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PLC Training For Beginners – Part 2 of 4: PLC Hardware Components

Welcome to the second part of our PLC training for beginners! in this part, we will specify the hardware parts of a PLC in detail.

PLC Training For Beginners Part 2

This training is structured into four parts. You are now in Part 2. If you landed here and want to start at the beginning, feel free to jump to Part 1: PLC Basics.

  • History of the PLC
  • PLC Architecture
  • Rockwell automation PLCs

  • Central Processing Unit- CPU
  • Power Supply
  • Digital And Analog IO
  • Communication
  • Choose suitable PLC hardware for your application

  • Studio 5000 Introduction
  • Studio 5000 Installation and Activation
  • Get Started with Studio 5000
  • Ladder Diagram Basic Instructions
  • IO Configuration

  • Let's develop a basic program...
  • Configure the controller in Studio5000 
  • Download logic to the controller

Though this is general PLC training for beginners, it will be focused on Rockwell automation PLCs and software in the examples.

Part 2: PLC Hardware Components

To proceed to the practical training, you first must learn to construct a basic PLC component list with all its components.

In part 1, we briefly described the PLC architecture. Then, hare, we will break up every component.

PLC Architecture
Figure 1: Architecture for a typical PLC

The scan starts from the field, a measurement performed by the digital and analog sensors.

Examples of analog sensors are temperature, level, flow, pressure, and vibration.

Examples of digital sensors are pushbuttons, limit switches, motors feedback, and valve feedback.

The Input modules on the rack receive the signals from the sensors as electric voltage or current value.

The CPU process the inputs signals and runs a code (or logic) with the inputs values and values stored in a memory.

The result of the code is reflected as digital and analog signals that send back to the field devices (motors activate, valve open) through the outputs modules.

The scan is cyclic and will begin again automatically

There are a few more processes in addition to the primary flow we described:

  • Diagnostic – the CPU performs a diagnostic check for all components every scan.
  • Communication – The CPU with the communication modules handles the communication to more devices like other PLCs, VFDs, etc.
  • SCADA – A connection to a SCADA is also running in the background, Inputs, and setpoints from the user.

The power supply provides power to all components on the specific rack.

PLC CPU

The CPU of a PLC is like a typical CPU for a personal computer, which controls all the functions of the computer.

However, different CPUs differ with respect to their handling speeds and storage. 

The memory unit stores the PLC control programs and other numerical data electronically. 

The CPU operates on 5V, where the holdup time for a PLC refers to how long it can bear a loss of power, typically 10ms to 3 seconds. 

The CPU of a PLC might contain more than one processor, with all individual processors having their own memory to be able to function independently. 

PLC’s CPUs have a key switch, which provides functions

  • RUN position – implementation of ladder logic program and operation of output instruments.
  • PROG position – for editing special properties and firmware updates. The logic is not running in this state, and outputs are not active (by default).
  • REM position – remote RUN or remote PROG will be controlled from the software.

PLC CPUs can interact with existing SCADA networks and complex decision-making aptitudes, equipped with extended commands for quick operation and execution of complex tasks such as calculation, indication control, latching, and other complicated mathematical endeavors. 

PLC CPUs
Figure 2: PLC CPU

PLC Power Supply

The PLC power supply converts a line voltage, typically 120V AC or 220/240V AC, into a usable 24V DC, used to power the PLC and its components. 

The power supply typically has three stages: the step–down transformer stage, the rectification stage, and the filtering stage. 

On certain PLC types, as shown in Fig 3, the power supply also comes as part of the rack.

Thus the power supply provides power to all other connected components in the rack through a BUS system in the rack. 

However, this is not standard as some modules do not have integrated power systems and must be wired individually to components requiring a power supply. 

The current rating for PLCs can range from 2 to 10A for smaller PLCs to up to 50A for more powerful and more significant programmable logic controllers. 

Usually, there is a backup battery as well, as this will provide power to the memory of the PLC in the event of a power outage.

PLC Power Supply
Figure 3: PLC Power Supply

PLC Digital and Analog IO

The most common PLC input and output type are the discrete I/O, which typically means signals with only two logical states 1/0 or ON/OFF. Light switches, pushbuttons, and proximity switches are all discrete I/O sources. 

However, we also have the analog I/O, which refers to signals with a range of values greater than just 1 or 0.  

I/O modules are either monolithic or modular (rack-based)

The rack-based input/output module has the advantage of being easier to troubleshoot and replace as opposed to the monolithic, which also suffers from having limited I/O ports. 

The I/O modules are always protected from the power supply by optocoupler circuits which require isolation. 

Discrete output cards can have 4, 8, 16, or 32 channels connected to current sourcing or current sinking devices. 

Different signal types can be interfaced to an analog IO module, such as voltages between 0-10V, currents between 0-20mA or 4-20mA, thermocouples, and RTDs.

PLC IOs
Figure 4: PLC Digital and Analog IO

PLC Communication

The communication rule in the PLC architecture is increasingly important.

Different protocols are used for various purposes. Rockwell’s PLCs mainly depend on the EthernetIP protocol for communication with other RA PLCs, RemoteIOs, VFDs, HMIs, etc.

Most communication modules support DLR – Device-level ring topology, which provides a single-fault tolerant ring network.

In the following figure, you can see an example of a typical EthernetIP network.

Network EthernepIP Example
Figure 5: Typical EthernetIP Network

 

If a different protocol is needed, like ModbusTCP or IEC81650, vendors like Prosoft are partnered with Rockwell and a few more brands and manufacturing communication modules for different protocols.

PLC Comm
Figure 6: PLC Communication

Choose the best suitable PLC hardware for your application

In every project, the first step will be choosing the most suitable hardware for the task from the application size and budget point of view.

Now you are familiar with the component required for your project. Next, you can go to Rockwell’s Selection Guides to explore the properties of every element.

Whats Now

Congrats! You finished the second part of our PLC training. The next part of this training is PLC Training For Beginners – Part 3 of 4: PLC Programming. In this part, we will explore the different programming languages and the studio5000 PLC programming software.

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