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What is a PLC (Programmable Logic Controller)?

By Haley Zeigler

April 10, 2024


A PLC (Programmable Logic Controller) is like a mini computer used to control machines and processes in places like factories. It's made to handle conditions like extreme temperatures, electric interference, and shaking, which are common in industrial settings.

A PLC works by gathering information from different sensors. It then follows pre-set instructions to control a process or lets you know what's happening so you can make changes. Because it's controlled by software, it's easy to change how it works. This makes it both flexible and cost-effective for managing different tasks.

In this article, I'll explain more about how PLCs work, how to program a PLC, and potential alternatives to PLCs. Picking the right tech will boost how well things run, cut down costs, and make everything more reliable, leading your projects to success.

What Does a PLC Do?

How PLCs work diagram

Imagine a PLC is like the brain that tells factory equipment what to do and when to do it. They collect real-time data to make sure everything runs smoothly, checking on things like how long a device has been running, the temperature, and how much it's being used.

To make sure everything in the process is working right, a PLC checks data from different sensors. It then uses its programmed instructions to decide what action to take. For example, it may adjust the machine or give real-time updates to help you make quick decisions. This is all done with software, making it easy to change things up fast and without spending a lot, to keep up with new needs.

PLCs are great at making automation smoother and making operations run better. They're also made to work well with other control systems, like SCADA, which helps manage and gather data. Through something called a Human Machine Interface (HMI), people like operators or managers can interact with the PLC in real-time, using a dashboard to watch and control the industrial activities and machinery.

How Much do PLCs Cost?

The average price of a PLC itself sits around $100-$200. The thing that is more costly about using a PLC is the time required to manually program each unit.

When to use a PLC?

PLCs are best used when there are many machines working together. For example, in factories with many moving parts. However, PLCs can also be used for smaller scale projects like controlling home automation systems.

Here's a list of facilities where PLCs are commonly deployed:

  • Manufacturing Plants
  • Oil and Gas Industry
  • Power Generation
  • Automotive Industry
  • Food and Beverage Production
  • Pharmaceuticals
  • Warehouse and Logistics

By integrating PLCs or an alternative system (like an RTU), you can increase operational efficiency, safety, and reliability.

How Are PLCs Programmed?

A Programmable Logic Controller (PLC) can be programmed in different ways. Ladder logic is the most popular language for programming PLCs because it looks like electrical circuits, making it easier to understand and visualize how things work in the PLC. This makes controlling processes simpler. Besides ladder logic, there are other programming languages that let you customize and design control systems, offering flexibility in how you set things up.

What Programming Languages Can I Use to Program a PLC?

Programming languages commonly used for programming PLCs (Programmable Logic Controllers) include:

  • Ladder Logic (LAD): Ladder Logic is the most widely used programming language for PLCs. It resembles electrical relay logic diagrams and is easy to understand for engineers with a background in electrical engineering. Ladder Logic is well-suited for designing simple to moderately complex control logic.
  • Structured Text (ST): Structured Text is a high-level programming language that resembles Pascal or C programming languages. It allows for more complex control algorithms and is suitable for tasks requiring mathematical calculations or data manipulation.
  • Function Block Diagram (FBD): Function Block Diagram is another graphical programming language used in PLC programming. It allows for the creation of reusable function blocks, making it suitable for modular programming and complex control tasks.
  • Sequential Function Chart (SFC): Sequential Function Chart is used for sequential control applications where a process needs to follow a specific sequence of steps. It is often used in conjunction with other programming languages for designing complex control systems.
  • Instruction List (IL): Instruction List is a low-level programming language that uses mnemonic codes to represent PLC instructions. It is less common than Ladder Logic or Structured Text but may be preferred by programmers with a background in assembly language programming.
  • Structured Control Language (SCL): Structured Control Language is a textual programming language similar to Structured Text but with additional features for control system programming.
  • Graphical Function Block Diagram (GFBD): GFBD is a graphical programming language used in some PLC programming environments. It allows for the creation of function blocks using graphical elements and connecting them to create control logic.

Different PLC manufacturers may support different programming languages, so the choice of language may depend on the specific PLC model and manufacturer's software environment. Additionally, some PLC programming environments may support multiple languages, allowing programmers to choose the most appropriate language for their application.

Advantages of PLCs

  • Flexibility: Easily updating PLCs means you can change production processes quickly, without losing much time.
  • Reliability: PLCs work great in tough conditions without breaking down, saving the need for frequent replacements.
  • Efficiency: Using PLCs cuts down on mistakes and boosts how well and consistently factories run.
  • Communication: PLCs can talk to other systems to help manage and make better decisions in factories.
  • Cost-Effective: Even though they're expensive at first, PLCs save money over time by reducing errors and maintenance costs.
PLCs image
PLCs are best implemented in controlled environments such as factories, with a knowedgeable programmer to configure it.

