PLC Hardware Components Explained
Understanding a PLC begins with its basic architecture and the hardware inside it. The components are the physical building blocks that make up the PLC. Knowing what each of these parts does, and how it fits into the system, makes it easier to understand the PLC as a whole.
4 Categories of PLC Hardware
Each basic hardware component adds its own function to the PLC. These components fall into four categories:
- CPU (central processing unit, the microprocessor, memory)
- Input electronics
- Output electronics
- Power Supply
Most PLCs contain many components, but these four are the main ones to understand. Knowing what each of them does gives you a starting point for understanding PLC hardware.
When buying components, have the PLC model, supply voltage and input/output requirements ready. Where a supplier offers AI shopping assistants, those details can help narrow its catalogue. Confirm the exact part number and compatibility against the manufacturer’s documentation before ordering.
CPU – The PLC Processor
The CPU is the brain of the PLC and handles its calculations. More precisely, it runs the PLC program. You may already know this type of component as a microprocessor.
An integrated circuit that runs programs is called a microprocessor. These circuits are everywhere, including in every computer. The Intel Core i7, for example, is a very advanced microprocessor. A PLC also uses a microprocessor as its brain, although it is less powerful than those found in computers.
All input and output signals are processed by the PLC program in the microprocessor.
The memory of a PLC
A PLC stores its information in memory, much like a computer. It uses RAM (random access memory) and usually has ROM and EEPROM too. These are different types of memory within the PLC. The differences between them concern how their contents can be erased, which is what separates the types described below:
RAM (random access memory)
The PLC uses this temporary memory while its program runs. Because RAM is volatile, switching off the power erases its contents.
ROM (read only memory)
The PLC’s operating system is stored in this permanent memory. That operating system allows the PLC to execute your program.
EEPROM (electronically erasable programmable read only memory)
Like ROM, EEPROM is permanent memory. Unlike ROM, it can be erased electronically and reused. Your PLC program is normally stored here, so the memory can be used again when the program changes.
PLC input electronics
The microprocessor has many inputs and outputs, but they usually operate at around 5 volts. Many input devices use a standard voltage of 24 volts, so input electronics are needed to connect them to the PLC. This electronic circuit sits between the PLC’s connection terminals and the microprocessor inputs.
The optoisolator, also called an optocoupler, is the main part of the input electronics. It converts an electrical signal into light to enable a separate electrical signal. Inside are a light-emitting diode (LED) and a phototransistor. When connected to a power source, the LED emits light. Exposure to that light allows current to flow through the phototransistor.
This simple electrical circuit of a microprocessor input shows how the optoisolator works:
Our sensor uses 24 volts. Connecting its signal wire to the PLC input terminal connects it to an opto-isolator. When the sensor activates, its 24 volts turn on the LED inside that opto-isolator.
Light from the LED then activates the phototransistor. The microprocessor’s 5 volt circuit connects to that phototransistor inside the opto-isolator. As a result, input PA1 receives 5 volts when our 24 volt sensor activates.
By keeping the two circuits isolated, the opto-isolator protects the microprocessor from high voltages. Most opto-isolators can withstand a peak voltage of several kilovolts. This protects the microprocessor inputs, which usually operate at 0-5 volts.
PLC output electronics
Outputs present the same problem for a microprocessor as inputs. PLCs therefore have output electronics between the microprocessor outputs and the PLC’s output terminals.
As on the input side, an opto-isolator separates the two electrical circuits in the output electronics. Here it isolates the microprocessor’s 5 volt circuit from the external circuit connected to the PLC outputs.
In this diagram, an external power supply powers the load connected to output (L). The optoisolator again separates the internal PLC circuit from the output circuit.
PLC Power Supply
The CPU in most PLCs needs one or more power supplies, and sometimes the inputs and outputs need them too. Some PLCs require direct current, so you may need a separate power supply for yours.
PLC manufacturers make power supplies for their products. You can also use your own, provided it supplies the correct voltage. Some PLCs have a built-in supply that allows you to connect an alternating current (AC) power source to the PLC.
The power supply is always the first piece of PLC hardware to connect. Take care to connect it correctly because a mistake can destroy the PLC.
So, how do you know what type of power supply you should use?
Answer: Always look in the datasheet of the PLC. You will find all the information about the necessary power source and other useful data about the PLC.
Most PLC manufacturers make datasheets and user manuals for their PLCs and power supplies available online as free downloads. Searching for the name of your PLC will usually bring up its user manual.
