However, with more powerful PLCs and so-called programmable automation controllers or PACs available today, the line between a PLC and DCS continue to blur. A DCS is likely to be more reliable as well, ensuring the continuous operation of a process or system. In fact, they are often used to control entire systems within a factory. On the other hand, a DCS can control many more machines and processes at the same time. A typical DCS shows the different communication layers from the field device level up to enterprise systems. PLCs are generally flexible and more readily customizable as well. PLC processing times are also quicker because they are naturally closer to the devices they control (motors, pumps, switches, etc.), so they are more responsive than a larger DCS would be because it is controlling much more than a single machine. This simple OS structure means they can execute programs quickly because there are not many other processes running in the background and demanding time from the processor. One hallmark of PLCs is their relatively simple operating system, designed to do a minimum of tasks like scanning inputs and updating outputs. PLCs still are used to control individual machines, while a DCS can control a larger number of machines or processes within a factory or plant. For the most part, the PLC/DCS split still follows this historical pattern. Meanwhile, a DCS was the predominant choice for process control in applications such as oil and gas and chemical plants.

In the early days of automation, PLCs dominated machine control, that is, applications calling for discrete control of machine processes. While there are differences between a programmable logic controller (PLC) and a distributed control system (DCS), there has also been a move towards greater convergence of the two technologies.

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