PLCs and DCSs cannot be directly compared. A PLC is a controller, an isolated product, while a DCS is a system. However, a PLC can be compared to a DCS control station. A PLC's cycle time is around 10 milliseconds, while a DCS control station's is around 500 milliseconds. PLCs are more open and have a stronger ability to work independently as a product.
DCS is a system that includes host software, network and controller, while PLC is just a controller. To form a complete system, a host SCADA system and a network connected to it are also required.
For PID loop control, Mitsubishi's process controllers can now be programmed using FBD, similar to SAMA configuration. DCS systems are larger, control more loops, and have more control methods and algorithms, enabling more complex inter-loop control. Hardware reliability is roughly the same. DCS can achieve I/O redundancy, while PLCs cannot. Relatively speaking, PLC-based systems are less expensive.
DCS is a "distributed control system". In terms of hardware, it includes field controllers, operator station computers, engineer station computers, and the network system connecting them. In terms of software, DCS is a holistic solution that solves all the technical problems of a system, and the various parts of the system are closely integrated.
A PLC is a device, which in hardware is equivalent to a field controller in a DCS; in software, it is a local solution with loose organization between stations.
The key differences between DCS and PLC lie in two points: first, DCS is distributed control with a global database; second, PLC uses a sequential scanning mechanism, while DCS is time-based control. Our system meets the first point; for example, a modification to an I/O tag is simultaneously reflected in the HMI.
PLCs have evolved from on/off control to sequential control and data transfer, offering multiple functions including bottom-up continuous PID control, with PID controllers located in interrupt stations. A single PC can be used as the master station, with multiple identical PLCs as slave stations. Alternatively, one PLC can be the master station, with multiple identical PLCs as slave stations, forming a PLC network. This is more convenient than using a PC as the master station because users don't need to know the communication protocol; they only need to follow the instruction manual's format. A PLC network can function as an independent DCS or as a subsystem of a DCS. PLCs are primarily used for sequential control in industrial processes, but newer PLCs also offer closed-loop control capabilities.
DCS: Distributed Control System. DCS is a monitoring technology integrating 4C (Communication, Computer, Control, CRT) technologies. It's a large, top-down, tree-structured system, with communication being crucial. PID controllers reside in interrupt stations, which connect the computer to field instruments and control devices in a tree-structured, parallel, and continuous link structure. Numerous cables also run parallel from relay stations to field instruments. Analog signals are used, including A/D-D/A converters and microprocessor-based hybrid signals. Each instrument has a pair of wires connected to its I/O pins, which are then connected to a LAN from the control station. DCS has a three-tiered structure: control (engineer station), operation (operator station), and field instruments (field monitoring and control stations). It's used for large-scale continuous process control, such as in petrochemical industries.
