Choose the right programming language

Which language should you choose for use with your programmable controller? Among the five languages defined in IEC 61131-3, generally ladder diagrams or ladder logic is most widely applied in North America. Other languages have practical applications and should not be overlooked. Choice depends on programmer's skill, the programming task, the level and structure of the problem and control syst...

07/01/2003


Which language should you choose for use with your programmable controller? Among the five languages defined in IEC 61131-3, generally ladder diagrams or ladder logic is most widely applied in North America. Other languages have practical applications and should not be overlooked. Choice depends on programmer's skill, the programming task, the level and structure of the problem and control system, who needs to interact with the program, and, perhaps, how often it's modified.

Since inception in 1992, PLCopen has helped promote and support programming standards, which allows, the association says, for less training, more logical organization, modularization, and use of modern software techniques. "Each program is structured, increasing its reusability, reducing errors, and increasing programming and user efficiency," according to the group.


Four languages describe the same piece of code.

"Also, the standard allows two ways of developing your program: top down and bottom up. Either you specify your whole application and divide it into subparts, declare your variables, and so on. Or you start programming your application at the bottom, for instance via derived functions and function blocks. Whichever you choose, the development environment will help you through the whole process," PLCopen says.

The five elements of IEC 61131-3 are:

  • Sequential function charts (SFC)—Rather than a language, SFC is more of a graphical method of organizing control programs.

  • Ladder diagram (LD)—most used in North America, it graphically represents rungs of contacts, coils, and special instruction blocks. Its origin is relay-ladder logic.

  • Instruction list (IL)—a text-based language similar to assembler. This is the European counterpart to LD.

  • Structured text (ST)—a text-based language similar to Pascal.

  • Function block diagram (FBD)—a graphical language corresponding to a circuit diagram. FBD is widely used in process industries.

Other resources

Several IEC standards provide more information about function blocks: IEC 61499 and IEC 61804, which focuses on the process industry. Function blocks encapsulate algorithms so they can be more easily understood and applied by those who aren't software specialists.

For more on function blocks, see www.controleng.com/issues , 2002, September, "Component Automation Enables Modeling and Control." Also, in the August 2003 issue, look for an article on multiple-platform programming software. See related items at www.controleng.com/tutorials.

IEC, at www.iec.org , publishes "Programmable controllers - Part 3: Programming languages." IEC 61131-3 "specifies syntax and semantics of programming languages for programmable controllers as defined in part 1 of IEC 61131." Price is about $205. A related IEC publication is "Programmable controllers - Part 8: Guidelines for the application and implementation of programming languages."

PLCopen, at www.PLCopen.org focuses on control programming and participates on technical committees to evolve programming standards.

Comments email MHoske@cfemedia.com



Pros and cons of IEC 61131-3

Benefits and drawbacks of IEC 61131-3 include the following, according to Wolfgang Langer, Schneider Electric software product manager:

Benefits are:

Less retraining costs—IEC looks and feel similar among vendors;

Greater focus on problem solving and software reusability;

Fewer misunderstandings and errors in programming when switching between languages; all IEC languages work the same; and

Greater community—many tools from independent suppliers can be used. PLCopen drives IEC standard with vendors to comply with interoperability standard.

Drawbacks include:

Too many optional features;

Does not define implementation limitations for the size of a page value to define a program;

Doesn't define the minimum of subsets needed to implement;

Doesn't define minimum limitations while implementing, such as how many rungs must be supported in one section; and

Doesn't define conversion among languages.

David Greenfield dgreenfield@reedbusiness.com



No comments
The Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by...
Each year, a panel of Control Engineering editors and industry expert judges select the System Integrator of the Year Award winners.
Control Engineering Leaders Under 40 identifies and gives recognition to young engineers who...
Learn more about methods used to ensure that the integration between the safety system and the process control...
Adding industrial toughness and reliability to Ethernet eGuide
Technological advances like multiple-in-multiple-out (MIMO) transmitting and receiving
Big plans for small nuclear reactors: Simpler, safer control designs; Smarter manufacturing; Industrial cloud; Mobile HMI; Controls convergence
Virtualization advice: 4 ways splitting servers can help manufacturing; Efficient motion controls; Fill the brain drain; Learn from the HART Plant of the Year
Two sides to process safety: Combining human and technical factors in your program; Preparing HMI graphics for migrations; Mechatronics and safety; Engineers' Choice Awards
The Ask Control Engineering blog covers all aspects of automation, including motors, drives, sensors, motion control, machine control, and embedded systems.
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
News and comments from Control Engineering process industries editor, Peter Welander.
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
This is a blog from the trenches – written by engineers who are implementing and upgrading control systems every day across every industry.
Anthony Baker is a fictitious aggregation of experts from Callisto Integration, providing manufacturing consulting and systems integration.
Integrator Guide

Integrator Guide

Search the online Automation Integrator Guide
 

Create New Listing

Visit the System Integrators page to view past winners of Control Engineering's System Integrator of the Year Award and learn how to enter the competition. You will also find more information on system integrators and Control System Integrators Association.

Case Study Database

Case Study Database

Get more exposure for your case study by uploading it to the Control Engineering case study database, where end-users can identify relevant solutions and explore what the experts are doing to effectively implement a variety of technology and productivity related projects.

These case studies provide examples of how knowledgeable solution providers have used technology, processes and people to create effective and successful implementations in real-world situations. Case studies can be completed by filling out a simple online form where you can outline the project title, abstract, and full story in 1500 words or less; upload photos, videos and a logo.

Click here to visit the Case Study Database and upload your case study.