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Introduction to Express Interface™ (Xi)
White Paper: Introduction to Express Interface™ (Xi)
Xi is a new Microsoft .NET based interface designed for secure and reliable access to real-time and historical process
automation system data. Xi provides a standard, .Net based interface for “classic” OPC server functionality, OPC Data Access
(DA), OPC Alarms & Events (A&E) and OPC Historical Data Access (HDA) and represents a natural progression of Microsoft
communication technology from Microsoft Component Object Model (COM) to .Net.
Xi provides the same functionality as the classic OPC servers while addressing some of the known shortcomings of classic
OPC. The classic OPC servers are based on COM communications, which was state of the art when the OPC specifications
were created, but since the introduction of OPC DA in 1996, Microsoft has moved from COM to .Net communications. COM is
efficient for local server access but can be difficult to configure for remote server access and problematic for communication
through firewalls.
In addition to classic OPC functionality, Xi provides the following features:
Secure communications. Xi delivers a firewall friendly interface and patented security features to provide secure
communications between process automation systems and higher level manufacturing execution systems (MES) and
enterprise resources planning (ERP) systems.
Robust connectivity. Xi increases client connectivity robustness by maintaining the state of the client connection, allowing the
client to reconnect quickly and easily if communications with the server were lost.
Plug and play. Xi provides automatic server discovery which allows Xi clients to automatically discover and connect to Xi
servers.
One interface for real-time, historical and event data transfer. Xi provides a single interface to read and write real-time process
data, subscribe to real-time alarms and events and read and write historical data.
Based on open, industry standards. Xi is based on Windows Communication Foundation (WCF), the latest communications
technology available from Microsoft. Using WCF, a Xi server is able to offer industry standard communication protocols, such
as the Transmission Control Protocol (TCP), the Hypertext Transfer Protocol (HTTP) and Secure Hypertext Transfer Protocol
(HTTPS).
Easy to implement. Xi provides .Net client applications a simple .Net interface for easy access to classic OPC servers. Xi
client applications can take advantage of new user interface technologies available in Windows Presentation Foundation
(WPF).
Access data from anywhere. Xi delivers a secure and robust .Net interface that supports interprocess and TCP
communications for .Net based Xi clients and a Web service interface for Internet based or non-Microsoft based Xi clients.
Future proof. As new WCF interface technologies
become available, Xi can use these new interfaces to
provide additional connectivity options. In addition,
because Xi has been designed as a native .Net
interface, additional protocols, such as OPC UA can
be easily layered on top of Xi to extend the Xi
functionality and increase the types of clients that can
connect.
Easy migration from classic OPC. Xi products can
wrap classic OPC clients and servers to provide a
quick and easy migration path from OPC to Xi. Xi is
also network compatible with classic OPC servers and
thus may be used in the same system as classic OPC
clients and servers.
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Introduction to Express Interface™ (Xi)
Technical Overview
Xi is defined as a set of integrated Windows Communication Foundation (WCF) interfaces that provide the equivalent
capabilities defined by the OPC Data Access, Alarms and Events, Historical Data Access and Complex Data specifications.
The Xi interfaces are implemented using the WCF Endpoints described below, beginning with the Server Discovery endpoint.
The Server Discovery endpoint is used for plug-and-play. It provides URLs of Xi servers. Discovery servers consolidate these
URLs and make them available to clients in a single access. Clients use them to access server metadata, automatically
generate proxies, and connect to servers using these proxies. This permits servers to individually customize their connection
properties (e.g the use of encryption) for their own environments.
Automatic discovery of Server Discovery endpoints is performed using the Microsoft Peer Name Resolution Protocol (PNRP).
PNRP is new and may not be available on some of the older Microsoft operating systems. When not available, an alternate
means of configuring clients and/or Discovery servers with the appropriate URLs will be necessary.
The figure below illustrates the endpoints supported by Xi servers for client access.
The Manage endpoint, or more correctly, the Resource Management endpoint is used for browsing the server, creating and
maintaining lists of run-time and historical data, events, and alarms, opening Read, Write, and Subscribe endpoints, and for
assigning lists to these endpoints.
The server provides an additional level of security by restricting access to the Read, Write, and Subscribe based on factors
other than the user identity (user privileges are handled through traditional mechanisms), such as the location and name of the
client application, the workstation where it runs, and the protocol used for access.
The Read endpoint is used for reading runtime and historical data and alarms from lists assigned to it. Reading an alarm list
returns alarms that are active or that were active but have not yet been acknowledged by an operator.
The Write endpoint is used for updating the state of the system (e.g. writing a setpoint, acknowledging an alarm, or correcting
entries in the history database.
The Subscribe endpoint is used to receive updated values, new alarms/events, and alarm state change via polling or
callbacks. Polling is used with HTTP for access through firewalls, and callbacks are used with TCP on local networks and with
Named Pipes when the client and server are running on the same platform.
In addition to endpoints and the resources accessible through them, the figure above also illustrates the client-side
architecture of Xi, composed of the client-specific code that communicates via the Xi Proxy. Clients not wishing to manage the
interface with the server directly may use the Client API designed to client application development.
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Introduction to Express Interface™ (Xi)
What others are saying about Xi
“Working with the new OPC Xi technology provides our customers with a natural path to more secure and compatible WCF
communication on the Microsoft platform.”
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Russ Agrusa, President, ICONICS
"OPC is great technology that enables effective integration of process data in manufacturing and into the business. It is great
to see this technology, Xi, address the few issues with OPC; specifically security and data unification. These enhancements
address the underlying issues while moving us forward with technology that preserves the immense investment companies,
including OSIsoft, have made in OPC. Xi also protects our customer’s investment in OPC and allows them to meet their
business requirements with little or no changes to their underlying systems."
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Jon Peterson, Vice President Marketing OSIsoft, LLC.
"Interoperability with Xi has proven much simpler than our initial deployment of OPC Data Access due to the DCOM security
issues. Since Xi is built on the standard Microsoft .Net Framework, includes a well tuned set of commands and provides OPC
DA, A&E and HDA server functionality, we feel Xi has the key components for customer acceptance as a new automation
system protocol."
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Delcio Prizon, R&D Manager, SMAR
“Xi was designed using current Microsoft supported standards in mind. Because of this, InduSoft Web Studio can be used as a
gateway to other communications standards or networks. This allows customers to keep existing infrastructure in place and
not incur added costs. From here, customers can share this information to any type of communication standard; this could be
on Windows 7, a mobile device or even a PC running an embedded operating system.
Xi was very easy to integrate into our standard product - it only took our developers a matter of a few weeks to get it
communicating to test servers over the Internet at a remote location.”
-
Scott Kortier, Marketing Communications Manager, Indusoft, Inc.
“Mobiform’s 'Status Vision Designer' is a powerful data visualization software package that serves as a sophisticated HMI
(Human Machine Interface) designer for manufacturing and other process control industries. Solutions from Status can be run
as a WPF Windows application or from a Web Browser using Microsoft Silverlight. Having XI as a supported protocol within
Status adds powerful visualization with flexible and secure communications, giving the user an easy to manage and secure
platform.”
-
Brian Thornton, Software Engineer, Mobiform Software, Inc.
Author Information:
Lee Neitzel
Senior Technologist
Emerson Process Management
Email: [email protected]
Chris Felts
Product Stragist
Emerson Process Management
Email: [email protected]
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