The changing landscape of control architecture

One distinct advantage to a long career in automation is to witness, firsthand, the evolution of technology. Looking at the technology of today, it is hard to imagine how we got things done 40 years ago, but, clearly, we did.

My first experience was on a MAC. No, not the one by Apple but an early programmable controller by Allen-Bradley. The Bulletin 1742 modular automation controller (MAC) was basically a motherboard bolted to the back panel of a control cabinet with eight reserved spots on the circuit board to which input and output modules could be plugged in to interact with 120 Vac or 24 Vdc I/O.

Each module could process four I/O points. The controller was programmed using a Bulletin 1740 operator terminal, which was a handheld programmer with small display through which simple ladder logic could be entered. There were modes of operation that allowed for single-scan troubleshooting and diagnostics.

The next step on my evolution as a programmer was the early Omron SysMAC programmable logic controller (PLC) series. These were brick PLCs where they came with a predetermined set of inputs and outputs on base module that also contained the processor.

Additional modules could be added to the base brick to expand the I/O and capabilities. It was on this platform that I cut my teeth in automation, and it launched me into what is a nearly 40-year career. From those early experiences, I quickly graduated to a variety of PLCs by Rockwell Automation’s Allen-Bradley, Omron, Schneider Electric’s Modicon and Siemens.

As most of us are aware, PLCs have evolved into current technology where the programmable automation controller (PAC) is now the king of the hill. This platform combines the Boolean logic of the PLC with the ability to program in advanced text-based languages like Visual Basic, various versions of C and other advanced programming languages.

Furthermore, the PAC also has an architecture where it is possible to have more than one processor on the logical backplane, and each processor can have assigned I/O modules that are exclusive to that processor or can “listen” to I/O that has been assigned to other processors in the backplane.

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The latest evolution of the PAC is to have both the main processor and a dedicated safety processor in the same physical module. In addition to a cost savings, the safety processor has specific, safety-rated I/O modules that permit building a safety environment in the graphical programming format commonly used for a standalone safety PLC, while programming the normal, non-safety application in the usual programming languages of ladder logic (LL or LD), structured text (ST), function block diagram (FBD), sequential flow chart (SFC) or instruction list (IL).

For smaller applications, hardware manufacturers came up with smart relays. These were basically a relay with, as the name suggests, some smart functions that could be configured by the user. Physically similar in size to a control relay, these smart relays could add function without having to involve a PLC and the associated cost.

Smart relays quickly evolved into something more than a smart relay. Single-input/single-output became a few inputs and outputs that could be programmed with an interface cable from relay to laptop. These became my favorite tool for junction box operations where I might want to add some time or count functionality to a smaller application without having to build a control box with a PLC.

These smart relays also came in handy to adapt a negative-positive-negative, or sinking, system to a positive-negative-positive, or sourcing, system and vice versa, without converting the whole machine or process over to the matching wiring system.

Each manufacturer seems to have a version of a smart relay and the low cost and built-in flexibility has led to an evolution of the original smart relay into something much more powerful. Progressive smart relays have come out with a few more I/O and a few more commands to the point where the smart relay has become an entirely new platform of PLC.

Small footprint and low or no cost for the programming software made the smart relay extremely popular. In a few short years, we have seen the gap between smart relay and micro-PLC pretty much evaporate.