By Dave Harrold
Dick Morley, the "father" of the PLC explained in a recent exclusive interview, "Our design philosophy was simple; put a bunch of relays in a box!"
At the time, Morley was working at Bedford Associates, and because this was the 84th Bedford Associates project, the first Programmable Logic Controller (PLC) was dubbed "084." Eventually the team named its invention the MOdular DIgital CONtroller—Modicon for short—and the rest is a well-documented history of the PLC.
The PLC was born in the early years of microprocessor development, when all sorts of software development languages were emerging, so the question occurs: "Why choose Relay Ladder Logic (RLL) as the programming language for the PLC?"
A Language for All Seasons
To that question Morley is quick to reply, "Technology doesn't count. It's what you do with the technology that counts. Because our philosophy was to put a bunch of relays in a box, we felt that ‘our box' was more likely to secure user acceptance if it ‘felt' more like what plant-floor users already had accepted and understood, and that was RLL. You see, at the time we created the PLC, we viewed, and still do, that PLC users are really the plant floor electricians and instrument technicians. Since these folks rely almost exclusively on RLL schematics to do their troubleshooting, it made sense then, and it makes sense now, that we provide them access to technology in a way that they are already comfortable using, and that is RLL."
Obviously, the decision of Morley and his buddies working on project 084 was the correct choice because, despite the existence of the programming standard IEC 61131-3 and its inclusion of Sequential Function Chart language and four interoperable programming languages (Instruction List, Ladder Diagram, Function Block Diagram and Structured Text), RLL remains the programming language of choice among users and integrators worldwide for most PLC applications.
The original creator of RLL schematics is lost forever in the chronicles of history, but we do know that during the 19th Century's Industrial Revolution, ladder logic schematics became the preferred graphical illustration method for a wide variety of electrical control systems. Today RLL schematics are found glued to the back of your dishwasher and clothes dryer, included in your refrigerator and garage door opener user manuals, and tucked into panels throughout processing and manufacturing plants worldwide.
Why? Because a recent discussion about RLL's popularity on the plctalk.net forum produced similar responses from Bob in Australia, Cameltoe in Tonga, Nathan in Korea, Thomas in U.S. and Sergei in Canada. Each said in their own way that RLL enjoys worldwide acceptance because it's an intuitive, graphical means of depicting both simple and complex control logic.
Jean Pierre Vandecandelaere, a trainer with VDAB Competentie Centrum in Belgium, explains that, "I teach seven different brands of PLC, and the elements of relay ladder logic schematics differs very little from brand to brand."
Mark Muhaw, an automation engineer with Ind-Concepts (http://plctrainer.net/) in South Carolina, provides another legitimate perspective about RLL's universal appeal, "My clients expect me to turn over PLC implementations that don't require them to contact me for on-going support. If they ever do need my support, I find that RLL makes it much easier for me to get myself back-up-to-speed on a particular implementation months or even years later."
So there you have them. The three top reasons why relay ladder logic remains the control language of choice of PLC users worldwide: it's easy to learn; robust to use; and is transparent across a huge number of platforms.
Figure 1. A typical relay ladder logic schematic for starting and stopping a motor.
Easy to Learn
RLL is simple. Relay ladder logic functions are illustrated with two vertical rails connected by several horizontal rungs, thus the name "ladder logic." Typically, the two vertical rail are powered such that together they represent an electrical circuit. As with any electrical circuit, each rung must include some sort of electrical load (i.e., relay coil, indicator lamp, etc.) On/off control of each rung is achieved through one or more sets of contacts (i.e., relay contacts, pushbuttons, etc.) In a ladder logic circuit, normally open (NO) contacts are type-A contacts, and normally closed (NC) contacts are type-B contacts. Connecting multiple contacts in series where all contacts must be closed in order to complete the circuit forms an "AND" function. Connecting multiple contacts in parallel provides a means of completing the circuit when any one of the contacts closes, thus forming a logical "OR" function. Relay coils, whether real or a "virtual" device mimicked by the PLCs internal software, control one or more sets of contacts. When a relay coil is energized or "picked up," its NO contacts close, and its NC contacts open. Conversely, when the coil is de-energized or "dropped out," its contacts return to their normal (shelf) state (Figure 1).
Instructor Bill Simpson of The Learning Pit advises, "Based on teaching PLCs for 15-plus years, there's one thing that I can guarantee that you will find out about PLCs and particularly about RLL. The simple programs and instructions you learn early in your training are by far the hardest to grasp and clearly understand. As you advance, you'll be amazed at how much easier it all seems to get! So, take heart, take it slow, and don't skip to those advanced instructions until you know the simple ones by heart. Yes, they really are that important!"