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A Short History of Standards for Controllers

(Sidebar to "Gleaning Efficiency from Global Standards" from the July 2007 issue of Automation World)

The evolution of standards for controllers has been a steady march toward open systems. For the machine tool industry, the march began about 40 years ago with the development of RS274 standard for G and M-codes for programming computerized numerical controls (CNCs). It was a boon for the industry. “The simple, clear structure made it easy to train people, so it became a heavily used standard in NC programming,” says Karl Rapp, automation and machine tool branch manager at vendor Bosch Rexroth Corp., in Hoffman Estates, Ill. 


He doubts that the newer Step-NC standards developed to accommodate the many more functions on today’s CNCs will enjoy the same success. “It will take much longer to get the same acceptance on a wide level, since it deals with more complex rules and syntax definitions made for CAD/CAM (computer-aided design/computer-aided manufacturing) software,” he says.

In motion control, the 10-Volt interface between controllers and drives became the de facto standard quickly because it was easy to learn and troubleshoot. As higher-resolution converters were needed for performance, the IEC 61491 digital SERCOS interface standard (for Serial Real-time Control System) supplanted it in the late 1980s for digital communications hardware and fiber cables. Not only did the standard save money, but the 32-bit interface resolution also boosted productivity, says Rapp.

For programmable logic controllers (PLCs), the IEC 61131-3 standard was significant. “Until 1990, PLC users knew only vendor-specific programming,” explains Rapp. This standard, promulgated by the International Electrotechnical Commission, emerged from European original equipment manufacturers’ (OEMs) and users’ agitating for vendor-neutral programming standards. It defines programming terminology, display of program languages, structures and methods.

Since then, the PLCopen organization has built upon the IEC 61131-3 programming languages by creating standard motion programming blocks and programming methods. “These solutions define the functioning across industries so that vendors, OEMs, and end-users can reduce integration, training, and troubleshooting time,” says Rapp. Because of the success, many vendors now provide libraries of them.

Despite the growing interest in open standards developed by international bodies, de facto standards still emerge. OPC communication is a case in point. This standard governs communications between applications running on a personal computer (PC) operating system. “Many users are benefiting from it by linking OPC-compliant products from different vendors without the need for extensive training,”
says Rapp.

The trend in industrial Ethernet-based communications is toward unifying input/output (I/O), motion control, and safety into one bus. About a dozen factions are petitioning the IEC to make their Ethernet-based buses a standard. “Each claims technical advantages, but not all are backed by a large number of product suppliers,” says Rapp.


To see the main story this sidebar was taken from - "Gleaning Efficiency from Global Standards" - please visit

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