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PLC Ladder Logic's Future Role in the Automation Industry

Ladder Logic or LL is the primary PLC or Programmable Logic Controller programming language. It is simple and represented graphically as coils and relay contacts. The biggest question is, "How well can these ladder logics serve complex automation that is needed by industries today or in the future, compared to other conventional programming languages?" The answer to this question is pretty complicated than just coils and contacts.

The origin of ladder logic

Automation has been in every industry way before Programmable Logic Controllers, usually using networks of relays in the brain of individual applications. There was not a programming role; these were engineers who design circuits and electricians to assemble these systems. These circuits would be appropriately documented in standard structures that resembled a ladder, hence the name ladder diagram.

When automated logic controllers arrived, it promised a lot of benefits when it comes to the automation industry, but nobody knew how to use them. Then, the experts got an idea to program Programmable Logic Controllers with ladder diagrams. It allowed the automation industry to repurpose existing skills for programming.

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Electricians and engineers could understand and read programming, and automation designs can mostly follow the same patterns that were used with relays. Before the advent of PLCs, the system relays took up most of the physical space in the cabinets. It cost a lot of money to purchase and required a lot of time to wire and setup.

These relays are also limited to on and off functions without capabilities in math, data collection or analog in relay logic (aside from some counting functions and sparse in special relays). Even the most complex and most extensive automation systems were limited by realities of physical relays, the language of LL did not have to do that much.

Today, processor chips inside PLCs can do more than relay information, at a lower cost of money, space, or implementation time. In turn, these programming languages used PLCs have grown to help reflect its increased capabilities. LL does a lot more in today's world compared to what it used to be. Programmable Logic Controllers are usually utilized for analog control, controlling motion, tracking part data (test results, bar codes, calibration), as well as a plethora of other tasks.

A lot of experts still considers ladder logic as the most dominant language in this industry. As languages evolved and automation industry has become more complicated by the minute programming Programmable Logic Controller has become a specialized job. Ladder logic is no longer a convenient language for engineers or electricians maintain and read, not it is a broadly existing skill set in the incoming workforce.

Other Programmable Logic Controller languages

Two communities influence programming in the industrial world: standard International Electrotechnical Commission 61131-3, or the industrial control standard used by International Electrotechnical Commission; and the PC programming. Both are usually seen as a complementary system to ladder logic, instead of directly opposed.

International Electrotechnical Commission 61131-3

As logic controllers become pretty popular, companies found the need for standards to help guide the programming industry. The International Electrotechnical Commission's committee addressed the demand with a standard IEC 61131, and other codes in part three.

It defined the four codes that usually were interchangeable: function block and ladder diagram, instructional list, and Structured Text, as well as a program organization language called the sequential function charts. Each language has its own weakness and strengths, and all of them can be used together within one program to support different functions.

There are certainly debates on which language is suited to certain tasks. Generally speaking, processes will operate as continue or discrete. The automation industry also tends to lean towards analog Input and Output (I/O) or mostly digital Input and Output.

PC languages

Languages that are drawn into automation from the PC industry tend to be used for peripheral/specialty apps. That may include programming automation peripherals like robots and cameras, as well as interfacing with databases. Structured Text or ST is very similar to Pascal, one of the most common PC codes at the time International Electrotechnical Commission 61131 was introduced.

Pascal has been out of the game for many years now, but it is an excellent example of how the automation world has borrowed from the PC industry. Current examples of these languages in automation include Java, C#, and C++/ These systems are pretty powerful but have not found a firm footing in the automation industry.

Common questions

Before comparing LL to other options, it is imperative to understand what programs really need to do. These programs have to do automation work, which most of the systems can do, but the world puts other demands from them as well. It boils down to the program readability. Ask these critical questions about these programming languages.

Can maintenance technicians or programmers look at codes and understand what these codes do?
Can problems be found immediately to help minimize downtime of systems?
When functionalities are desired, how easy is it to extend programs?

There are features of different codes that can affect a technician or programmer's ability to make a good and readable program. There are areas, in particular, can influence the selection of language.

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General knowledge

LL is a widely used system, which makes it a lot easier to find experts who can read the system. Other International Electrotechnical Commission 61131 languages show some preference within specific industries, but are not as widely understand or use. The PC systems that are used in most automation businesses tend to be pretty popular and have many support resources, but not always in industrial communities which diminishes potential advantages and benefits.

Internal documentation

Most, if not all, systems available are a way for programmers to explain their intent in plain English. In LL, it is mostly done with variables or tag comments and rung comments, which work well for simple rung comments, but can be less helpful for more extensive math or rungs. Other International Electrotechnical Commission 61131 languages differ but are usually excellent for documenting things they are good at.

For instance, Structured Text allows excellent documentations of loop structures and math. PC languages used to have a lot of functionality available with a lot of options for internal documentation. Sometimes even generate documentation automatically from the source code, creating manuals for programmers and technicians to understand and reference the big picture a lot quicker.

Loops and decision structures

At simple levels, programming comes down to "do these steps if, this situation happens." This type of decision structure is called "if-then," and it is pretty easy to understand and use in every language discussed. But what if the technician or programmer wants to do something multiple times? These structures are not well supported by LL, resulting in codes with poor readability. PC languages and Structured Text, on the other hand, used to be pretty good at all loop structures and common decisions.


Subtraction and addition are very easy, and it works fine in any language, but what happens when complex math is involved? LL supports arithmetic and essential functions like square root or absolute value, but as math calculations include more pieces, it tends to be less readable by users.

Comments get separated from the things they are documenting, as well as doing the same mathematical operation more than once is not good, since loops are not pretty. Structured Text, as well as PC languages, used to be more graceful with more prominent math functions.

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