One of the most common questions in industrial control and automation systems is “what’s the difference between a PLC and a DCS? In order to fully understand the difference between these two systems, we must look at the history of Programmable Logic Controllers (PLC) and Distributed Control Systems (DCS), and break down their internal components.
The main difference between PLC and DCS is that a PLC controls individual machines while a DCS controls multiple machines within a factory or plant. However, this is just a basic explanation. Let’s go a little deeper and examine the important differences that are less commonly known.
PLCs were invented to replace the older version of a control system that relied on relays and contactors in order to function. DCS, or distributed control systems, were invented to accommodate additional needs within the automation industry. They were not introduced to replace PLCs, but rather coexist with them and enhance the functionality of a plant or factory.
The need for PLCs sprung up because the old systems lacked flexibility and required an inordinate amount of time to fix issues. The outdated systems were difficult to troubleshoot due to dirty connections, loose wires and cryptic documentation. Control systems were still handled by using relay controls, which meant that entire rooms had walls covered in relays, terminal blocks and cords. Many of the issues with the relays and contactors sparked the old saying “five hours to find it, five minutes to fix it.”
People needed a much more efficient model. They needed something that could reduce machine downtime, work in an industrial environment that was notorious for its dirt, moisture and debris, and also allow for modular expansion. In 1971, Allen-Bradley created a new design called the Bulletin 1774 PLC. While other companies were designing their own prototypes and making waves, Allen-Bradely became synonymous with PLCs for their innovation. However, Richard Morely, who worked with Modicon, is generally referred to as the father of the PLC. He’s credited with designing the original ladder-logic programmable controller, and the creating ladder logic programming,
Process control was still expanding in the early 1960s. The biggest change occured when the company Imperial Chemical Industries replaced their analog control system with a computer. This meant that a computer could actually perform the control tasks, instead of an analog system supervising the tasks. This concept became known as Direct Digital Control (DDC) and proved to be more cost-effective as well as more efficient. DDC languages emerged in large numbers. Because of its simplicity, it became known for its use in building automation tasks like heating and ventilation. Even today, DCS systems are rooted in the early DDC languages.
Up until the 1980s, technological advances had mainly been focused on replacing analog control systems with computers. While this was revolutionary, the next step was to create a centralized system where everything could be controlled. Midac was one of the first companies to achieve this, creating an advanced building automation system for the University of Melbourne. The system had a system with microprocessors sharing tasks and memory.
However, in the 90s with the domination of Microsoft, DCS companies learned to become more software oriented, understanding that continuing to produce some of the hardware was no longer viable—and this is still going on today.
PLCs have a fairly simple operating system. They can complete tasks fairly quickly because there are no other programs in the background demanding its attention. PLCs also tend to be quicker because they’re close to the device that they’re meant to control, such as motors and pumps. PLCs work to control a single machine; it can focus its attention on a single task and operate machine processes. This is much different than a DCS, which controls multiple machines at once in a large setting.
On top of that, PLCs tend to be more flexible and can be customized to suit a specific need, where DCS generally cannot. PLCs tend to use lower level languages. They are simple yet offer fewer functions. This includes ladder logic, function block diagram, and structured text language. The generated codes are markedly smaller compared to the codes the DCS languages create. DCS uses high-level programming languages like CFC or “continuous functioning chart” to issue its programming and monitoring. This is very different from a PLC, which uses lower level language. The downside of this is that high-level languages generate lots of codes after being compiled, which puts a strain on the CPU. This causes a delayed response to important inputs.
PLC monitoring systems differ from DCS even more because there’s no relationship between the programming and the monitoring environments. They each need their own time to be created and developed. The DCS also has a different monitoring system than the PLC. The DCS has an integrated software package in which there is a robust set of monitoring facilities. The control functions are within the programming environment, meaning the corresponding graphics will automatically be added to the HMI page. The monitoring systems function in a way that saves precious time.
The most basic difference between a PLC and a DCS is that PLCs were designed to replace relay-based controls and analog equipment, controlling one machine. DCS, on the other hand, are used to control entire plants, buildings, or processes that are vast and interconnected. While the main difference between DCS and PLC is how they’re used and what they’re used for, there are other less obvious differences that you should be aware of if you’re interested in this world.
As a PLC technician, understanding these differences will make you stronger in your career and build up your expertise. If you want to learn more and expand your knowledge about the exciting world of PLCs, check out our PLC Technician Training Program. The program prepares students for a career as a PLC technician and is great for those who are new to PLCs or who already have experience in the field.
With the advancement of microprocessors and automation technology, the lines have blurred between the Programmable Logic Controller (PLC) and Distributed Control Systems (DCS). It is more challenging than ever for process manufacturers to select the best technology for their application.
