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:

Figure 7.1 – Relays...

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...

Exploring PLC types

Based on the hardware specifications, there are two basic types of PLC:

• Compact
• Modular

Let’s have a look at each of the types.

Compact PLC

In this, the CPU, input module, output module, and sometimes the power supply are integrated into a single module.

It has a fixed/limited number of I/O by default, but some are designed to support expansion modules, which can be added to increase the number of inputs and outputs. A compact PLC can also be referred to as a fixed or integrated PLC. S7-200 and S7-1200 are examples of compact PLCs by Siemens, while FX 1N, FX2N, and others are examples of compact PLCs from Mitsubishi. Other PLC manufacturers also have their compact PLC series/models.

The following figure shows a compact PLC from Siemens (S7-1200, CPU 1211C, AC/DC/Rly):

Figure 7.7 – Compact PLC from Siemens

The credit for this figure goes to Showlight Technologies Ltd. (www.showlight.com.ng...

Understanding the PLC scan cycle

The PLC scan cycle is a good way to explain how the PLC works. Let’s have a look at it to have a better understanding of the operation of a PLC.

The PLC scan cycle is the cycle in which the PLC reads the inputs, runs the PLC programs, performs diagnostic and communication tasks, and updates the output. It’s a repetitive process. The time taken by the PLC to complete one scan cycle is referred to as the scan time and is measured in milliseconds.

The following is a PLC scan cycle:

Figure 7.9 – PLC scan cycle

PLC scan cycle may vary slightly depending on the PLC in use. Basically, the PLC starts by reading the inputs, that is, checking the state of the connected switches and sensors to know which one is ON or OFF or the current value if it’s analog. It then executes the user program based on the state of the inputs. Next, the PLC performs communication tasks, which include communicating necessary...

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

Here, in this section, we will look into the connection (wiring) of input devices (switches) and output devices (lights and actuators) to the PLC. Sinking and sourcing are two important terms in PLC wiring. They are used to describe the direction of the conventional current flow between two circuits or devices. A conventional current always flows from positive potential (+) to negative potential (-). Anytime you have a current flowing between two devices, one will be sourcing while the other will be sinking.

The sourcing device is the device that provides the current (positive), while the sinking device is the device that absorbs the current (that is, the device connected to negative or ground).

In PLC wiring, the two devices in question are input devices and input modules or output devices and output modules. PLC manufacturers usually produce input/output modules that you can either sink or source. There...

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

We also discussed photoelectric sensors in Chapter 2, Switches and Sensors – Working Principles, Applications, and Wiring; in this section, we will learn how to wire a photoelectric sensor to an input module.

Figure 2.49 in Chapter 2 shows the wiring diagram of a retro-reflective photoelectric sensor.

The retro-reflective photoelectric sensor described in Chapter 2 can be made to switch to positive or negative depending on whether the input module to be used with it is sinking or sourcing. If the White (WH) is connected to positive, the Black (BK) will have an output (+ve) when an object is detected, and if the WH is connected to negative, the BK will have negative when an object is detected.

The following circuit diagram shows the wiring of a photoelectric sensor connection to a sinking input module:

Figure 7.23 – Photoelectric sensor connection to a sinking...

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

The PLC wiring explained in this chapter provides the basic skill needed to get PLCs wired to sensors, lights, and actuators. It is advisable to consult the manual of any PLC you want to wire and follow the wiring/guides provided in the manual. The following diagram shows the wiring of the Siemens S7-1200 PLC (CPU 1211C AC/DC/Relay) in the manual:

Figure 7.25 – Siemens S7 1200 PLC wiring (CPU 1211C AC/DC/Relay)

The pictorial diagram shown as follows will give a better understanding of the wiring of the Siemens S7-1200 PLC (CPU 1211C AC/DC/Relay):

Figure 7.26 – Pictorial diagram of the Siemens S7-1200 PLC wiring (CPU 1211C AC/DC/Relay)

The credit for this figure goes to Showlight Technologies Ltd. (www.showlight.com.ng).

Explanation of the wiring

In Figure 7.26, a circuit breaker connects to the AC supply (220V AC) and is used to supply AC power to...

Questions

The following are questions to test your understanding of this chapter. Ensure you have read and understood the topics in this chapter before attempting the questions:

1. __________________ is an industrial computer that consists of both hardware and software used for automating industrial processes.
2. PLC is an acronym for _______________________________.
3. The basic modules that make up a PLC are: ________________, _________________, ________________, and __________________.
4. __________________ can be regarded as the brain of the PLC system.
5. The module or part that connects all the input devices (switches and sensors) to the PLC is called _____________.
6. The module or part that connects the PLC to the output devices or actuators is called ______________.
7. _______________ is the cycle in which the PLC reads the inputs, runs the PLC programs, performs diagnostic and communication tasks, and updates the output.
8. The time taken by the PLC to complete...