Cost is a Factor to Choose PLC or DCS
In the past it was fairly easy to determine whether a PLC or a DCS was right for an application but in recent years this has become more difficult. It is argued that more powerful PLC product coupled with the new software tools provide an integrated process control system rivaling a Distributed Control System (DCS) for process control applications.
The architecture of a PLC and DCS system look strikingly similar on a system layout drawing with the same basic components: field devices, input/output modules, controllers, and Human Machine Interface (HMI).
Today with open technologies, DCS system are competitively priced with PLCs, in fact, if you consider the cost of implementing the system and the cost of making changes to the system over time, in addition to the initial purchase price, the DCS can be much less expensive. The total project costs include the expenses required to build a working solution that accomplishes the long-term goal of effective process control.
One must consider maintenance and changes to accommodate growth over time. These total costs are lower than applying PLCs because the built-in functions and inherent integration available in a DCS enable implementation and maintenance of a more effective system with less labor.