01 April 2002
OPC DX is the soft gateway
By Jonas Berge
This 'babblefish' overcomes hardware gateway problems
As with field-level networking, where several protocols-Foundation fieldbus H1, Profibus-PA, HART, DeviceNet, and others-share the market, it is already clear that for host-level networking there will also be several coexisting options.
OPC-Data Exchange for Ethernet is a lock
Since its announced development at ISA 2001 in Houston this past September, OPC-DX has been a shoe-in for success because it's backed-indeed, pushed-by the biggest control system suppliers out there: Emerson, Rockwell, and Siemens.
As well, the technology rides on Microsoft's OLE (now Active X), COM, and DCOM.
ControlNet International, the Fieldbus Foundation, the Open DeviceNet Vendor Association, and Profibus International agree on the initiative.
What of the user? Anecdotally, InTech magazine and the World Bus Journal receive very positive feedback from their sources regarding OPC. In that DX is not available-the specification was to become available December 2001 and may be so when people read this article-reports on it don't exist.
The OPC Foundation, keeper and promoter of the technology, collects application stories on its Web site. While this nonprofit organization probably won't be posting any tales of monumental failure there, it is a good place for the prospective user of OPC and OPC-DX to start when evaluating the products. The foundation's charter is to develop a worldwide, industry-standard spec for multivendor interoperability in the manufacturing and process industries.
-Nicholas Sheble, firstname.lastname@example.org
A plant that purchases different subsystems from diverse manufacturers may end up with islands of automation. Gateway hardware to convert among the different protocols is difficult if not impossible to come by and requires a tremendous configuration effort if the amount of data is large.
However, using the OLE for process control-Data Exchange (OPC-DX) technology, a plant can link subsystems together, despite various Ethernet flavors or other protocols.
OPC-DX works like a soft gateway. Linking systems or protocols has in the past been very difficult because gateways are usually not available for the combinations of networks required. In the few cases where gateways are available, they typically support only a part of the network functionality, making it impossible to benefit fully.
Third parties often develop gateways, and they may not keep pace technologically with industry advances. The latest and greatest features of the networks may not get support. As new versions come to market, incompatibilities may also exist. As there's no independent interoperability testing, these incompatibilities may surface only during the late stages of a project.
Lastly, because few units of gateway hardware for each combination of a multitude of protocols sell, the price is usually very high. Indeed, the word gateway has bad connotations for many.
The OPC-DX software gateway scheme overcomes these hardware gateway problems.
How OPC-DX works
OPC-DX is a server-to-server communication mechanism. Devices, networks, or subsystems connect to OPC-DA servers. OPC-DA servers can, in turn, connect one another directly using OPC-DX interfaces communicating peer to peer, eliminating the need for intermediate bridge or mirror applications to transfer the data.
The DX interface is thus both client and server at the same time, acting as both source and sink for data.
An OPC-DX gateway has an OPC-DA server part and an OPC-DA client part with a DX interface. The server part produces data for other applications, and the client part consumes data from other applications. The OPC-DA server part delivers the data the other applications need.
The connections establish from a configuration tool that uses the standard OPC browser mechanism to locate the data source and sink in devices. For the most part, linking data in one device to another will now be a simple drag-and-drop operation.
OPC-DX will be easier to configure than traditional gateways, but certain data configurations will remain complex.
Replace industrial Ethernet?
Industrial Ethernet protocols and OPC-DX have some mutually exclusive characteristics that make them suitable for different tasks.
OPC-DX is a complement to industrial Ethernet protocols, not a replacement, at least at this point in time. Whether OPC-DX can eventually replace the many different industrial Ethernet protocols depends on several critical factors: safety, availability, interoperability, and openness.
For the foreseeable future, devices will use embedded industrial Ethernet communications, while at the same time they'll come with OPC-DA server and DX gateway software to enable linking to other protocols.
Safety: OPC-DX has basic mechanisms for detecting failed communication links based on time-out that provide shutdown by substituting a preconfigured fail-safe value.
The mechanism for quality/status is not mandatory and not well defined. However, an industrial Ethernet such as Foundation fieldbus HSE has a programming language with a very well-defined shutdown mechanism, which is also interoperability tested.
Availability: OPC-DX addresses availability by providing the option to fail operational by holding the last good value should a connection fail. The industrial Ethernets, however provide for varying degrees of redundancy that increase availability.
Foundation fieldbus HSE, for instance, has a sophisticated redundancy scheme providing also for complete network redundancy, device redundancy, and port redundancy.
Interoperability: OPC-DX handles simple data-type conversion such as integer to floating point. However, some devices operate in percentages, while others use engineering units. In such cases, scaling has to happen elsewhere.
Similarly, different devices or systems may have entirely different ways of representing certain process functions. Conversion of one set of semantics to another requires more configuration, more checking, and documentation.
The advantage of using a single protocol as far as possible is that data maintains the same meaning throughout the system. Using devices based on the same protocol without gateways always provides the best integration.
Openness: All forms of OPC are based on the Microsoft DCOM technology found only in Microsoft Windows and perhaps one or more embedded operating systems. This means a vast majority of devices today are not able to run OPC-DX embedded and require a Windows server to act as a proxy.
Many users are uncomfortable passing real-time process information used in closed loops and interlocks through a Windows machine and therefore prefer an industrial Ethernet protocol.
OPC-DX is ideal for server-to-server communication and offers plenty of benefits over traditional hardware gateways. It is a complement to the various industrial Ethernets, not a replacement.
The greatest advantage of OPC-DX is that one can integrate existing distributed control systems with modern Ethernet/fieldbus-based systems. Similarly other subsystems for advanced control, paper quality control, turbine control, emergency shutdown, and others can tie in, too. WBJ
Behind the byline
Jonas Berge is a general manager at Smar's Asia-Pacific HQ in Singapore. He has 14 years of experience in development and application in the field of instrumentation. Mr. Berge is a senior member of ISA, and his book Fieldbuses for Process Control: Engineering, Operation, and Maintenance came on the market this year.