1.Definition of PLC and DSC on Wikipedia：
PLC(Programmable logic controller): An industrial digital computer which has been ruggedized and adapted for the control of manufacturing processes, such as assembly lines, or robotic devices, or any activity that requires high reliability, ease of programming and process fault diagnosis.
DCS(Distributed Control systems): A computerized control system for a process or plant usually with many control loops, in which autonomous controllers are distributed throughout the system, but there is no central operator supervisory control. This is in contrast to systems that use centralized controllers; either discrete controllers located at a central control room or within a central computer.
2.Previous And Present Life of PLC and DCS
2.1 Previous Life
At first, about 40 years ago the PLC was used primarily for discrete controls. The programming of the PLC’s was primarily in ladder logic, which is a format that is very similar to a schematic.Receiving device information from the field, solving the logic and then energizing the outputs to produce the desired effect, PLC play an important role in manufacturing processes.Essentially, the PLC was invented to perform repetitive tasks in a reliable and durable manner.
As for the DCS, it was invented because of the increasing use of microcomputers. One of the primary advantage of DCS is that it have the integrated monitoring and control system similar to today’s SCADA systems. With an entire tag base which already created for the process control, the DCS system is available to use on the monitoring and control screens. Another advantage of DCS is that an entire plant could be connected via proprietary communications and controlled by a distributed system. Beside,DCS’s also had function block programming.Function block programming which is a section or several lines of code behind a single interface.By handling the manual and automatic operation of a valve, Function block programming can save a lot of time and redundant programming.
2.2 At Present
Today’s PLC systems can have nearly the same as the DCS, excluding the supervisory control and data acquisition (SCADA). The DCS and PLC are quite similar, save for the integrated monitoring and control nowaday. With open source communications, fiber optics, Ethernet and the like, many PLC’s can now communicate with each other and be autonomous PLC that communicate over the network to other autonomous controllers. That wide communication would allow for single or multiple processes being controlled by one PLC to communicate with another PLC. With today’s technologies, a wide and robust PLC system could do virtuall the same thing that the DCS’s can do.
With a PLC system (multiple PLC’s in a plant structure), you still need to create the supervisory and control system. The entire DCS database would be available for the creation of the monitoring and system, the PLC systems individual PLC databases would need to be created in the SCADA system software. There are more programmers available for hire in the PLC arena and with the new programming languages such as function block, sequential function, etc., the advantage of function block programming is no longer exclusive to the DCS.
But DCS system still have the advantage of installation costs and the onboard monitoring and control system. It can save some costs because of the location of the autonomous controller to the process can be close in vicinity versus pulling long runs of I/O wire across a plant.
However,the DCS’s is the scarcity of programmers that have some experience with a DCS’s. Most plant floor technicians are familiar with ladder logic programming, the DCS programmers and technicians typically need more specialized experience in database functions as well as IT-related networking knowledge.
In summation, the DCS has autonomous controllers dispersed throughout the entire plant. If a controller fails, the entire plant doesn’t necessarily get impacted. It also has the onboard monitoring and control that saves development time. A single PLC is a single point of failure. You surely wouldn’t want to control an entire plant with a single PLC,however; a connected PLC system can have nearly the same security and robustness as a DCS.