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Choosing the Right PLC: 6 Things to Consider

Mike Jeffries | February 15, 2021


A programmable logic controller (PLC) is a small, sturdy industrial computer designed for control of automated industrial processes and machines. PLCs are used almost everywhere. They are most frequently used in factories and industrial plants to control pumps, motors, lights, circuit breakers, fans, and other machinery. In everyday life, these controllers are utilized in elevators, automatic car washes, traffic signals, amusement park rides, among other equipment that we otherwise take for granted.

Related Blog: In What Application Processes are PLC's Most Beneficial

The reliability, control, and ease of programming that a PLC provides while functioning in harsh conditions make it a practical choice for many applications. But before you start thinking about the factors in selecting an effective PLC, it's useful to have a general understanding of the architecture and the basic function of a PLC.

The Architecture of a PLC

A PLC consists of the following components:

  • Input/Output (I/O) section. The input section or module connects with sensors, switches, control panels, and other input sources. The output section or module can be a motor, lamp, solenoid, heater or another output device that is controlled by varying the input signals.
  • Central Processing Unit (CPU). The "brain" of the PLC that performs the programming instructions to process the input signals and control the output signals.
  • Programming Device. A platform where the program or control logic is stored, usually in the form of a handheld device, laptop or PC. 
  • Power Supply. The component that supplies power to input and output devices.
  • Memory. This component stores the control program that the CPU uses and the data from the devices that the PLC is connected to.

The Basic Function of a PLC

Because the PLC is, in fact, a small computer, it uses the same simple steps to process information the way our laptops and desktops do. From a functional standpoint, its main parts consist of:

  • CPU
  • Inputs
  • Outputs

A PLC takes in information in the form of inputs, performs logic on the inputs in the CPU, and then switches outputs on or off based on that logic. 

Here's a simple example of how a PLC controls an industrial valve:

  1. The CPU checks the status of the inputs: the valve is 30% open.
  2. The CPU takes this information about the inputs and performs logic on them.
  3. The CPU operates the logic of the output to open the valve.

Now, let's look at six of the most important considerations in choosing a PLC: system requirements, environmental requirements, inputs and outputs (I/Os), CPU speed, communication protocol types, and programming.

1. System Requirements

The tasks that your system is required to carry out will determine the kind of PLC you need. Another deciding factor is whether your system is to be built from scratch or if existing products are already installed. These factors are important because you will want to match a PLC's functionality with the task at hand and with any existing products that are installed.

2. Environmental Requirements

Although most PLCs are ruggedized for tough physical environments, issues such as dust, vibration, temperature or specific facility codes may affect your application. Temperature extremes, for example, may be problematic. The typical operating range for PLCs is 0-55 degrees Celsius (32-130 degrees Fahrenheit), but if your facility's ambient temperature falls outside of that range or has other atypical environmental requirements, you will need to research other potential PLC candidates more thoroughly.

3. Inputs and Outputs (I/Os)

As we've seen so far, inputs and outputs are fundamental to the operation of a PLC. Two key factors to consider in choosing the right PLC are the number of I/Os and their location.

Because a PLC controls a sizable part of a process, you will want to make sure that it can handle multiple I/Os and I/Os of different types. The number of both analog and discrete devices that your system has will impact this decision as well. Keep in mind that the number of I/Os will also determine the size of your PLC's chassis.

The location of I/Os also makes a difference in your selection. Will your system require a local I/O, or will you need both local and remote I/Os? The answer to this question depends on whether your application will have subsystems that are a long distance from the CPU. You will also need to investigate whether the speeds and distances of the remote I/O are sufficient for your application.

4. Speed of CPU

The processing of the CPU must be quick enough to handle multiple I/O's and the types of data collection required for your application. Having enough memory is essential too, and it will depend on how many devices you have. Another consideration is the scan time, which is the amount of time it takes for the CPU to perform one cycle of gathering inputs, running the PLC program, and updating the outputs. Also, program memory for the CPU will be determined by the type of program and instructions you plan to use.

5. Type of Communication Protocols

The type of communication protocols your system will use is another factor in PLC selection. What kind of networks or devices will the PLC be communicating with? Sometimes PLCs are equipped with communication ports, but others require additional communication modules. Other options are to communicate remotely via Ethernet or to build upon multiple types of communications as your system requires.

6. Programming

Find out what your programming requirements are: will you need basic programming instructions, or will you require special programming instructions? Are you accustomed to using assembly language or would you prefer to use a higher-level language such as C or BASIC? Do you want to use tag name based control, where you assign a specific amount of memory for a certain data type? Or, are you more comfortable using fixed memory addressing that does the assigning for you. The programming characteristics of the PLC you decide on should give you the most ease-of-use for development, troubleshooting, and maintenance.

Choosing the most effective PLC for your application takes some careful thought and research in order to save money and prevent problems in the long run. Mader Electric stocks a wide selection of controls, including PLCs. We can provide the knowledge and expertise you need when it's time to make a selection. For more information about our PLCs or other controls, contact us today.

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