The difference between PLC vs DCS – DCS is a complete control system, while PLC is one control system component. In the past, the difference between the systems was significant. However, today’s PLC systems combined with a SCADA system and a few more tools are almost equivalent to a homogeneous DCS system.
This article will answer one of the lingering questions in industrial automation: What is the Difference between DCS vs. PLC?
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PLC and DCS comparison
One of the most frequently asked questions in industrial automation is related to the core components used for automating a machine or process – Programmable Logic Controllers (PLC) and Distributed Control Systems (DCS). Specifically, the difference between these automation products. This is usually asked when someone is assigned to select or identify which best suits an industrial application.
To answer this question, What is the difference between PLC vs DCS? It is necessary to compare both systems based on the usage and application, system composition and features, engineering and design, and cost-effectiveness.
Answers and explanations related to these aspects should provide the main comparison points in identifying the primary differences and give reference in choosing which automation component is practical for a particular industrial process.
Usage and Applications
Small to large applications
Hardware, Visualization Software, Reports, Historian
Cheaper, depending on the application size
Engineering and Deployment
Fast - Milliseconds per scan
Slow - 1 Min per scan
Limited - Can be solved by adding more PLC's
Almost unlimited - Also require more hardware
Exist on bigger PLC's
Exist by default
Low to High - depending on the application size
High - Therefore not suitable to small applications
High - Full flexibility
Low - DCS systems are more "close" for changes
Frequent process changes
High - PLC's can handle frequent changes
Low - due to the overall system complexity
The PLC vendor support only the PLC, Other component supported by other vendors
One vendor support the all system
What is the Difference between PLC vs DCS?
In the past, the difference between DCS and PLC systems was significant. PLCs were used only for fast real-time preprocess like safety systems, motors control, or material handling systems. Also, The PLCs were used only with Digital Inputs IOs because the technology for analog IO wasn’t there yet.
On the other hand, the DCS systems were used only for big, slow, and complex processes like heavy chemical plants. The use of Analog IOs was kept only for DCS systems. When a DCS needs a safety system, a dedicated PLC has to be added.
The technology in the industrial control sphere had shifted a few levels from then. Today a PLC system uses Analog IO without a problem, and PLCs are integrated successfully as the central control system in chemical plants and other complex processes. Thus, the two systems become very similar.
There are still a few differences that we will review:
Usage and Application
As defined by National Electrical Manufacturer’s Association (NEMA) standard ICS-1978, A PLC is a digitally operating electronic apparatus that uses a programming memory for the internal storage of instructions for implementing specific functions such as logic, sequencing, timing, counting, and arithmetic to control through digital or analog modules, various types of machines or process.
In simpler terms, a PLC is basically a computer designed to implement manual and automated machine or process control by executing the user logic downloaded to its programmable memory.
For example, Siemens and Rockwell Automation / Allen-Bradley both these brands manufacture a wide range of PLCs designed to cater to different application sizes such as small, medium, and large-scale applications.
Rockwell Automation / Allen-Bradley has the Logix automation platform, which includes MicroLogix, CompactLogix, ControlLogix, and the recently introduced Micro 800 controllers.
Siemens has the Simatic products, including S7-1200, S7-1500, S7-300, and S7-400, as active product offerings.
A Distributed Control System (DCS), on the other hand, is a fully functional system. It is a collection of hardware and software products explicitly used for medium to large-scale applications.
This is because DCS not only implements the same capabilities a PLC does but also provides Process Visualization, Data Acquisition, Historian and network security in one package.
Additionally, as applications become more complex and more critical areas are involved, DCS may easily be the only choice. The packaged components of a DCS may be engineered quickly to form a robust architecture with a high level of integrity and availability.
System Composition and features
A PLC system may be composed of a variety of Hardware components to function appropriately. This hardware may vary depending on the application.
However, most commonly used include a Discrete Input (DI), Discrete Output (DO), Analog Input (AI), Analog Output (AO), and communication modules aside from the mandatory required modules such as Power Supply and Central Processing Unit (CPU).
On the other hand, A DCS makes use of a PLC as a component only. As previously mentioned, DCS is a collection of hardware and software products.
It leverages the flexibility of a PLC to implement control and combines this with other built-in functionalities of the system such as Visualization, Integration, Data Acquisition and Historian software, and communication management and commissioning tools.
It is also important to mention that while some manufacturers such as Siemens and Rockwell Automation use their PLC products as components of their own DCS platform, only a specific PLC category is qualified for use.
For instance, Siemens’ PCS7 DCS uses only S7-400 controllers, while Rockwell Automation’s Plant PAx utilizes the ControlLogix L7/L8 series controller. While this is true for Siemens and Rockwell Automation, Foxboro and Delta V DCS have different implementations as they have specific “PLCs” or controllers for their DCS.