With most discrete automation applications, the PLC is the heart of the system. It contains the logic to move the products through the machining or assembly line. The PLC can handle high-speed applications that require scan rates of 10 milliseconds or less, including operations involving motion control, high-speed interlocking, or control of motors and drives. Fast scan rates are necessary to be able to effectively control these devices. The Human Machine Interface (HMI) provides the operator with supplemental information or exception data.
In process automation where the environment can be volatile and dangerous and where operators can’t see the actual product, the HMI is considered the heart of the system. It provides a window into the process and operators play a more active role utilizing the HMI to monitor and control the process. The DCS is not designed to handle high-speed applications like the PLC can. The regulatory control loops normally scan in the 100 to 500 millisecond range. To have the control logic execute faster could cause excessive wear on final control elements such as valves, resulting in premature maintenance and process issues.
PLC’s high level programming languages allow for creating custom logic. The PLC comes with an extensive variety of functions and elemental building blocks that can be custom developed and chained together. Creation of the custom code from scratch allows for integration of functions and products into a seamless architecture.
A traditional DCS does not have PLC like flexibility. The traditional DCS has pre-engineered solutions consisting of standards, templates, and extensive libraries “out-of-the-box”. A DCS trades unlimited functionality and customization for repeatability and dependability. The system is expected to function as a complete solution with standard functions.
The PLC has flexibility and open architecture whereas the DCS has an intuitive programming platform, which utilizes predefined and pretested functions to save time and drive repeatability. The PLC’s ladder logic is best for discrete control applications. The DCS’s function block diagram is preferred for continuous control and implementing alarming schemes.
If your application would be best served using both PLC and DCS you may need a process control system for hybrid applications. In the old days if you chose to go with a DCS you were locked into that company and the flexibility of integration of the PLC open architecture world was not an option. Not true today, as we look at the Siemens SIMATIC PCS 7.
The open architecture of SIMATIC PCS 7 process control system enables full integration of all the automation systems in your plant: process, batch, discrete and safety, and all the field devices; instrumentation, analytics, motors, drives, and safety into a single platform, with common tools for engineering, visualization, and facility-wide asset and maintenance management.
The PCS7 platform is essentially software extensions for the Siemens S7-400 PLC and Siemens HMI running WinCC. These software extensions come in the form of function libraries and software tools that are used to create a high-level software solution that when compiled runs on the same hardware platform as a traditional PLC. The controller software and HMI software are closely integrated and much of the HMI functionality is automatically generated based on the content of the controller program.
From a development standpoint, there is an extensive and comprehensive software library that is used to develop the overall software solution. Very little, if any, lower-level programming is required. Development time is reduced because of the software library and the fact that much of the HMI functionality is automatically generated. From a safety standpoint, hardware and software redundancy is fully supported by PCS7.
The standardization of much of the control logic may reduce the time/cost of a new engineer picking up an existing project and understanding it. There could be significant cost savings from systems with heavy redundancy or duplicate hardware and process.
While Siemens SIMATIC PCS 7 distributed control system has been a proven workhorse trusted by process industries for decades, Siemens has continuously updated the robust system architecture including creating an additional building block in route for the Digital Enterprise. The latest version of SIMATIC PCS 7 is Version 9.1.
SIMATIC PCS 7 Version 9.1 now provides users with an updated master data library, called the Advanced Process Libraries (APLs), which includes a variety of standardized and system-tested Control Module Types (CMT) and Equipment Module Types (EMT). As opposed to the first block templates, these new modules can increase the potential and flexibility of the single CMTs. While some of the APLs are specifically built for larger customers, most of them are general APLs for general application, which include functions, function blocks, data types, and data blocks for various continuous and sequential function charts (SFC).
With the end of support for some operating systems, plant owners and operators face increased security risks. SIMATIC PCS 7 Version 9.1 tackles this challenge by supporting Windows Server 2019 Standard Edition and Windows 10 Enterprise 2019 LTSC, ensuring compatibility with current workstation Industrial PCs (IPCs). This will make sure that the installation of the latest Microsoft updates are more simple and convenient. Additionally, SIMATIC PCS 7 V9.1 is now friendly with Windows Defender Antivirus, better protecting the system from attacks.
PCS7 was developed specifically for process control applications. The PCS7 is not used in place of a PLC, but it is a tool or programming method for using a PLC. A PLC is still used in conjunction with the process control system, but PCS7 is used to build the control software and interrelations. PCS7 is more likely to replace a system HMI than the PLC. PCS7 uses WinCC to provide a SCADA solution for supervisory control and the acquisition of data for a process environment. In general, the types of projects you would use PCS7 would be Chemical, Energy, Utilities, Food and Beverage, and Pharmaceutical.
As a Siemens Solution Partner, Patti Engineering has the expertise to help you evaluate the automation technologies available to help you determine the best solution for your business to operate at peak efficiency. The right solution can make all the difference to your return on investment and bottom-line profits.
Editor’s Note: This newsletter was originally published in November 2011 and has been updated for comprehensiveness and clarity.
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