Extending the reach of OPC

InTech, Oct 2001 by Wacker, Paul

Device servers help

bridge a gap to

Ethernet

Over the past several years,

advancements in the Windows operating system (OS) have provided new mechanisms for the exchange of information among desktop applications. Building on these capabilities, OLE for process control (OPC) has greatly simplified access to real-time process information for users of human-machine interface (HMI), supervisory control and data acquisition (SCADA), and PC-based visualization applications.

While OPC has eased access to process and production information, it hasn't solved the other half of the problem: making the connection between the desktop and remote manufacturing or production equipment.

OPC provides a mechanism to exchange data between applications in a standardized way. This allows hardware and software providers to focus on their core competencies. In doing so, the user benefits from a modular architecture, cross-vendor compatibility, and a broader range of choices for developing integrated manufacturing systems.

Easy does It

While some OPC servers connect to a particular process control device, most use a device driver to handle the lowlevel communications protocols and interface with the underlying communications adapter or serial port. For users, this architecture makes connecting a new device as simple as installing a new printer to a desktop computer.

The device driver links the OPC server to the process control device. It is the software component of the device interface and relies on a communications adapter to make the physical device connection. Depending on the application, the adapter may be a standard serial port (RS-232), a specialized serial adapter (RS-422/485), a multiport serial adapter, or an industrial network adapter (i.e., DeviceNet, Profibus, and Foundation fieldbus).

Industrial network connections are convenient because a single network adapter connects a PC to dozens of pieces of equipment. Unfortunately, the required network adapters can be expensive. Direct network connections can also be a problem where the network coordinates an operation among different pieces of equipment. And networked systems are often highly optimized, with limited ability to handle extra traffic. This prohibits their connection to enterprise business systems.

Fortunately, most devices come with one or more serial ports. Due to their ubiquitous nature, they are often a common choice for connecting PCs to process control devices. The technology is generally well understood and flexible enough to meet the needs of a variety of applications; most PCs have serial ports as standard hardware; and serial ports are typically the only interface available on installed and legacy equipment. They have their limitations though, primarily when attaching more than a few devices or when the devices are more than 100 feet (30.5 meters) from the PC.

When more than a few serial devices need to be connected, additional hardware is generally required. Multiport serial adapters are available that provide up to 16 ports per adapter. Should a user need more ports, he adds adapters to expand system capability, provided there are available slots in the PC. Some adapters require special drivers to overcome OS limitations. For some, this configuration might seem convenient, until they realize the management problems in dealing with all of the cables fanning out from the PC. So some multiport serial adapters ship with an "octopus" cable to help connect the various devices to a single card.

Long run

Cabling issues aren't limited to connections with a PC; there are some inherent problems in connecting serial devices over long distances. RS-232 is generally limited to cable runs of approximately 100 feet (30.5 meters) and has limited noise immunity, due to the use of a common ground signal. RS-422 and RS485 have improved noise immunity through the use of differential signaling over twisted-pair cabling and may be suited for runs up to 4,000 feet (1219.2 meters). In either case, users can overcome some of these noise issues and distance limitations with repeaters and modems, but at additional expense. Similarly, the use of RS-422 and RS-485 devices may require signal converters at the PC, in cases where only an RS-232 interface is available.

An emerging technology is out there that will extend the distance of serial connections and provide access to data in the attached device from multiple locations. This technology takes the serial port out of the PC and puts it next to the process control device, using openstandard Ethernet networking. At the center is the device server.

Device servers are specialized computers that include a CPU, a real-time OS, a TCP/IP stack, and an Ethernet connection to provide a bridge to serial devices with RS-232, RS-422, and RS-485 connections. By encapsulating serial data in network packets and transporting them over an Ethernet network, device servers allow virtual serial links to establish over extended distances.

Using Ethernet as the connecting media has its advantages. Standard 10 megabits per second (Mbps) Ethernet (10Base-T) is more than capable of handling the traffic of hundreds of serial devices simultaneously. Ethernet gives you room to grow and the ability to easily scale to 100 Mbps Fast Ethernet (100Base-TX) and 1,000 Mbps Gigabit Ethernet (1000Base-TX). Ethernet also offers a wide choice of topologies (arrangement of devices on the network) and media types (coaxial, twistedpair, and fiber optic cable), allowing the right configuration for the job at hand.