For example, the latest Foxboro Evo DCS system boasts a control system that evolved directly from the previous Foxboro I/A series. At the same time, Delta V DCS uses the M-Series and the PK controller hardware. It is not common to find this hardware in non-DCS applications.
Engineering and Deployment
A DCS system is typically composed of hardware and software components of the same brand. This uniformity is accurate for Foxboro, DeltaV, Siemens, Rockwell Automation, and other brands. With this, compatibility among other components across the architecture is guaranteed, contributing to faster engineering.
One example of this compatibility is the built-in, pre-defined logic and visualization templates available for deployment within the PLC program and visualization design. The logic and visualization templates, being of the same brand, are designed to complement each other and may be re-used in multiple instances allowing fast and rapid integration – ideal for large-scale and critical applications.
A PLC system, being only hardware, depends on the supplied block library and doesn’t have an equivalent visualization template. Without this feature, forming a fully functional system requires separate addition of visualization software, which could be of the same or different brand.
This affects product compatibility and ease of integration as different visualization software tends to have other methods, templates, and functionality, thus, increasing engineering time.
In terms of data acquisition and integration to Visualization software, the integrated software components of a DCS simplify the collection of data – from acquisition to monitoring and control points. A control system engineer can seamlessly integrate logic and visualization templates, and communication management easily connects both ends. However, integration and data acquisition methods vary depending on the selected visualization component when using a PLC.
Workflow is new but also an important aspect nowadays. Foxboro Evo and Emerson DeltaV implement this for a faster and more efficient resolution of the control system issues. Designing a near fool-proof workflow is always achievable with modern DCS.
A workflow can be configured for different process events such as alarm, maintenance, and other specific instances. This ensures that concerned personnel will be notified accordingly based on hierarchy and level of criticality while giving ample time for each person to respond. With a PLC, workflow is still possible. However, it depends on the external workflow provider and the depth of support for a particular PLC brand.
Both PLC and DCS are designed and implemented by a Control System Engineer.
Distributed Control system (DCS) hardware is typically supplied with hardware components of higher specifications, a stronger CPU processing, and larger internal memory. In addition, as DCS is commonly utilized for large, critical, and demanding applications, most DCS systems use a redundant CPU or processor and redundant communication modules to establish high availability. Aside from this, I/O devices are designed to form redundancy, further increasing system uptime.
PLC, on the other hand, is entirely different. Therefore, one can choose a PLC with minimum specifications for small-scale and simple applications and higher specifications for larger applications. This is one of the primary reasons why PLCs come in different sizes.
For instance, Siemens S7-1200 is used for small-scale applications, while S7-1500 and S7-300 are compatible with medium-scale applications. Similarly, some hardware within the S7-1500 hardware range and the entire S7-400 product family are ideally used for large-scale applications.
Though PLCs come in different forms to cater to different application sizes, they do not come with visualization programs, communication management, integration, Data acquisition, and Historian software.
Considering the limited features of a PLC, can we say that a PLC has a lower investment cost than DCS? The answer is a YES and a NO.
YES, because PLCs can cater to small-scale applications. It is not practical to implement DCS in this application size as the embedded functionalities of a DCS may be of disadvantage and incur unnecessary costs.
No, because in highly critical and demanding requirements, including data management, high availability, security, and integrity, DCS is the ideal choice as the built-in functions of a DCS may cater to these requirements out-of-the-box without additional costs.
Distributed Control Systems (DCS) and Programmable Logic Controllers (PLC) are industrial automation components that cater to specific application areas.
DCS is a complete system, while PLC is essentially a component of a DCS.
The embedded features of DCS benefit the user by ensuring compatibility and enabling seamless integration for fast and efficient engineering of medium to large, critical applications.
However, PLCs cater to a broader application range of criticality. Combined with a SCADA system and a few more tools (like Report tool and Historian) from the same vendor (recommended) or as 3rd party solutions, they are almost equivalent to a homogeneous DCS system.
When using a PLC system with SCADA, the user can enjoy the best solutions in the market for each system component – for example, Using Controllogix PLC with Ignition SCADA and with OSI Historian. The DCS system doesn’t have this level of flexibility.
We at PLCynergy have a lot of experience with PLC systems or DCS based on PLCs, like Rockwell automation Plant PAx.
Yes, Most DCS systems support industrial protocols like Ethernet/IP, Profinet, and ModbusTCP. Therefore a PLC can be connected to a DCS system through those networks. Another option is connecting the PLC IO to a DCS IO.
No, Usually, a DCS will use Function Blocks Logic.
The DCS architecture is the way the component of the DCS system is connected and ware selected. From the number and size of the CPUs, the type and number of distributed IO to the number of user displays. The DCS architecture also refers to the network topology of the DCS System.
The two most significant advantages of DCS are:
- Scale: Suitable for big applications.
- Compatibility: All the components are from the same vendor.
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