June 15, 2017

How the concept of PLCs was introduced – Knowledge bytes from AAG Engineering


Before PLC was even introduced, all connections were made through hardware panels. These hardware panels required a huge amount of time to be wired. The challenges were not limited, and every change was a time-consuming process. In case if a bug was reported then debugging would become a very cumbersome process too.

To overcome above challenges, PLC or Programmable Logic controller was introduced in the late 1960’s.

It was developed to offer the same functionality as the relay logic systems that were existing at that time.

But some features of PLC made it outstanding from other logic controllers. Those are:

  • PLC was in solid-state and not mechanical
  • It enabled ease in modifying input and output devices
  • Even plant electricians were able to program and maintain it with ease
  • It can withstand a harsh industrial environment
  • It had no hard drive
  • It had battery backup
  • It can start in few seconds
  • It used ladder logic for programming

With these features, PLCs started finding a place in almost every industry. With the technological advancements, these features were even improvised to suit the requirements of the end customer.

Today, PLC has reached a level where it can be customized to cater to the personal needs of each and every plant or industry.

Moreover, it can be interfaced with all types of sensors and output devices like

  • Proximity or photoelectric sensors
  • Limit switches
  • Pushbuttons

These output or input devices can be analog as well as digital and can be single or multiples with

  • Extended temperature ranges
  • Immunity to electrical noise
  • Resistance to vibration and impact

Therefore PLCs run a four-step operational process in a repeating loop:

  1. Input Scan: Looks for all connected input devices
  2. Program Scan: Executes user-created program logic
  3. Output Scan: Runs or shuts off all connected output devices
  4. Housekeeping: communicates with programming devices and performs internal diagnostics

Thus it makes sure that it has taken input from all devices and has performed as programmed with all output devices.

At AAG Engineering Technologies, we ensure that our supplied PLC’s are programmed to meet the requirements of our end customers.

With our experience and an expert team of professionals, we analyze the requirements of our client and even visit the place to take a closer look at all specifications and accordingly design the PLC that can meet the requirements.

If you wish to install PLC at your unit, you can enquire our sales team at or have a better look at all our services and products at



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