OPC: Not as bad as it seems

WE'VE HEARD a few scary stories recently warning about the “seven flavors of OPC,” the inability of certain OPC clients and servers to com­municate, and the difficulties of implementing an OPC solution. If you aren’t careful when selecting OPC products, problems certainly can arise. However, by exercising a little due diligence, you can bring an OPC application up with no problems.

“The difficulty in configuring and setting up an OPC system essentially lies with the developers of OPC servers and clients,” says Scott Saunders, vice president of sales and marketing at Moore Industries. “How difficult the job is depends on how closely the developers followed the OPC standard, and how easy they made it for a user to configure an OPC system. Easy-to-decipher documentation and application examples, including screen shots, always assist in configuration.”

Any time you set up an OPC system, the user first has to set up the channels manually in the OPC server, states Arun Veeramani, product manager at National Instruments. “Then he has to go into the client to configure the tags,” he adds. “To alleviate this, companies like NI publish all the channels. This way people don’t spend time configuring the channels. They can go directly into the OPC client and pull in the tags they want.”

This is called “mapping,” because it identifies the locations of various inputs, outputs and variables, and it has to be done no matter what kind of interface you use. In some cases, you have to build a tag database in the OPC server, says John Weber, president of Software Toolbox. “Many OPC server vendors allow the user to set up the OPC server, so it knows the COM port and node address of the target device for serial connections and the Ethernet address for network connections, and then simply pass in PLC addresses to read from the HMI/SCADA system,” says Weber.

“Setting up an OPC system requires about two minutes,” says Stefan Hoppe of Beckhoff Automation and member of the OPC European Steering Committee. “The OPC system user only has to install the OPC server once. Also, configuration only needs to be done once, with software devices available and easily accessible for this purpose via the OPC Server interface.”

As for the “seven flavors” of OPC, it appears this really isn’t a problem, unless you’re doing specialized work. “Most vendors are OPC DA-compliant,” explains Veeramani. “While there are other flavors like OPC Alarms & Events, OPC Batch, OPC Historical Data Access, OPC Data eXchange, OPC Security, OPC XML-DA and OPC Complex Data, they aren’t as widely adopted as OPC DA. So, when a vendor claims to be OPC-compliant, it generally means OPC DA.”

Other problems we’ve heard concern OPC’s speed and the DCOM connection. “While OPC is not fast enough for high-speed data acquisition, it satisfies the most common use case of moving real-time data from PLCs and DCSs to HMI/SCADA software,” says Veeramani.

If you require blazing speed, OPC might not be the ideal solution. “We support OPC in our software,” says Marcia Gadbois, vice president of business development at Indusoft, “But if an application requires a very-fast, real-time response, we suggest that our clients use our dedicated device drivers to eliminate the overhead in OPC.”

DCOM, the Microsoft protocol that OPC servers and clients use to talk to each other when they’re on different PCs, has its issues, notes Weber. “Config­uration can be tricky, and it doesn’t pass through routers and firewalls well,” he states. “If you’re using DCOM, and both PCs are in the same domain, it’s a fast and good connection once you get the system up and running. The challenge can be in the initial configuration and if the network connection isn’t solid.”

The latest version of OPC United Archi­tec­ture (UA), released in June by the OPC Foundation, addresses the DCOM problem, though it will be a while before we know to what degree. Another solution is to buy “OPC tunneling” software, which bypasses DCOM.

All our experts suggest that, before you buy an OPC product, due diligence is in order. “Make sure the vendor is an OPC member, check their profile at www.opcfoundation.org to be sure the product is certified, and make sure the vendor has attended OPC Interoperability meetings hosted by OPC Foundation,” advises Tim Donaldson, director of marketing at Iconics.

“My recommendation would be to try it before you buy it,” advises Saunders.