![]() With the larger, more complex systems in use today, however, this old-school approach doesn’t work.ĭistinguishing between Location of Control and Mode of Control, Niemi noted, leads to control switches that can accommodate a wide range of systems. In older DCS (Distributed Control System) technology, this may not have been much of a problem because here may have only been a single device switch (there weren’t multiple ways to control it) and the term Hand meant Location of Control and Mode of Control. The inherent problem with the term HOA, according to Niemi, is that it combines Location of Control (the designation for hand/manual operation) with Mode of Control (the designation for automated operation) on the same switch. Using the terms Location of Control and Mode of Control, he suggested, could help clear it up. Niemi believes this lingering confusion may stem from use of the single term HOA. Does Auto translate as “controlled by the SCADA system?” If so, what term should be used if the SCADA system allows personnel to manually set a fixed PLC output to control the field device rather than allowing the PLC itself to automatically set the output? 2016 blog post, when a PLC or SCADA system is involved. The problem increases, he wrote in a Feb. (), this is a recipe for confusion: If one switch is in the Hand position and the other is in the Auto position, how is the device going to be controlled? According to Josh Niemi, P.E., of Normal, IL-based SCADAware Inc. The problem is exacerbated by the fact that a site often employs multiple switches in different areas of the plant to control a single field device. For plant-floor personnel, the HOA (Hand-Off-Auto) switch represents one of the most misunderstood components of an industrial-control system. ![]()
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