Understanding PLC Hardware and Wiring

Industrial automation has been in existence since before the programmable logic controller (PLC) was developed in the late 1960s. Relays and timers were used for automating manufacturing processes prior to the introduction of PLCs but they had so many challenges. Some of these challenges included the large space occupied by relays and timers, the amount of time required to troubleshoot the system, and making changes to the system was very difficult due to the fact that so many relays were hardwired together in a specific order for the machine to operate. A PLC provides a solution to all these challenges by needing less space, being easy to troubleshoot, and changes can easily be made to a control system that uses a PLC. The following photograph shows a relay room with so many relays that take up much space, are not easy to troubleshoot, and changes are not easy to make on the control system:

This chapter focuses on the hardware aspect of PLCs. We will understand what a PLC really is, the  various parts that make up a PLC, the operation of a PLC, types of PLCs, PLC manufacturers/vendors,  and how to wire sensors and actuators learned in the previous chapter to a PLC.
 

We are going to cover the following main topics in this chapter: 

• Introducing a PLC 

• Exploring PLC modules 

• Exploring PLC types 

• Understanding the PLC scan cycle 

• Knowing about PLC vendors 

• PLC wiring (1) – Wiring of switches, lights, and actuators to a PLC 

• PLC wiring (2) – Wiring of proximity sensors (3-wire)

 
• PLC wiring (3) – Wiring of photoelectric sensors (retro-reflective) 

• Wiring of the Siemens S7-1200 PLC (CPU 1211C AC/DC/Relay)

 Technical requirements 

While every part of this book is valuable, Chapter 2, Switches and Sensors – Working Principles, Applications, and Wiring, and Chapter 3, Actuators and Their Applications in Industrial Automation,  are very important to have a better understanding of this chapter. 

Introducing a PLC 

A PLC is an industrial computer that consists of both hardware and software used for automating  industrial processes. They are mostly used in manufacturing industries and can be found in other  industries including transportation, manufacturing, warehousing, oil and gas, building, and so on.  A PLC can also be referred to as an electronic device that accepts inputs from sensors and switches,  
processes them, and gives out signals to control actuators. PLCs take data from the plant floor through  switches and sensors, they then execute the program logic written into them and give an output signal  based on the result of the program logic to control actuators or machines connected to them. PLCs  
are the brains behind almost all modern automated machines in industries.
 

Let’s now have a look at the parts that make up a PLC. Exploring PLC modules The hardware part of a PLC consists of various modules. The basic modules that make up a PLC  include the following: 

• The power supply 

• The CPU

 

 • Input module 

• Output module 

The following figure shows the block diagram of a PLC:

 

 In the next section, we will be looking at each module.
 

The power supply 

The power supply part of a PLC provides the appropriate voltage and current to other parts/modules of  the PLC. Most PLCs use 24V DC and the supply from the grid is usually 110V or 220V AC, depending  on the country. A power supply takes the 220V or 110V AC as input and provides the required voltage  (usually 24V DC) and current as output to other parts of a PLC. The current rating of power supplies  can range from 2 to about 50 amps, depending on the PLC size. The following figure shows a Siemens  power supply module:

 

The power supply module of a PLC provides the energy required for the PLC to function. A PLC  
without a power supply is just like a vehicle without fuel. The power supply works by stepping down  
the line voltage of usually 110V or 220V AC to a lower AC voltage with the aid of a transformer.  
 

A rectifier then converts the lower voltage AC to DC. A capacitor smooths or filters the DC, and a  regulator circuit within the unit provides a regulated DC output of usually 24V DC. Let’s now have a look at the next module in our list, the CPU module.

 

 The CPU module 

The central processing unit (CPU) can be regarded as the brain of the PLC system. It consists of  a processor, memory, and other integrated circuits that handle program execution, storage, and communication to other external devices including a programming device or personal computer
(PC), a Human-Machine Interface (HMI), a variable frequency drive (VFD), and so on. The CPU  controls and coordinates the entire operation of the PLC. The following figure shows the CPU of an  Siemens PLC (S7-300 CPU 313):

 

 

 The CPU of a PLC functions like the brain of a human. The entire range of human activities (including  walking, sitting, standing, and others) is controlled by the brain. Similarly, the entire operation of  the PLC is controlled by the CPU. The CPU of a PLC can also be linked to the CPU of a PC, which  controls all the activities of the computer. A computer without a CPU cannot function and likewise,  
a PLC without the CPU cannot function. The CPU executes the program written into the PLC and  makes the decisions needed by the PLC to automate or operate machinery and communicate with  other devices. Let’s have a look at the next module in our list, the input module.

The input module 

The input module is the part that connects all the input devices (switches and sensors) to the PLC. It  allows the PLC to monitor the current state of switches and sensors that are required to perform the  necessary control functions when executing a program. Input modules can basically be classified as  
digital and analog. Digital input modules are used when the input devices to be connected produce  a discrete signal (ON or OFF), while analog input modules are used when the input device to be  connected produces an analog signal (that is 0-5V, 0-10V, 0-20mA, or 4-20mA).
The following figure shows a digital input module for a Siemens PLC (SM321, DI 16X DC24V):

 

The input module receives signals from switches and sensors and sends them to the CPU for processing. Let’s have a look at the output module.

The output module 

This is the part that connects the PLC to the output devices or actuators. An output device can be a  light, relay, contactor, solenoid valve, control valve, and so on. The PLC output module operates or  controls the output devices based on the state of the inputs and the result of the written program in  
the CPU. Output modules can also be classified into two, namely digital output modules and analog  output modules. The digital output module can control output devices or loads that are either ON or  
OFF. Examples of such loads or devices include lights, relays, contactors, solenoid valves, and others.  
 

The digital output module produces binary outputs (1 or 0), which means ON or OFF for loads that  are either ON or OFF. Analog output modules, on the other hand, produce variable or changing  signals that can range from 0-5V, 0-10V, 0-20mA, or 4-20mA. They are used for controlling output  devices that require a control signal between full ON and full OFF. Examples of such loads or devices  
include analog voltmeters, analog ammeters, control valves, VFDs, and so on. For example, an output  module can be connected to a control valve to vary the opening or closing of the control valve slowly  between full ON and full OFF. You can decide to open a control valve to half or one-third of its full  
capacity depending on the need.
 

The output module of a PLC can be a transistor, a TRIAC, or a relay type, depending on the type of  component used in the output circuit to switch the load ON or OFF.
 

In a transistor output type, a transistor is in the output circuitry for switching loads ON or OFF. A  transistor type of output can only switch a DC load. A transistor-type output is fast, has a longer  lifetime, and switches small currents.
In a TRIAC output type, TRIAC is used in the output circuitry for switching loads ON and OFF. It  can only switch AC loads (that is, it cannot be used for DC loads). It also has a longer lifetime and is  faster when compared to the relay output type. 

In a relay type, relays are used in the output circuitry for switching loads. It can be used for both AC  and DC loads. It is the most common type of PLC output.
 

The following figure shows the Siemens digital output module:

The output module takes the processed signal from the CPU to control output devices such as lights,  relays, contactors, and so on.
Let’s have a look at other PLC modules.