7 Questions to consider when selecting PLC-SCADA or DCS

The evolving landscape of industrial automation has blurred the lines between Programmable Logic Controllers (PLC), Supervisory Control and Data Acquisition (SCADA) systems, and Distributed Control Systems (DCS). As technologies advance and integration becomes more seamless, choosing the right system can dramatically affect efficiency and performance. Here are seven crucial questions to consider when making the decision between PLC-SCADA and DCS.

Want more information on plc screen? Feel free to contact us.

Supervisory Control and Data Acquisition (SCADA)

This discussion focuses on SCADA packages used as HMIs above PLCs, rather than Enterprise SCADA systems. Typically, Enterprise SCADA systems are much larger and are used in applications such as public transport, water distribution, and pipeline monitoring, requiring tens of thousands of I/O points and multiple user interfaces.

Convergence

Historically, PLC, SCADA, and DCS systems were supplied by separate manufacturers. However, the top PLC companies—Siemens, Rockwell, and Schneider—are now also leading suppliers of SCADA and DCS. This convergence has unified I/O, controllers, and software, complicating the selection process. Companies like Honeywell, ABB, and Emerson have released PLC-equivalent products, while others continue to adapt to smaller DCS systems to compete in PLC markets.

Another notable shift is in the implementation. Traditionally, PLC-SCADA systems were installed by Systems Integrators (SIs), while DCS implementations were handled by the manufacturers themselves. However, this too is evolving with the emergence of certified SI partner networks by companies like Siemens. Enhanced capabilities and easier configuration make modern PLCs a formidable option, although players like Mitsubishi and Omron have chosen not to enter the DCS market directly.

The next 7 questions serve as guidelines to aid in decision-making between PLC-SCADA and DCS systems.

QUESTION 1 – What is being manufactured and how?

1. Manufacturing or assembly of specific items
2. Visible product movement through the process
3. High-speed logic control (such as motors)
4. Simple batch control
5. Often referred to as Factory Automation

These points lean towards PLC-SCADA, whereas:

1. Involves the combination and/or transformation of raw materials
2. Invisible product movement through the process
3. Regulatory/Analog (loop) control
4. Fluid dynamics
5. Complex Batch Control
6. Often referred to as Process Automation

Lean towards DCS.

QUESTION 2 - What is the value of the product being manufactured and the cost of downtime?

1. Value of each individual component being manufactured is relatively low
2. Downtime mainly results in lost production
3. Downtime does not typically damage the process equipment or is unsafe to personnel

These points lean towards PLC-SCADA, whereas:

1. Value of continuous product or batch can be very high
2. Downtime not only results in lost production, it can result in dangerous conditions
3. Downtime can result in process equipment damage (product hardens, etc.)

Lean towards DCS.

QUESTION 3 - What is viewed as the heart of the system?

If the controller is the focal point, this points to PLC-SCADA, which can operate independently without an HMI. Conversely, if the HMI is the heart, this would indicate a preference for DCS. DCS are typically integrated systems that include both controller(s) and HMI(s).

QUESTION 4 - What does the operator need to be successful?

1. The operator's primary role is to handle exceptions
2. Status information (on/off, run/stop) is critical information for the operator
3. Exception-based alarming is key information for the operator
4. Manufacturing could run without the operator and the HMI

These points lean towards PLC-SCADA, whereas:

1. The operator's interaction is typically required to keep the process in its target performance range
2. Face-plates and analog trends are critical to "see" what is happening to the process
3. Alarm management is key to safe operation of the process and for responding effectively during plant upset conditions - typically meeting EEMUA 191
4. Failure of the HMI could force the shutdown of the process

Lean towards DCS.

QUESTION 5 - What system performance is required?

1. Fast logic scan (approx. 10ms) is required to perform motor or motion control
2. Bumpless redundancy is not normally cost justified
3. System can be taken offline to make configuration changes
4. Simple PID control
5. Diagnostics to tell you when something is broken

These points lean towards PLC-SCADA, whereas:

1. Control loops require a deterministic scan execution at a speed of 100 to 500ms
2. Bumpless system redundancy is often required at many levels
3. Online configuration changes are often required
4. Advanced PID control up to advanced process control and self-tuning
5. Asset management alerts user to what might break before it does - predictive maintenance

Lean towards DCS.

QUESTION 6 - What are the engineering expectations?

If you are looking for more details, kindly visit Android Industrial Mini Pc.

