Making sense of standards

By Jim Montague, Executive Editor

Standards are organizational tools, but like any tools, they’re only good if they’re built right and applied correctly. Similar to legislative statutes and grammatical rules, they’re basically a popularity contest of agreed upon societal norms designed to make hardware and procedures uniform, so they can be consistently applied by users.   

Ironically, it seems some of the most talked about and anticipated standards during the 10 years Control Design has been around, such as the International Electrotechnical Commission’s (IEC) 61158 eight-part standard, often ended up having the least practical influence. However, those that grew up organically in peripheral areas, such as Ethernet and NFPA 79, had the most functional impact. Some technologies, such as Ethernet and its many offspring, have such momentum that they become de facto standards. Others wax and wane until they’re needed, while some simply dry up and blow away like many obsolete technologies. 

For users, it’s important to understand that different standards coexist at various stages of development and adoption. Some were already long established—often in Europe—and revised over the past 10 years, while others popped up only recently to help bring some order to galloping technological advances. It seems some Holy Grails just take longer to find than others.

NFPA 79: Safety Aids Simplification 
Though not directly governing control or automation functions, the National Fire Protection Association’s (NFPA) safety rules have just as much influence on machine builders as more closely related standards. In 2002, the organization revised its NFPA 79 regulations to allow higher-voltage power, typically 30 A and 600 V, to be supplied via soft-wiring with factory-applied, molded connectors. This design simplification reduces labor and potential errors, and also fits with how cables and connectors have evolved in recent years.

NFPA 79’s revision was the first for the standard since 1997, and it included several changes that help implement programmable safety systems. The main clauses that changed in NFPA 79 include:

• 9.2.5.4.1.4—when a Category 0 stop is used for the emergency stop function, it shall have only hardwired electromechanical components. 
• 9.4.3—in the event of any single failure, control systems incorporating software and firmware-based controllers performing safety-related functions shall: lead to safe-state system shutdown; prevent operation until the component failure is corrected; prevent unintended startup upon failure correction; provide protection equivalent to control systems with hardwired/hardware; and be designed in conformance with an approved standard that provides requirements for such systems.
• 11.3.4—software and firmware controllers used in safety-related functions shall be listed for use in safety-related functions.

Ethernet Ascendant
Ethernet in industrial networking, defined under the Institute of Electrical and Electronics Engineers’ (IEEE) 802.3, emerged on the plant floor, gained dominance, and has been characterized in the past 10 years by the quasi-proprietary icing that many suppliers spread on top of the TCP/IP protocol. However, some newer players have been getting into the act, and using Ethernet to enable higher-speed, multi-axis, and other motion control-related technologies.

One notable effort in this area comes from the SERCOS Trade Organization, which added Ethernet capabilities to its 20-year-old protocol. The organization released its specification for the controller-to-controller (C2C) synchronization and communication profile for interconnecting motion controllers using SERCOS III industrial Ethernet-based standard for motion control. This profile defines mechanisms to interconnect distributed control functions, and synchronizes distributed motion controls in modular machines and systems via SERCOS III, which offers hardware redundancy, hot-plugging, and cross-communication.

Another trend hastening Ethernet’s adoption found users gathering around two connector types. RJ45 is already the most-used Ethernet connector worldwide, but now these connectors are being made more rugged for industrial settings, and even acquiring over-molding for especially harsh applications. Users also are adopting traditional, round M12 and M8 four and eight-pole connectors for installing Ethernet on the plant floor. This has triggered an increase in demand for two-pair Ethernet cable, which M12 connectors require, rather than the four-pair Ethernet cables that don’t match these connectors.

Programming with IEC 61131
One standard devised in Europe and slowly gaining traction in North America over the past decade is IEC 61131-3. Designed as universal programming language, it’s intended to create commonality in programming controllers by harmonizing the programming interface, including the definition of five languages. These languages are Sequential Function Charts (SFC), used to build the internal organization of a program, and four interoperable languages: Instruction List (IL), Ladder Diagram (LD), Function Block Diagram (FBD) and Structured Text (ST).

IEC 61131 was developed by users with expertise in different areas. Ladder Logic was required by the North American representatives, while Instruction List was required by the German representatives. The standard applies to instruction sets, program organizational units, data types, and the software model. It doesn’t apply to execution, display presentation, and how-to methods. In fact, IEC 61131 allows extensions to the standard, which creates incompatibilities between vendor offerings.