Industrial Networks
By John C. Colwell, Bishop & Associates Inc.

Today’s state-of-the-art industrial control automation began in the 1950s, with the introduction of the 4-20 mA current loop. The 4-20 mA systems employed a star wiring topology which required that each sensor or actuator be home-run wired to its controller. While the current loop represented a vast improvement over earlier 3-15 psi pneumatic systems, it still lacked the flexibility to accommodate frequent line changeovers. In the mid-1980s, the Hart (Highway Addressable Remote Transducer) protocol was introduced. The Hart system utilized the Bell 202 frequency shift keying (FSK) protocol to provide a digital overlay on the 4-20 mA control networks. Developed by Rosemount Inc., the Hart system would link a total of 15 devices on a single twisted-pair wire run, thus improving flexibility and reducing wiring cost and complexity. The Hart system marked the beginnings of modern fieldbus technology.

In the industrial process control world, the two predominant fieldbus standards are the Foundation Fieldbus, designed to be compatible with the IEC 61158 (ISA/SP-50) standards project, and Profibus, developed by the German Department of Education and Research in 1989. Profibus PA, optimized for process automation, was introduced in 1996. Foundation Fieldbus differs from Profibus in that Foundation Fieldbus relies on more widely distributed intelligence among process elements, whereas Profibus relies on a more centralized intelligence. As a result, Foundation Fieldbus is favored by oil refinery and chemical processing industries, while Profibus is favored by the water treatment, pharmaceutical, chemical, and food and beverage industries. Both standards share common physical interfaces. Also, it should be noted that in most large-scale process control systems, a mix of 4-20 mA, Hart, and modern fieldbus networks is common.

In the factory automation space, leading fieldbus standards include Foundation Fieldbus, Profibus, DeviceNet, Interbus-S, ModBus, Control Net, CANopen, CC-Link, Seriplex, Hart, AS-I, Ethernet/IP™, and others. With the except of Control Net, which uses RG-6 coaxial media with BNC connectors, most fieldbus systems offer choices of shielded and unshielded twisted-pair media, along with a choice of sealed or non-sealed connectors. The unsealed interconnections are typically found within a protected control panel. Fixed and pluggable terminal blocks are typically found in this application.

Outside the protected environment, the “Brad Harrison” M12 and, to a lesser extent, the M8-type circular connectors, rule. The latest innovation in this class of connectors is represented by the Molex Ultra-Lock™ connection system, which employs a “push-to-lock” method that provides a fast, simple, and secure connection between the I/O module and I/O devices. Other circular connectors include the M16, M23, and M40 types. Illustrated, above right, is a M12 fieldbus connector by Tyco Electronics. This particular connector features three or four status-indicating LEDs.

Fieldbus cable assemblies are typically insert-molded types, connectorized at both ends. Single-ended pigtails are also available. Both types are provided in standard lengths. A variety of cable insulations are available to meet special application requirements, however, polyvinylchloride (PVC) and polyurethane (PUR) are most common. Major industrial cable manufacturers, like Belden, offer a broad range of insulation materials that meet almost any environmental requirement. The insert-molded cable assemblies meet IP67 protection class requirements. Field serviceable assemblies, having mechanical backshell hardware, meet IP68 requirements.

An AS-I fieldbus diagram (above right) from Turck illustrating A thru D, respectively, a master controller, AS-I power supply, AS-I cables, and AS-I modules. This is typical of the physical simplicity and flexibility of the fieldbus topology.

Accessories include I/O modules of various types. Most of these employ the M12 connectors. The smaller M8 connectors are often employed at the device end of the cable, where miniature sensors and proximity switches are connected. Molex (Woodhead) has an Ethernet discrete I/O module (right), which feature its RJ Lynx™ IP67 modular Ethernet connectors and circular M12 types.

Like enterprise networks, an industrial network can be described as a network of networks. As the above diagram from Belden illustrates, the industrial network environment is a multi-tiered and multi-functional system with a variety of environmental challenges, particularly at the fieldbus level, that are not generally found in the traditional enterprise space. These include mechanical challenges, such as crushing, cut-through, abrasion, water immersion, UV exposure, oil exposure, high-temperature exposure, cold bend and cold impact, as well as electrical challenges such as severe EMI exposure. The cable assemblies appear relatively simple; however, the underlying material technology can become very sophisticated.

The fieldbus typically connects sensors, actuators, and controllers. The backbone network for the fieldbus networks is generally based on Ethernet. However, Ethernet TCP/IP is a best-effort protocol and not well suited to the real-time data transmission demands of factory automation. As a result, real-time extensions, Real Time Transport Protocol (RTP), User Datagram Protocol (UDP), and similar extensions have been developed creating what is often referred to as “Ethernet on steroids.” EtherNet/IP and EtherCAT are examples. The Open DeviceNet Vendors Association (ODVA) has developed a Common Industrial Protocol (CIP) which provides a comprehensive suite of messages and services for various manufacturing automation applications, including control, safety, synchronization, motion, configuration, and information. CIP is a powerful tool that enables greater integration of the factory and enterprise networks.

Industrial Ethernets typically employ hardened connectors and cable assemblies. At the upper left is an Amphenol/Alden T568/RJ-45 type connector imbedded in an IP67 circular connector. At the upper right, is an Amphenol/Socapex T568/RJ-45 type connector imbedded in an IP67 rectangular connector.

At right is an M12 connector with the D-coded keying arrangement.

It is estimated that fieldbus technology is employed in 7.5 to 10 percent of all existing factory automation and process control environments. However, for new installations, more than 50 percent are expected to employ fieldbus systems.

The market for insert-molded fieldbus cable assemblies is estimated at $340 million in 2007, and is expected to reach $500 million in 2012, representing a five-year CAGR of eight percent.

The market for mechanical backshell fieldbus cable assemblies is estimated at $110 million in 2007, and is expected to grow at a five-year CAGR of 6.4 percent, reaching $150 million in 2012. Industrial Ethernet cables, including imbedded T568/RJ-45 types and M12 D-coded, are valued at $180 million in 2007, and are expected to reach $270 million in 2012, representing a five-year CAGR of 8.4 percent. The total value of these three cable assemblies is estimated at $630 million in 2007. The combined five-year CAGR is 7.9 percent. This compares favorably with the aggregate CAGR of 5.5 percent for all categories of industrial cable assemblies.

John Colwell Director, Telecom, Medical and Instrumentation, Bishop & Associates Inc. John Colwell’s background includes 10 years at Nortel Networks‑Cable Group, where he directed the U.S. premises cable marketing effort. In addition, Colwell worked with Nortel's global product development group. Prior to joining Nortel, Colwell held positions in engineering, business planning and development at Amphenol Corporation.