ETAT Glossary

There are currently 18 names in this directory beginning with the letter O.
Object Linking and Embedding
(OLE) A set of system services that provides a powerful means for applications to interact and inter-operate and is based on the underlying Component Object Model. Through OLE Automation, an application can dynamically identify and use the services of other applications.

Overall Equipment Effectiveness is the result of the KPI when combined together in a mathematical formula. The OEE percentage when compared to the target rate can tell machine operators whether or not the machine is performing effectively, or under par. Features to create OEE solutions are built into many SCADA software packages like InduSoft Web Studio.

online editing
Editing a program that is being executed without stopping PLC operation.

OPC (pronounced as separate letters) is an industrial connectivity standard that enables the transfer of automation data between automation hardware and software. The OPC Foundation ( is responsible for defining and maintaining all OPC specifications. The goal of OPC is to make it possible for a software application to access automation data from any control and/or monitoring system, regardless of its vendor. OPC manages this by inserting a middle layer, called an OPC Server, between an application and its data source. The purpose of this layer is to translate the application's data queries into commands that the data source understands. For this to work, both the application and the Server must be OPC-compliant. That is, the application must be capable of issuing OPC commands and the data source must be capable of responding to them. Common data sources include a PLC, DCS, IED (Intelligent Electronic Device), scale, transmitter, etc. However, data sources can also include software applications such as an HMI, Historian, Advanced Process Control (APC), etc.

OPC Client
An OPC Client is a software module that enables applications to acquire data from an OPC Server or conduct supervisory control using an OPC Server. OPC is based on client/server architecture. OPC Client applications can be used for data acquisition. In this scenario, they simply subscribe to data (or poll data) from an OPC Server and either display or archive the data, or even both. Software applications can then archive data from a PLC, DCS, process analyzer, etc. OPC Client applications can also be used for supervisory control. HMI applications are typically programmed with OPC Clients. This enables the HMI to provide supervisory control setpoints in the OPC Server, which, in turn, sends the setpoints to the underlying control system (PLC or DCS). When OPC Clients and Servers are used together, they can be used for supervisory control and data acquisition (SCADA). OPC Clients can handle an unlimited number of OPC Server connections. In addition, OPC Clients can handle multiple OPC specifications simultaneously. For example, an OPC Client can support OPC DA (Data Access), OPC HDA (Historical Data Access), OPC A&E (Alarms & Events), and even OPC UA (Unified Architecture) inside the same software application.

OPC Commands
The OPC Commands specification defines a set of interfaces that enable OPC clients and servers to identify, send, and monitor control commands that execute on a device. The OPC Foundation ( is responsible for defining and maintaining the OPC Commands specification.

OPC Compliant
An OPC product (such as an OPC Client or Server) that passed the OPC Compliance testing. OPC Products that pass can display the OPC Compliance logo.

OPC Data Access (OPC DA) provides access to real time automation data. Using OPC DA, software applications can retrieve real-time data to enable them to monitor a given process. In that scenario, OPC DA is used for data acquisition. OPC DA can also be used to enable a software application to write data to a control system. This enables OPC DA applications to handle supervisory control. Thus, OPC DA is well suited for Supervisory Control and Data Acquisition (SCADA) scenarios. OPC DA is typically used by almost all process control applications that are used as an HMI, Historian, Advanced Process Control (APC), Maintenance, ERP (Enterprise Resource Planning), Analysis, Optimization, etc.

OPC Data eXchange
OPC Data eXchange (OPC DX) is a specification that defines how OPC servers exchange data with other OPC servers. While a small number of vendors incorporated this specification in their products, there are no commercial implementations of this technology. Instead, corporations are using various forms for OPC bridging products that transfer data between OPC Servers.

OPC Foundation
The OPC Foundation is responsible for developing and maintaining the OPC Specifications and ensuring product interoperability and compliance. The OPC Foundation is made up of over 350 companies, organizations and individuals.

OPC Server
An OPC Server is a software module that enables applications to provide their data to the outside world using OPC. OPC is based on client/server architecture. Many OPC Servers are used strictly as “protocol converters.” In other words, they connect to an automation controller (such as a PLC, DCS, RTU, etc), using one communication protocol, and convert the data and calls to OPC. OPC Servers enable application to read and write data. They enable users to conduct supervisory control and data acquisition (SCADA).

OPC Unified Architecture
OPC Unified Architecture (OPC UA) is the most recent of all OPC specifications and unifies them all into one. OPC UA provides all the capabilities of the current OPC specifications and is even backwards compatible with current specifications. OPC UA also adds various capabilities that were unavailable in previous specifications. OPC UA enables programmers to select various data transportation methods such as COM, DCOM, or .Net (using XML to enable communication with operating systems other than Windows, or binary encoding to gain significant speed).

The numbers and variables on which an instruction operates.

operating mode
The mode that determines the operation of the program in the CPU Unit, including PROGRAM mode, MONITOR mode, and RUN mode.

a symbol that performs a simple function on some code such as multiplying two numbers or comparing them to see if they are equal.

Open Systems Interconnection: open structure for networking devices which promotes interoperability between multiple vendors

OSI Model
A model for describing communications in a network in which hardware is divided into seven layers

outer loop
A loop that contains another loop.

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