1. Solution is generic in nature, to be applied on a wide variety of applications
2. System designed to be flexible
3. Use Ladder Logic to configure application
4. Database/Configuration separate for PLC and HMI and communications between the two must be configured
5. Customized routines usually required

These points lean towards PLC-SCADA, whereas:

1. Many algorithms (i.e. PID) are very complex and don’t vary application to application
2. Standard application libraries are expected (function blocks, face-plates, symbols) – significant out-of-the-box functionality – use of pre-defined, pre-tested functions saves time
3. The entire system (controller and HMI) is expected to function as a complete entity
4. Able to undertake upfront design/configuration of complete system before implementation begins

Lean towards DCS.

QUESTION 7 - What are the integration requirements?

1. Application is homogeneous i.e. all control or all safety (unless for machine safety)

Additional criteria:

1. Signals are safe area, simple instruments or mostly digital
2. A small number of third-party PLCs or other systems link to the main PLC

Points to PLC-SCADA, whereas:

1. Application is mixed control and safety on a common platform
2. Signals are hazardous area and smart instruments - remote central diagnostics/configuration is required
3. Integration of a large number of soft I/O from other PLCs and third-party systems is required

Lean towards DCS.

Conclusions

1. Traditionally, PLC-SCADA systems have been used for smaller, mainly digital applications implemented by SIs. However, PLCs have become more powerful, capable of handling larger and more complex analog applications.
2. Similarly, DCS has been the go-to for larger, mainly analog applications, typically implemented by the DCS manufacturer. However, DCS companies have introduced "entry-level" systems and PLC equivalents that can handle smaller applications, recruiting a network of SI partners to deliver these systems.
3. While PLC-SCADA hardware and licenses have traditionally been cheaper, DCS companies argue this is offset by reduced engineering hours due to integrated controller/HMI databases, libraries, symbols, face-plates, and other pre-configured elements. Nevertheless, DCS configuration is often considered more structured and easier for future engineers to understand.
4. The automation market has seen significant consolidation, with smaller SCADA, PLC, and DCS companies being acquired. Major players like Siemens and Schneider have established their presence across all technologies, with experienced SI partner networks. Rockwell is strong in PLC-SCADA but newer to DCS, while Honeywell, Emerson, ABB, and Yokogawa dominate the DCS sphere but continue developing smaller automation solutions and partner networks.
5. The lines between PLC-SCADA and DCS blur further when PLC and SCADA are sourced from the same manufacturer. Integrated configuration before hardware delivery, using libraries, Ladder, FBD/STL, and higher-level languages like JavaScript and Graph, is possible. Examples include the Siemens TIA Portal, which offers a cost-effective simulation package for virtual testing and commissioning. Most of these benefits are lost when buying PLCs and SCADA from different manufacturers.

PLC, SCADA and DCS Systems and Companies

References

PLCs Programmable Logic Controllers - A Complete Guide

PLC Programming

Different models are suitable for use with varying programming languages. Common options include:

• ST: Structured Text (text-based)
• FBD: Function Block Diagram (graphic-based)
• LAD: Ladder logic (graphic-based)
• STL: Statement List (text-based)
• SCL: Structured Control Language (graphic-based)

PLC programming courses and tutorials are widely available if you are new to the world of logic and programming languages. FBD is a popular choice for beginners as it utilizes drag-and-drop functionality, making I/O behavior easy to understand. Visual, image-based languages such as Ladder logic can aid in understanding and improving processes. Conversely, SCL is primarily used for more complex and demanding programming requirements.

Although programming is usually done via a laptop and software, some older models can be used with handheld programmers.

PLC Software

The control program is created on a computer or via a connected control panel and loaded onto the controller via an interface. The defined switching of output devices and input devices is then handled autonomously by the PLC, which has its own power supply.

Specific functions of PLCs include:

• Flow control
• Link control
• Number-based functions including time and counting

Programmable logic controller software can be categorized as follows:

• Cycle-oriented PLCs - Follow the basic principle of data processing with input, processing, and output. Inputs are queried, control is passed to the user program, and the process restarts after control signals are transferred to the outputs
• Cyclic PLCs with interrupt - Programmed to run an additional program loop adapted to a different situation if the connected sensor's status changes. The main program continues once the additional loop completes
• Event-controlled PLCs - Designed to process specific pre-programmed tasks following a status change of the connected sensor(s)

Note that PLC programming software is not standardized—it is specific to a manufacturer, and often a specific model. This means proprietary serial cables or other accessories may be required.

Contact us to discuss your requirements for a medical grade computer. Our experienced sales team can help you identify the options that best suit your needs.