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Industry News

May 19 2017

PLC Programming (Prelude) - A Brief History of PLCs

Posted by RF Jordan in Industry News

To gain an appreciation for the difference between where we are in creating PLC applications and where we should be, requires a reflection on the origin and evolution of parallel technologies.  In this article, we will compare how the functionality of PLC technology has kept pace with telephony.


Dick Morley was instrumental in bringing computer technology into the industrial world in a very practical way, through the introduction of the Programmable Logic Controller (PLC).  He is often referred to as the “Father of the PLC.”  He did this in the late 1960’s.  He called his industrial programmable computer the MOdular DIgital CONtroller, or MODICON.  To this day, Schneider Electric continues to manufacture PLCs with this moniker.   In the waning days for the industrial revolution, this technology transformed the way industry approached the manufacturing process.  Though the early PLCs were expensive, astute manufacturers recognized immense cost savings through the reduction of time it took to design and commission control systems.

At the time that PLCs arrived on the high technology horizon, there was another technology that was (it seemed) fully mature.  The telephone was patented in 1876 by Alexander Graham Bell and within about a decade, evolved into a global network communication system.  So, what does this have to do with PLCs?  Consider the scope of telephone technology at the time of the introduction of the PLC which really hadn’t changed that much from the late 1880’s.  In the late 1960s, there was some touch-tone capability with telephones, but most people had “rotary” phones.  Many consumers shared “party-lines” where you could inadvertently become an interloper into your neighbors’ conversation.  Also, the costs associated with long-distance calling were significant.


Now, think about what the telephone has developed into.  Not only do we not concern ourselves with distance, we are not bound by physical phone-lines and corded handsets.  We are not limited to voice communications either.  Today our phones are capable of taking and sending pictures to our family and friends, sending e-mails, browsing the Internet, manage your finances, get weather updates, and don’t forget the barrage of Apps that you can put on your phone.


In light of considering the capabilities of your phone, think about the integration of PLCs in industry over the same time period.  During its’ infancy, Morley developed programming tools that mimicked the symbolism used by plant-floor electricians.  Trade schools, then as now, teach tradesmen the schematic representation of the electromechanical relays that, up to that time, was the only effective way of control their machines.  Morley realized that, for his invention to gain industry acceptance, he would have to use terms and images that were familiar to his target audience.  So, in the decade that followed, “Ladder Logic” programming became the predominant tool for programming PLCs (in the United States anyway).


If we compare the advancements in PLC programming, over the same time period that we considered for the telephone, what would you expect to see?  You guessed it, Ladder Logic.  After forty years the predominant programming tool used today, is still Ladder Logic.  You may ask, “Why hasn’t PLC technology advanced?”  The truth of the matter is, that is has.  Back in the 1990’s, the consortium known as PLCOpen released a standard that defined five different programming tools, and tools that incorporated current computer design and development methods, for PLCs.  All of these advancements together allow the programmer to apply high-level object-oriented programming techniques to industrial applications.  This includes the use of structured data-types, data and object abstraction, encapsulation, and hierarchical objects that inherit properties from parent objects.  Properly applied, these tools could have a significant impact on reducing the program development time and provide standardization at an object level rather than at a program level.  This leads to portability of code among various programming environments, even among various manufacturers.


So why don’t we take advantage of these tools that have been in the marketplace for over twenty-five years?  The basic elements for programming in ladder logic are simple enough that even those untrained in electronics can understand it.  Unfortunately, it takes a lot of these elements to create logic to meet the ever-increasing demands of today’s applications.  Ladder logic is not well equipped to manipulate data in the Information Age. Many engineering firms don’t believe they have time to learn the new tools.  By missing the opportunity to implement the more powerful programming tools they extend the development, commissioning, and trouble-shooting time and, consistently, miss their deadlines because of their antiquated methods.  For some reason, the people who seemed to be able to learn how to capitalize on the advancements in telephone technology can’t seem to get beyond the contacts and coils in their PLCs.