The 7 Essential Parts of a PLC System

When I was in school PLC’s were just in their infancy. We were taught that the PLC consisted of the central processing unit (CPU), analog and digital inputs and outputs. Everything was programmed with dedicated handheld devices and/or software devices on specialized hardware. We now have modern PLC systems that are capable of so much more. Let’s look at how we can now break up these modern PLC system into the seven essential components.

Inputs and Outputs (I/O)
Analog I/O
Specialty I/O
Programming Tools

CPUCentral Processing Unit
PLC manufactures use proprietary computer chips in their controllers. These are the brains in the controller to do the work we want. There has been an effort to standardize the programming languages of the PLC. IEC 61131-3 specifies the five different languages being:
• Structured Text (ST)
• Function Block Diagram (FBD)
• Sequential Function Chart (SFC)
• Instruction List (IL)
• Ladder Diagram (LD)
Support for a language does not mean that the key stokes for writing and implementing the logic is the same in each manufacturer‘s controller. Every manufacturer will have a different method of implementation. Additional information about programming languages can be obtained by viewing ‘What everybody ought to know about PLC programming languages.’
Every manufacturer will try to set themselves apart by quoting execution time of the program code. This is the speed at which the program is looked at and the physical hardware is scanned or set. The following posts will explain the PLC program scan.

Who else wants to know how a PLC Scans?
Understanding the PLC Program Scan.

People / organizations get ‘locked’ into one manufacturer of PLC by having knowledge and hardware only of one supplier. Training of personal throughout the organization on programming and troubleshooting can be expensive. This is also true of the actual hardware already installed and running on the plant floor.
Sizing and actual model number of a PLC is determined by all of the rest of the parts of the PLC system.

Inputs and Outputs (I/O)
Sizing of the PLC was traditionally done by the number of inputs and outputs that the system will control. When we talk of I/O, this is referring to discrete on/off devices. Inputs would be items like pushbuttons, switches, sensors, etc. Outputs would be items like motors, lights, solenoid, etc.
How PLC Inputs Work
How PLC Outputs Work

Analog I/O
The analog inputs and outputs on a PLC system refer to an input or output that gives us a range. An example of an input would be the weight of a product, speed of the motor, etc. Analog outputs would include setting the speed of the motor, controlling a proportional valve, etc.
How PLC Inputs Work
How PLC Outputs Work

Specialty I/O
High speed counters, position control units (2 axis or more interpolated), Basic card modules, thermocouple / RTD input cards, etc. These are just a few samples of specialty cards that are available for specific tasks that the PLC may have to control. Manufactures have started to incorporate allot of these features within the standard PLC unit.

Programming Tools
Small portable dedicated devices are sometimes available to program and troubleshoot the PLC. However the majority of the time now you will be using a computer running the software from the manufacture to program and troubleshoot the PLC. The computer is usually a windows based machine.
Simulators are also very common now when developing and testing your logic. You can limit the amount of troubleshooting required before installing on the factory floor when using a simulator. This may not be available on all software packages. Please refer to the unit you are using.
Here is an example: AdvancedHMI to communicate Modbus TCP to the Automation Direct Do-More Designer Software Simulator. The following is the sequence of operation:
Watch on YouTube : Running the Cylinder Sequence (PLC / HMI)
Note: All of the programs used are provided free of charge and are an excellent way to learn PLC / HMI programming.
Each manufacturer will have different specifications for the computer hardware and software required to run their package. (Example: Communication ports and Operating systems)

The HMI (Human Machine Interface) is a method in which the operator can control, adjust or troubleshoot the machine. It usually involves a visual screen to display information to the operator and/or touch screen/ keys for input back from the operator. HMI ‘s can be built into the controller or as a separate item through networking.
Horner XL4 System Hardware

Networking is the ability to communicate to other devices in the system. This will involve the physical and software methods. Physical will be items such as the cable, signal voltages and configuration. Software will consist of the protocol (language) used to communicate so each device understands the other. 

Peer to Peer Communications
Modern controllers will often have the ability to communicate to each other in the system by sharing a common memory block. Each PLC will have the ability to write to a certain section and read the entire memory block. This works well in applications where you need to decentralize the control system. (Smaller controllers networked together in a system.) A decentralized system is generally more flexible and quicker.

SCADA – Supervisory Control and Data Acquisition – This term is used to describe the ability to retrieve data from the system, make decisions, and store information.
Modern controllers will now have open protocols. This means that the information is published on how to communicate on the network.

Networking is a large topic. Here are a couple of posts that dive into some of the details.
How to Send Email to SMTP Server (VB)
Here’s is a quick way to send email (text) messages from the PLC
How to Implement the Omron PLC Host Link Protocol (VB)
How to Implement the Omron Host Link Protocol Part 2 – VBA
Implementing the Omron CX Server DDE and Excel
Modbus TCP Data Logging to Database
Creating a HMI Login Screen on AdvancedHMI
Deploying an AdvancedHMI Project
What Everybody Ought to Know About IP Addressing

The seven parts of the PLC system gives you a good understanding on what to look for and ask about when specifying a PLC. Manufactures will assist you in specifying the controller by giving you information. The following is the system builder for Automation Direct.
Ask your local PLC suppler on sizing your next PLC project.

Watch on YouTube : The 7 Essential Parts of a PLC System
If you have any questions or need further information please contact me.
Thank you,

If you’re like most of my readers, you’re committed to learning about technology. Numbering systems used in PLC’s are not difficult to learn and understand. We will walk through the numbering systems used in PLCs. This includes Bits, Decimal, Hexadecimal, ASCII and Floating Point.

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