What You Might Not Know About SoftPLCs




What You Might Not Know About SoftPLCs

There are misconceptions about what SoftPLCs can and cannot do.

A SoftPLC is a software version of a programmable logic controller (PLC). The term SoftPLC has been around for many years, and like many technical terms, it means different things to different people.


IEC 61131-3 standard


For many years, the control system market has seen the proliferation of a variety of programming languages ​​and development environments, different for each manufacturer. IEC 61131, the international standard for programmable logic controllers, was first published in 1992 (as IEC 1131). IEC 61131 standardizes programmable logic controller technology and covers equipment requirements, programming languages, guidelines for use, communications and functional safety.

The third part of IEC 61131 (IEC 61131-3) deals with the programming languages ​​used in programmable logic controllers. The standard originally defined five programming languages:


  • Ladder diagram, based on the most common form of PLC programming language.
  • Structured text, similar to high-level information technology (IT) programming languages ​​such as C or Pascal.
  • Function Block Diagram, a graphical language where users connect blocks of functionality together to create a program.
  • Sequential Function Diagram, another graphical language that allows the execution of a finite state machine.
  • Instruction List, another text-based language similar to assembly language. This is now obsolete in the standard.


The IEC 61131-3 standard defines a series of functions and data types that must be supported by all compliant programmable logic controllers. Functions are the basic building blocks of all programs and include arithmetic operations (eg, addition, subtraction), Boolean logic (eg, AND, OR, NOT), and programming structures such as loops, comparisons and decisions.

One of the results of the adoption of IEC 61131 was the recognition that the software and hardware elements of programmable logic controllers could be considered separately. This in turn created the SoftPLC concept.


Soft PLC Features


A SoftPLC combines the functions of conventional PLCs with those of data loggers, communication gateways and other elements such as human-machine interfaces (HMI) and web servers. In the early days of SoftPLCs, it was common to use industrial computer hardware as the platform. This approach is one reason why there are many misconceptions about what SoftPLCs can and cannot do, including:


  • Non-deterministic operation: Early industrial computer hardware tended to run Microsoft Windows operating systems. Because they are not deterministic, many people have decided that a SoftPLC could never be used in place of a conventional PLC.
  • Reliability and robustness: Experience with the reliability of the Windows operating system, combined with the potentially lower environmental specifications of PC hardware, has led many to discount this solution for industrial applications.
  • Security: Once industrial control system security became an issue, maintaining outdated operating systems, such as Windows NT on SoftPLC hardware, was a potentially limiting factor.


However, there are many variations of PC hardware and operating systems that can make up a SoftPLC platform. Real-time operating systems such as RTLinux, QNX, RTAI, Intime, FreeRTOS, and VXWorks provide the fault-tolerant, deterministic behavior expected in a conventional PLC.

Many PLCs that users think of as conventional are actually built on SoftPLC technology. The primary business model for SoftPLCs relies on original equipment manufacturers (OEMs) porting the SoftPLC runtime to their platforms and integrating it seamlessly into their configuration and monitoring ecosystems. wider.


Reasons to use SoftPLCs


Although there are some misconceptions, there are many other potential benefits of SoftPLC technology, some related to the OEM and some to the end user:


  • Time to market: OEMs can develop their applications and select or design their hardware systems in parallel, and finally port the SoftPLC runtime environment quickly and easily.
  • Versatility: The same SoftPLC application can be ported to different hardware and operating system combinations. This means that users can potentially develop and maintain a common application (or at least a library of common functions) that can run in different environments if needed. This allows end users to choose specific hardware solutions according to their needs, saving money on input/output (I/O) and interfaces.
  • Scalability: The abstraction of software and hardware allows the OEM to more easily update the hardware and operating system while maintaining software and application backward compatibility.
  • Standardization: Compliance with IEC 61131-3 simplifies user training and supports greater consistency in application development, helping to reduce ongoing maintenance costs. Even if a user changes a SoftPLC solution, using IEC 61131-3 compliant application code can significantly reduce the cost of migration. This also applies to communication protocols (such as IEC 61850, DNP3), which are often integrated into SoftPLC solutions.
  • Product differentiation: Many SoftPLC solutions have functionality in addition to IEC 61131-3. Examples include support for industrial protocols, built-in redundancy, and distributed operation capabilities.


Safety and Security Concerns


Safety and security will always be top concerns for any automation system component. As stated earlier, misconceptions regarding the security and safety of SoftPLC are mainly the result of early systems running on non-deterministic operating systems. It is possible to develop a resilient, secure, and safe solution using SoftPLC technology, and many OEMs have done so. When the OEM has all the source code, everything is under their control.

An advantage of SoftPLC solutions is that hardware and operating system choices can be made independently, depending on safety and security requirements.

Some SoftPLC solutions have even developed runtimes that OEMs can integrate into systems targeted for IEC 61508 certification. develop the runtime itself using approved methods.


Final Thoughts


For those who haven’t looked at SoftPLC solutions in the past few years, it’s worth revisiting. Much has changed in the 30 years since the inception of IEC 61131-3, and many popular “conventional” PLCs run on SoftPLC technology. As with any solution choice, requirements should guide the response, but SoftPLC-based approaches should be included in any consideration.

This feature was originally published in the February 2022 issue of InTech magazine.



About the Author



Steve Mustard, CAP, has worked in the software development field for over 25 years, including embedded software and hardware development for military applications and product development for industrial automation and control systems. Mustard is a member of the AF government relations committee and the ISA99 committee.

Pascal Girerd is a business developer at Straton Automation, which provides a software-based API with many benefits for industries such as utilities and embedded automation.






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