What is a PLC (Programmable Logic Controller)?

A PLC (Programmable Logic Controller) is like a mini computer. These devices 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 one, and potential alternatives to these devices. 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?

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. To do so, they check 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. Operators and users can use the dashboard to watch and control the industrial activities and machinery.

What are the Components of a PLC?

PLCs are made up of several parts. The key components include:

• Central Processing Unit (CPU): The brain of the device, where the control program is stored and executed. It processes inputs, runs the logic, and sends commands to outputs.
• Power Supply: Provides the necessary electrical power to the PLC and its components, upholding stable operations.
• I/O Modules: Interface components that connect the PLC to external devices and manage both the inputs and outputs. This allows the PLC to interact with its environment.
• Memory: Stores the control program, data, and other essential information that the CPU uses to execute tasks.
• Communication Interface: Allows the PLC to communicate with other devices, such as other PLCs, computers, or Human-Machine Interfaces (HMIs). This communication is often via standard protocols like Ethernet or Modbus.
• Programming Device: A tool - often a computer with specialized software - used to write, upload, and modify the control program that runs on the PLC.
• Chassis or Rack: The physical framework that houses and connects all the PLC components. This provides a structured and organized layout.

The components of a PLC work together to automate and control industrial processes. At the core of the system is the CPU, which executes the control logic stored in the device's memory.

I/O modules facilitate interaction with external devices. They receive inputs and send outputs to control various operations. The power supply ensures all components receive the necessary electrical power.

At the same time, the communication interface allows the PLC to connect with other systems for monitoring and control. All these components are typically housed in a chassis or rack, providing a compact and organized structure.

Expanding the Role of PLCs with Communication Protocols and Integrated Devices

PLCs are no longer standalone automation tools. They're integral parts of complex, interconnected systems that demand smooth communication and interaction. The need for flexibility, scalability, and real-time responsiveness in industrial processes has driven this evolution.

Communication Protocols: The Backbone of Integration

Modern PLCs utilize advanced communication protocols such as Modbus, Ethernet/IP, and Profibus to interface with other devices and systems. These protocols enable real-time data exchange between the PLC and input/output devices, SCADA systems, and even cloud-based platforms. This connectivity allows operators to monitor processes remotely, reduce downtime, and enhance operational efficiency.

Input and Output Devices: Essential Connections

Input and output (I/O) devices serve as the physical connection points for PLCs, allowing them to gather data and control equipment. Inputs include sensors that measure parameters like temperature, pressure, and flow rate, while outputs drive actuators such as motors, valves, and lights. By integrating intelligent I/O modules with communication capabilities, PLCs can achieve distributed control. This improves system reliability and simplifies wiring complexity.

Enhancing Flexibility with PLC Integration

Integrating PLC systems with a strong communication network transforms how industries operate. For instance:

• Predictive maintenance becomes possible through continuous monitoring of machinery health.
• Energy efficiency improves with real-time adjustments to operating parameters.
• Production lines become more adaptable to new product designs without significant hardware changes.

With these advancements, PLCs remain at the forefront of automation, bridging the gap between traditional equipment and modern, IoT-driven industrial environments.

How Much do PLCs Cost?

The average price of the device 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?

These 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 They Programmed?

You can program a Programmable Logic Controller (PLC) in different ways. Ladder logic is the most popular language for programming them because it looks like electrical circuits. This makes it easier to understand and visualize how things work in the PLC.

Having that advantage 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 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.

Memory is Stored and Managed for Information Retention

Memory and storage management in a PLC determine how efficiently it can execute control programs and handle data. Memory management saves and organizes critical data to make sure information is retained. The types of Memory in a PLC include:

• Volatile Memory (RAM)
• Non-Volatile Memory (ROM/Flash Memory)
• EEPROM (Electrically Erasable Programmable Read-Only Memory)

Volatile memory is used for temporary storage while the PLC is running. It stores data related to the current operation, such as the values of inputs and outputs, timers, counters, and intermediate results of calculations. Since RAM is volatile, all data is lost when the power is turned off.

Non-volatile memory is used to store the PLC's firmware and the control program (ladder logic or other programming languages). Because it is non-volatile, data in this memory is retained even when the power is turned off. Flash memory is often used for program storage because it allows easy updates and modifications to the control program.

EEPROM is another type of non-volatile memory used in some PLCs to store user programs, configuration data, and settings. Like flash memory, it retains data without power and allows for reprogramming without needing to remove the memory module.

These memory types are managed via:

• Program Storage: The control program is stored in non-volatile memory (typically flash memory). This ensures that the PLC can retain the program and continue operation even after a power cycle.
• Data Handling: Volatile memory (RAM) is used for handling dynamic data during the PLC's operation. This includes storing the state of I/O points, intermediate calculations, and temporary variables. Some also allow certain data areas in RAM to be backed up to non-volatile memory to preserve critical data across power cycles.
• Data Retention and Backup: In cases where it is crucial to retain certain data across power cycles (e.g., counters, timers, or specific variables), PLCs can be configured to save this data to non-volatile memory. This ensures that critical data is not lost during a shutdown or power outage.
• Memory Allocation: PLCs often allow the user to allocate memory resources to different tasks, such as logic execution, data storage, and communication buffers. This flexibility helps optimize the PLC's performance based on the specific application needs.

Overall, memory and storage in a PLC are carefully managed to maintain reliable operation, data integrity, and the ability to update or modify control programs as needed. This management is needed for maintaining the performance and longevity of PLC-controlled systems.

The Life Cycle Verifies Functionality

The life cycle of a PLC is a comprehensive process that verifies the system is designed, implemented, and maintained effectively to meet the demands of industrial automation. Each stage plays a crucial role in the overall performance and longevity of the PLC. The stages of their life cycle are:

1. Design and Planning: This initial stage involves defining system requirements, selecting the appropriate PLC, and designing the overall control system architecture.
2. Programming and Installation: Engineers develop the control program, install the PLC on-site, and thoroughly test the system to ensure it meets operational needs.
3. Operation and Maintenance: Once in operation, the it requires regular monitoring and maintenance to ensure it functions optimally and any issues are promptly addressed.
4. Upgrading and Replacement: As technology and system requirements evolve, the PLC may be updated or modified, and eventually, it will be replaced with newer technology to maintain efficiency.

By following these stages, a PLC can operate reliably and efficiently, adapting to new requirements and technologies as they emerge.

Advantages of PLCs

• Flexibility: Easily updating PLCs means you can change production processes quickly, without losing much time.
• Reliability: They 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: They can talk to other systems to help manage and make better decisions in factories.
• Cost-Effective: Even though they're expensive at first, they save money over time by reducing errors and maintenance costs.

PLCs are best implemented in controlled environments such as factories, with a knowledgeable 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.

Andrew Erickson

Andrew Erickson is an Application Engineer at DPS Telecom, a manufacturer of semi-custom remote alarm monitoring systems based in Fresno, California. Andrew brings more than 18 years of experience building site monitoring solutions, developing intuitive user interfaces and documentation, and opt...