Disadvantages of PLCs

  • Initial cost: Units are modestly priced, but expenses rise with programming, installation, and integration.
  • Specialized knowledge: Requires in-house expertise or hiring external professionals, increasing operational costs.
  • Vulnerability to extreme conditions: Despite robustness, PLCs can be damaged, leading to downtime and maintenance costs.
  • Obsolescence risk: Rapid technological advancements can make PLCs outdated, necessitating costly upgrades or replacements.

What are Some Alternatives to PLCs?

If all of that sounded like a foreign language to you, but you still need a way to monitor and control your remote gear, you're in luck. There are solutions that don't require a tech-savvy programmer to implement.

For individuals or industries seeking alternatives to programmable logic controllers (PLCs), the following options can offer simplified or tailored solutions for managing and controlling automated processes:

  • Remote Telemetry Units (RTUs): Designed for remote environments, RTUs efficiently gather and transmit data for monitoring and control. RTUs are already programmed and configured, and can be easily interfaced via web or a control master.
  • Microcontrollers: The core of modern electronic devices, offering a cost-effective solution for automated control in small devices.
  • Distributed Control Systems (DCS): Ideal for complex, large-scale industrial processes requiring centralized control.
  • Personal Computers (PCs) with Automation Software: PCs become versatile control systems with automation software for adaptable processes.
  • Smart Relays: Provide a simple, compact solution for basic automation tasks by combining relays, timers, and switches.
  • Programmable Automation Controllers (PACs): Combine PLC robustness with PC versatility for complex, real-time control.
  • Single-Board Computers (SBCs): Offer a DIY approach for a wide range of tasks with community support, suitable for non-industrial applications.

Each of these alternatives to PLCs comes with its own set of features, benefits, and best-use scenarios. You'll want to assess the specific needs of your project or operation before making a selection.

Many of these solutions work better for small-scale projects, and for someone with advanced programming skills who is able to program the device for monitoring and control.

Remote Telemetry Units (RTUs)

If you're looking for something simpler than the complex and code-heavy PLCs, Remote Telemetry Units (RTUs) might be just what you need. One of the greatest benefits of an RTU is that it is pre-configured, making them easy to implement and roll out at multiple sites, even without highly technical staff.

RTUs are great for collecting and sending data from unmanned sites, straight to a central system that controls everything. Unlike PLCs, which are good at controlling processes and automation, RTUs focus more on monitoring equipment and conditions from afar.

They're vital in industries like oil and gas, water management, and electricity distribution: anywhere you need to get real-time updates from equipment spread out over large areas. RTUs connect to sensors to pick up important info (temperature, pressure, how much of something is flowing, etc.) and then send this data using different ways of communication. This helps businesses run more smoothly, plan maintenance before things break down, and make better decisions, all with no programming required on your part.

Should I Choose an RTU Over a PLC?

Choosing between a Remote Telemetry Unit (RTU) and a Programmable Logic Controller (PLC) boils down to what your specific needs are. RTUs shine in scenarios where the primary goal is to monitor and collect data from various locations, especially those that are remote or difficult to access. Here are some key benefits of opting for an RTU over a PLC:

  • Ease of Use: RTUs are designed to be user-friendly, especially for individuals who may not have extensive programming skills. This means you can set up and manage your system without needing to become a coding expert.
  • Efficient Remote Monitoring: With an RTU, you can keep an eye on your equipment no matter where you are. This is invaluable for industries spread across large areas, like oil fields or power grids, where constant physical monitoring is impractical.
  • Durable and Reliable: RTUs are built to withstand harsh environments. They can operate in extreme temperatures, dust, and even explosive atmospheres, making them ideal for outdoor or industrial settings.
  • Real-Time Alerts and Data: RTUs provide real-time information, allowing for immediate response to any critical changes in the conditions they monitor. This can help in preventing equipment failures, reducing downtime, and making timely decisions.
  • Scalability: If your network grows, it's easier to add more RTUs to monitor additional points. This scalability ensures that your monitoring system can expand with your operational needs without significant overhauls.

When deciding between an RTU and a PLC, consider what's more important for your operation: the complex control and automation capabilities of a PLC, or the straightforward, robust monitoring and data collection features of an RTU. For many, the simplicity, remote capabilities, and reliability of RTUs make them ideal for keeping tabs on far-flung operations without needing deep programming knowledge.

Talk with the Experts at DPS Telecom

The choice of which device to choose is often confusing and conditional. Here at DPS, we are happy to provide personalized consultations in order to determine your needs.

The objective of our consultations is to find a long-term solution to your SCADA needs. Let us help you be proactive in your search to find effective remote monitoring systems at the capacity which will best fit your company.

If you would like more information regarding how you can decide on and implement the best equipment for you, give us a call at (800) 622-3314 today.

Haley Zeigler

Haley Zeigler

Haley is a Technical Marketing Writer at DPS Telecom. She works closely alongside the Sales and Marketing teams, as well as DPS engineers, resulting in a broad understanding of DPS products, clients, and the network monitoring industry.