devices, processors, Fieldbus Modules, data storage devices, and so on. Alternately, only display-specific equipment can be incorporated.

Modular Industrial Consoles (MICs) are ideal for supporting powerful multiple-screen, real-time display software interactions. This combination allows console resources to be optimally allocated to meet changing day-to-day needs.

Operator Interface

Operating in conjunction with human interface input/output components, the workstation processors serve as a link between the operator and other distributed processor modules.

They receive graphic and textual information both stored internally or from application processors and generate signals to display the information on a workstation display. Display formats and data files are available from bulk storage.

Live display information (distributed data objects) is available from any control -processor, or from shared system global data. The video information displayed can include free form combinations of text, graphic illustrations, charts, and control displays.

The workstation processors display textual information as 80 text characters per line, with four fonts. The processors provide resizable and restackable windows. Displays for all of the workstation processors may also be developed using the system software running in a compatible personal computer.

A workstation processor, together with its workstation monitor and input components, can be configured with combinations of peripherals to suit functions and user preferences.


The architecture of the DCS permits it to be connected to other foreign systems using a gateway module for adapting different communication protocols.