PLC 101: Functioning And Deployment

A PLC which stands for Programmable Logic Controller, is basically a technology used to control automated systems in various industries. It is an important part of process automation across industries these days and they constitute simple forms of control systems that have been increasingly replacing hard wired logical relays spread out on a large scale of late.

It is similar to typical PCs in certain hardware and terminologies such as central processing units, memory and communication. However a PLC is designed to operate in more rough and rugged industrial surroundings and is also more flexible in terms of interfacing with inputs fed in as well as outputs generated.

How a PLC works

The working or functioning of a PLC is dependent on its 3 main components. As mentioned before, just like how a normal PC has a CPU, a PLC also has similar parts that decide how it functions. They are:

–          The Central Processing Unit (CPU)

–          The power supply and rack

–          The input and output section

An additional segment can be identified for a PLC considering its industry-heavy application and that is the program section.

In brief terms, PLCs work as per the following sequence:

  • The input section first converts real time analogous electric signals that it receives into digital electric signals which the PLC uses with the help of certain connector rails
  • The CPU then stores these digital electric signals in the PLC external memory space. The locations are called bits.
  • In the next step, the programmable instructions are spelled out for the device in the form of mnemonics and symbols which are then stored in the user memory.
  • Then, just like in a typical PC, the CPU extracts these programmable instructions and executes them by manipulating and processing the inputs so as to control the output devices.
  • The results of the implementation get stored in the external memory space that controls the output devices

In addition to these, the CPU also performs the functions of updating input storage with changes attributable to the alterations in output content. At the same time it also performs internal functions like resetting the timer and keeping a check on the user memory.

What is PLC control?

PLC control is nothing but the control affected through the deployment of PLC automation software. From a general perspective, a PLC helps control different devices working in an industry without being too rigid with the process flow. For the same reason, it is often known as a method of “softwiring” the different devices involved in process automation.

They are known to provide this kind of flexibility without compromising on reliability and robustness. They eliminate program crashes or other mechanical level failures as well.

Moreover, as they are available in even small sizes, they are more compact and much more economical when compared to traditional control systems.

What is PLC training?

PLC training refers to the employee level skill development and training imparted in order to familiarize the concept and functioning of a PLC in any industry. As is the case with any new deployment in any industry or office, new process automation software deployment also requires the management to carry out specific training programs for the middle and lower level employees, who may be operating the system.

Additionally, PLC being an industry norm these days, there are also specialized PLC technician training programs offering professional qualification with valid certification for prospective PLC operators who may be interested in pursuing a career in the same. The program typically involves modules detailed on the basic knowledge and skills required to work with a PLC system within an industry.


An Overview of Programmable Logic Controllers Through Allen Bradley Software

PLC is a kind of digital computer that is utilized for the purpose of automation in the electromechanical processes in the plants. It is made for multiple outputs and inputs arrangements so that the data can be availed through the sensors, used, and the command can be issued to the actuators. When it comes to PLC, a foremost name that deserves a mention is that of the Allen-Bradley range of factory automation equipment by Rockwell Automation.

allen-bradley logoWhat is Allen Bradley?

Allen-Bradley is the name given to a range of factory automation equipment made by Rockwell Automation. The company deals in the manufacturing of PLC (programmable logic controllers), sensors, human-machine interfaces, safety systems and components, and many more of such high-end equipment. Allen Bradley is one of the premier PLC software manufactured by the company and adopted by the companies around the world.

What is Allen Bradley software?

The Allen Bradley software products are used to support the intelligent I/O modules, collectors, portable data, servo drives, power-flex drives, motor control centers, sensors, programmable controllers, and motors. The designs of this software ensure configuration and designing of the products such that they are able to deliver the work faster to offer accelerated content and provide value for the time through utilization of the high-end tools.

What is Allen Bradley PLC programming?

In the Allen Bradley PLC programming software, the connection between the computer and the controller is established through either Ethernet cable or serial cable. RSLogix is the name of the programming software of Allen Bradley’s PLC. Understanding this PLC software is suggested before taking any PLC training session and the basics course of PLC is based on the Allen Bradley PLC programming software. The knowledge that is acquired, thus, can be used in any PLC brand. Here are a few of the steps to set up a project on the PLC Allen Bradley using the Allen Bradley RSLogix 5000 software.

The configuration of the software

Connect with the controller using the Serial Port and start the configuration of the Serial driver. Connect the serial cable on one side and controller on another side of the PC. The next step is the configuration of a connection. In order to configure the connection, you can utilize the RSLinx Classic Lite from Rockwell Automation. RS232 DF1 Device driver is best to be used for the serial communications. After you have selected the configuration driver, the following are the steps that you need to undertake.

  1. Select the RS232 DF1 Device driver from the pull-down list having the names of the driver types.
  2. Select add new for adding the driver In the ‘add new’ dialog box, write the name of the driver and click on OK.
  3. To specify the settings for serial port:
  •   Select the option of the serial port on your workstation from the menu of Com Port. This is done to make sure that the cable is properly connected to the ports.
    Select the serial port from the pull-down menu for Devices
  • Click on Auto Configure. :

The automatic configuration should be successfully initiated after this, and if it fails to do so, then you must have selected the wrong port. Check that and initiate the configuration one more time. The configuration of the I/O modules is needed for the physical modules of the project and it constitutes of the simple steps of Ladder Logic programming and Logical Continuity.

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Difference between RS485 and Profibus

When it comes to industrial networking, there are many specifications that govern the smooth exchange of information across hosts, slaves and throughout the network. All of these may be familiar with people who have mastered the art of network communication, but anyone who is starting out could be easily confused between the technical terms and accessories.

One such commonly confused pair of technical terms is the RS485 and Profibus.

Now people who do have a fair idea about the whole data exchange system in industrial networking would know that these are two very important contributors for smooth process automation. But, from a more detailed perspective, these are two different aspects related to the same concept.

Let’s get in to what each of these is.


Profibus stands for Process Field Bus. It is basically a standard of fieldbus communication used in automation technology. And field bus refers to the network protocols used for real-time distributed control.

There are two versions of the standard being used in industries. One of them is Profibus DP which stands for Decentralised Peripherals and the other is Profibus PA which stands for Process Automation. Out of these the Profibus DP is more commonly used by industries while the Profibus PA is more application specific and is hence lesser used.

Profibus DP is meant to be used to control and operate sensors and actuators through a centralized platform for automation applications in production industries and factories. It includes diagnostic options as well.

Profibus PA on the other hand is mainly used to monitor measuring equipment through a process control system of course. It is specifically meant to be used in explosion prone or hazardous areas.


The RS485 is also a standard, but it defines the electrical signal levels in the communication between drivers and receivers or hosts and slaves in networking. In this case though, the electrical signalling is more balanced and multi point systems are also supported. For the same reason, the RS485 is a very useful and popular standard in industrial control systems. It is more so as it is capable of being connected with multiple receivers, whether in a linear or a multi drop bus and it supports both Local Area Networks as well as multi drop communication links and works with data rates up to 10 Mbits.

Difference between Profibus and RS485

Both the Profibus and the RS485 are standards used in communication during process automation. However, they are used in different aspects of communication.

Profibus is a very flexible and reliable communication technology where issues can be caused by small and simple errors. On the other hand, RS485 is a more balanced technology that makes use of a two-wire transmission system. In effect this makes it the difference in voltage between the two wires among which the communication or data exchange is happening. For the same reason that it doesn’t depend on the voltage directed to the ground, it is more stable. This also makes it less sensitive to interference when compared to a single-ended transmission.