PLCs are the most prevalent computers we have around us yet they are less exposed than your traditional PC. We encounter PLCs almost everywhere as we go about our usual business; when we press that up or down button on a lift and the lift and the door open, we then go ahead to press the button for the floor of our destination. The conveyor at the airport check where our luggage is scanned even for those us in the industry, that conveyor belt at a bottling plant and many more. All these area PLC applications in which an input like pushing a button registers with an input device and based on the installed program in the computer, it triggers a certain output like an elevator door opening for you to get in or get out once you get to your destination.

Figure 1. Clicking on an elevator.

A PLC (Programmable Logic Controllers) is an industrial computer used to monitor inputs and depending upon their state make decisions based on its program or logic, to control (turn on/off) it’s outputs to automate a machine or a process. PLC development began in 1968 in response to a request from General Motors and were first installed in industry in 1969 with communications abilities beginning around 1973. They could be used send and receive varying voltages to allow them to enter the analogue world.

Figure 2. PLCs work with inputs& outputs.

The 80′s saw an attempt to standardize communications with manufacturing automation protocol (MAP), reduce the size of the PLC, and making them software programmable through symbolic programming on personal computers instead of dedicated programming terminals or handheld programmers. The latest standard “IEC 61131-3″ has tried to merge PLC programming languages under one international standard. We now have PLCs that are programmable in Function Block Diagrams -FBD, Ladder Language -LAD Instruction Lists -IL, C and Structured Text Language -STL all at the same time.
Comments of a PLC include:
• Central Processing Unit
• Memory unit
• Power supply.
• Input/Output Modules.
• Programming device.

The need for automation is obviously growing hence the need to have a control system that is easily programmable, flexible, versatile and cost effective. Its evident that PLCs are being utilised in various industrial segments including Energy Efficiency, process control in manufacturing i.e. Food and Beverage lines, Assembly Lines, Power Generation and Port Operations just to mention a few. Infact the advent of Industry 4.0 (current trend of automation and data exchange in manufacturing technologies) concept has heightened the demand for PLCs.

PLC Operations

The PLC program is executed as part of a repetitive process referred to as a SCAN CYCLE.


A PLC scan starts with the CPU reading the status of inputs. The application program is executed using the status of the inputs. Once the program is completed, the CPU performs internal diagnostics and communication tasks. The scan cycle ends by updating the outputs, then starts over. The cycle time depends on the size of the program, the number of I/Os, and the amount of communication required. Typical SCAN CYCLE time is 100-150ms

Programming is done through programming terminal. Programming terminal translates
engineering language (logic control) to machine language (binary code). Most PLC manufacturers offer software packages that allow a standard computer to be used as a programming terminal:
Example of programs
• RS Logix for Allen Bradley
• GX Works 2 for Mitsubishi
• Unity pro for Schneider

Global Players

There are a number of PLC Manufacturers globally with Siemens being the most popular PLC you will find around:


Viscar Industrial Capacity Limited prides itself with expertise and capacity to offer you PLC Training for both Basic and Advanced levels tailored for different manufactures including: Siemens, Allen Bradley, Omron, Mitsubishi, GE Versa Max , Schneider Electric etc. We will be conducting a PLC level one training on the 7th to 11th May 2018 at the Viscar Knowledge Room. To learn more about this course click here