Changing of the Guard: The Programmable-Logic Controller

Long the mainstay of industrial process controls, the programmable-logic controller (PLC) is now emerging as a superior control device for life-safety systems...


Long the mainstay of industrial process controls, the programmable-logic controller (PLC) is now emerging as a superior control device for life-safety systems.

Safety-system designers have long relied upon hardwired logic panels-a myriad of relays, lights and alarms-to effect a series of yes/no decisions and annunciations. While building-automation systems have embraced the digital age such that front-end deployment of electrical-mechanical devices is rare, safety systems have just recently begun a similar migration to digital systems.

Reasons for the recent migration are twofold. First, insurance carriers have recognized that the reliability of a PLC, set up correctly, is much greater than that of a PC. Secondly, the PLC proactively "learns" the personality of a system and can anticipate a hazardous incident developing and react to prevent it. Consequently, decision-makers in building projects are extremely receptive to the anticipatory capabilities of a "smart PLC," especially when compared to the largely reactive capabilities of a hardwire-based system.

At the same time, insurance carriers and code officials have not accepted the PLC without qualifications. Namely, a minimum number of hardwired safeties must be maintained. This is typically specified as "safety-hardened PLCs with redundant I/O wiring for critical safety devices." By specifying emergency power in addition to the PLC, this now provides system designers with the best of all worlds: the ability to harness the power of a computer; to anticipate, read and thereby prevent a life safety incident; and also a fail-safe reliability status with respect to insurance carriers and code requirements.

Additional benefits of PLC technology are worth noting:

  • Now that PLCs are a mature technology, technician familiarity and parts support are no longer impediments. Additionally, process-data acquisition systems and vendors of other devices have interfaced with PLCs on a regular basis such that their interface software has been written, eliminating any roadblocks or surcharges to interface with a PLC.


  • Relative to hardwired alternatives, the PLC can easily reduce cabinet footprints and their associated service clearances by an order of magnitude.


  • A PLC solution greatly reduces the amount of wiring and moving parts and, unlike hardwired devices, does not rely on relays with springs that wear and contacts that stick.


  • Costs of PLCs, like those for the rest of the digital world, have dropped dramatically, such that the price associated with a dedicated PLC is easily justified.


  • PLCs can better handle analog I/O, which is central to allowing recognition of an impending event.

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