How do you choose between ladder logic and structured text for a PLC project?

The argument against ST that "you cannot see the status of each logic element" is not entirely correct. It's dependent upon the platform you're using. If you can't see real-time variable status in ST, then your platform is the problem, not the programming language. In that case, it's time to change your platform. ST is the way to go in most cases. It ports more easily to other platforms and is more suitable for complex coding algorithms. If one's argument is that electricians and technicians can more easily read and troubleshoot LD versus ST, then perhaps the company shouldn't be hiring these persons to do the job of a programmer. A real Controls/Automation programmer should be expected to be full-stack programmer, not just Ladder logic

As automation systems become more advanced and interconnected, it is important for technicians to learn and use structured text programming in addition to ladder logic. While ladder logic is still widely used and understood, it has limitations in handling complex calculations and logic functions. Structured text programming allows for more advanced and efficient programming, making it ideal for today's more complex automation systems. By learning both languages, technicians can expand their skillset and effectively program and troubleshoot a wider range of systems.

In the U.S., ladder logic is commonly used in controls engineering. I recall a situation at my job where we initially chose Structured Text for a control system. Despite its technical advantages and appeal among engineers, this choice soon proved problematic. The majority of end users, technicians, and engineers were more accustomed to Ladder Logic. We faced difficulties in finding experts for modifications and troubleshooting. Ultimately, the decision to use Structured Text led to extra costs, as we had to convert the system back to Ladder Logic, a more universally understood language in our industry.

I believe the choice between Ladder Logic and ST depends on the complexity of the control tasks in the automation system. Ladder logic is ideal for simple and sequential control tasks while ST is good for complex mathematical calculations and algorithms. In some applications a combination of both Ladder Logic and ST might be used within the same PLC.

An example of an advanced program that cannot be performed in ladder logic and requires structured text is implementing a model-based control strategy, such as a predictive or adaptive control algorithm. These algorithms require sophisticated mathematical calculations and data processing that cannot be easily achieved using ladder logic. Structured text, with its support for data types, arrays, and functions, allows the programmer to create complex algorithms that can learn and adapt to changing conditions and improve the performance of the automation system

Je ne suis pas d'accord, avec plus de 10 ans d'expériences et ayant mis en place et où modifier des systèmes entiers sur du process industriel complexe, tu pourras faire exactement la même chose avec le ladder ou FBD, la seule différence est que le ladder demande plus de ressources. Les avantages du ladder ou du FBD est que ce sont des langages axés pour des électriciens ou électroniciens avancés pouvant mettre en avant leurs logiques sur un programme, d'ailleurs c'est une évolution en soit , ?a été mon cas et pour pas mal de mes collègues. Tu peux absolument tout faire , que ce soit du calcul , du traitement de données, conversation, du pointage d'adresses pour envoyer et recevoir des datas et même créer des fonctions Complexes .

One potential issue with ladder logic is that it can be difficult to modify or update, especially as the system becomes more complex. Each rung and symbol must be manually edited or added, which can be time-consuming and error-prone. Additionally, ladder logic may not be as easy to read and understand as other programming languages, such as structured text or function block diagram, for more experienced programmers. Finally, ladder logic may not be the best choice for systems that require high-performance or real-time processing, as it can have longer scan times compared to other languages.

Additionally, writing structured text can be time-consuming, and it requires more technical knowledge and expertise in programming and software development. It can also be harder to read and understand by other technicians or stakeholders who are not familiar with the language. Therefore, the choice between ladder logic and structured text depends on the specific requirements, constraints, and goals of the automation system, as well as the skills and preferences of the designers and operators.

In many platforms Structure Text implementation you can't examine the code while it's running like you can with Function Block and Ladder Logic. You need to use on-line data displays to see your variables' values. For us then Structure Text is used to set variables used as settings (transmitter ranges; alarm and trip setpoints; and default on or off delay timers, such as for alarm/trip activation or delays for clearing alarm and trips).

It’s important to note that both ladder logic and structured text have their own unique strengths and weaknesses, and the decision to choose one language over the other should be based on a careful evaluation of the project requirements, constraints, and goals. Additionally, it’s worth considering the long-term maintainability and scalability of the code, as well as the availability and support of the language and platform in the industry. It may also be useful to consult with other engineers or experts in the field to gather insights and perspectives on the best approach for a given project.

Modern electronic control systems are becoming increasingly modular, meaning that faulty modules can be easily replaced rather than repaired. As a result, there is less of a need for electricians to troubleshoot and repair individual components using ladder logic. Additionally, as automation systems become more complex, structured text is becoming more necessary to program and control the advanced functions of these systems. As a result, it is becoming increasingly important for technicians and engineers to learn and use structured text in order to keep up with the demands of modern automation.