Control products include two main categories: DCS and PLC.
DCS (Distributed Control System) is also known as a distributed control system.
PLC (Program Logic Control)
What are the differences between DCS and PLC?
1. From the perspective of origin and development:
DCS evolved from traditional instrument panels. Therefore, DCS inherently focuses more on instrument control. For example, Honeywell's TPS. DCS systems don't even have a limit on the number of PID controllers.
PLCs evolved from traditional relay circuits, and the earliest PLCs didn't even have the ability to process analog signals. Therefore, from the beginning, PLCs emphasized logic operation capabilities.
2. From the perspective of system scalability and compatibility:
During the development of DCS, although each manufacturer has its own independent system, the internal communication protocols of most systems are not entirely the same. However, the operator-level network platform has adopted Ethernet, which allows for the addition or removal of nodes and the placement of nodes wherever there is network coverage. In addition, new general-purpose operator stations, based on open protocols such as OPC and DDE of Windows systems, can also facilitate communication between systems to achieve resource sharing.
PLC systems generally have little or no need for expansion because they are typically designed for specific devices. Compatibility requirements are also rare; for example, resource sharing between two or more systems is very difficult for a PLC. Furthermore, PLCs usually employ dedicated network architectures, such as Siemens' MFI bus network, making even adding an operator station difficult or costly. However, with technological advancements, the scalability of PLCs is becoming increasingly robust.
3. Regarding the database used:
DCS systems typically provide a unified database. This is used in various applications, such as configuration software, monitoring software, trend charts, and reports.
PLC systems typically use different databases; configuration software, monitoring software, and even archiving software each have their own databases. This is why it's often said that Siemens' S7-400 systems, from version 414 onwards, only qualify as DCS when they use the PCS7 system (a system using a unified database).
Similarities between DCS systems and PLC systems
Strictly speaking, DCS and PLC can no longer be clearly distinguished; the concepts between them have become blurred.
The use and development of control products must be based on meeting different process requirements. Automated production units will not use DCS simply because they want to.
