SCADA System Hardware

SCADA systems typically include the following equipment:

AXP host computers

Operator consoles

Remote terminal units

Communications equipment

Intersite communications equipment

In a multi-site system, one site is usually designated as the “host” site, while the others are called “remote” or “foreign” sites. The host site is usually the recipient of data transferred from remote sites. The host site also communicates to the eterracontrol computers to gather locally telemetered data.

In this document, the eterracontrol computer is also referred to as the eterrascada “Front End”. Each eterracontrol computer communicates with its remote terminal units (RTUs) by way of phone lines, microwaves, or fiber optic cable lines.

Computers

SCADA software runs on Microsoft Windows and Linux Operating Systems. The particular type of computer used depends greatly on the volume of data being retrieved, processed, and stored.

At a control site, SCADA usually runs on two computers. One performs real-time functions (that is, the gathering, processing, and displaying of real-time data). This computer is referred to as the “primary”.

The other computer acts as a backup; it is referred to as the “secondary”. Its purpose is to take over real-time operations should the primary computer fail.

The front-end computers also typically run in a dual primary/secondary mode, running on two separate computers.

Starting with eterrahabitat 5.9, multi-site systems are supported where each site can have a pair of eterraplatform or SCADA systems in a primary or secondary redundant configuration. The two sites are managed in an active or passive configuration where only one of the sites is allowed to be active at a time. For more information, refer to the Configuration Manager User’s Guide included with eterrahabitat.

Operator Consoles

Consoles are the connection between the computer and the operator. The console allows operators to view and interact with the SCADA system and to use SCADA displays (for a discussion of displays, see chapter 5 Displays and Associated Tasks).

Remote Terminal Units (RTUs)

RTUs connect the SCADA system to the monitored system. Usually, an RTU is wired to devices within close physical proximity, and it receives incoming measurements and transmits outgoing commands for those devices.

Whether the data is a measurement or a control command, each piece of data handled by an RTU is associated with an identifier known as an “address”. This term refers to a set of points retrieved in a single command to the RTU. Each address of an RTU can be scanned independently. Specific scanning instructions for SCADA (that is, which RTUs and addresses to scan) are maintained in the SCADAMOM database.

Data is transmitted to and from RTUs to the SCADA Front Ends via the “communications front ends” (CFEs). The communications path that links RTUs to a CFE consists of the following pieces of equipment:

CFE port

Communications line into the CFE port

Modems located at each CFE port and at the RTU

The status of each communications path in the system is displayed on an eterracontrol display. The operating states of CFEs and communication lines can be controlled from the eterracontrol displays.

The eterracontrol functionality and displays are discussed in the eterracontrol Display Reference Manual.

Computer-to-RTU Communications

The communications equipment and software that link the front-end computer(s) to the RTUs are referred to collectively as the “communications front end” (CFE). The main piece of hardware in the CFE is the “intelligent communications processor”.

The software that communicates between SCADA on a host and a CFE is eterracontrol, or the SCADA Front End. The eterracontrol software must run on a Windows system. If the host computer is also a Windows system, the eterracontrol software can reside on the host computer or be distributed on a separate computer.

In addition to communicating with one or more CFEs, the Front End maintains information regarding the configuration of the communication components, and it facilitates interfacing with devices having a variety of communication protocols.

In an SCADA system where each control site has two host computers, there are two redundant SCADA Front Ends, which are both in communication with the operational SCADA. Each SCADA Front End, with its CFEs, functions as an independent unit. This means that on host failover (where the role of a computer switches from Standby to Operational), the Front End(s) that are not on the host computer continue scanning, so that no data is lost.

The CFEs communicate with the RTUs and perform several functions to minimize data processing on the host computer. These functions include:

Maintaining scanning instructions provided to them by the host. This allows the host to request scanning with a minimum of communication with the CFEs.

Reporting communication errors when RTUs do not respond and when they respond with errors.

Checking incoming data so that the host receives only data that has changed since the previous scan. (This capability is often referred to as “reporting by exception”.)

Periodically reporting all data to the host regardless of whether the data has changed from a previous scan. (This capability is called “integrity scanning”.)

Performing data conversion to make the different RTU communication protocols appear the same to the host (in a system where different kinds of RTUs are used).

Intersite Communications

Sometimes, two SCADA sites need data about the same device. For example, it is common for data to be sent from a remote site to the host site for study at the host site. In this case, duplicate records representing the device are contained in the SCADAMOM databases at both sites. The site from which data is retrieved is called the “native” site where the measurement is initially retrieved in scanning.

The data is sent from the native site to the “receiving” site that needs the information. Whenever the measurement changes at the native site, the new value and its associated data quality indicators are transferred to the receiving site.