![a plc ladder logic program consists of a number of rungs with each rung controlling an input a plc ladder logic program consists of a number of rungs with each rung controlling an input](https://cdn.slidesharecdn.com/ss_thumbnails/plcprogrammingslideshare-190820191829-thumbnail-4.jpg)
- A PLC LADDER LOGIC PROGRAM CONSISTS OF A NUMBER OF RUNGS WITH EACH RUNG CONTROLLING AN INPUT CODE
- A PLC LADDER LOGIC PROGRAM CONSISTS OF A NUMBER OF RUNGS WITH EACH RUNG CONTROLLING AN INPUT SERIES
A PLC LADDER LOGIC PROGRAM CONSISTS OF A NUMBER OF RUNGS WITH EACH RUNG CONTROLLING AN INPUT CODE
What makes this unusual from a programming standpoint is that the logic is not executed in the normal line by line code that you would see in languages like “C” or “BASIC”, but are executed in an electrical “scan” where every rung of logic is “solved” many times per second, and each time it is “solved”, it is called a “logic scan”.
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![a plc ladder logic program consists of a number of rungs with each rung controlling an input a plc ladder logic program consists of a number of rungs with each rung controlling an input](https://forumautomation.com/uploads/default/original/2X/3/3d13a381651328597c8176ff6a198317ddc529cf.png)
A single line of logic is referred to as a “rung” of logic, a hold over from the “Ladder Logic” name where your program looks like an endless ladder on the screen. When viewed, it looks like the electrical drawings that the original designers would draw to create logic in the relay racks of yesteryear. In a normal ladder logic programming arrangement, each “line” of “logic” starts on the left side of the screen on the “power rail”, goes through several “contacts” (which we call inputs) and then typically ends in a “coil” (which we call outputs) and then it terminates with the “common rail” to complete the virtual circuit. This ladder logic programming language differs from normal instructional languages in several ways due to this legacy. PLCs in that era cost several times more than houses and took large teams of people to implement, but they still represented a significant cost savings relative to the relay racks they replaced. Furthermore, the graphical representation of the networks made troubleshooting much faster when diagnosing a failure. A single PLC could replace hundreds of relays, even thousands of relays and the space savings was immense. This is why the well-funded operations of their time went to programmable logic controllers back in the late 1970s and early 1980s. Even with very reliable relays, they were only good for opening and closing a few thousand times before one or more contacts were burned or the relay coil would simply short circuit and fail to close the contacts, often blowing fuses in the process. Some control systems were pretty basic while others were wonders of complexity and ingenuity.Īs you would expect, determining which relay was faulty was difficult and this approach led to an abundance of measurable downtime.
A PLC LADDER LOGIC PROGRAM CONSISTS OF A NUMBER OF RUNGS WITH EACH RUNG CONTROLLING AN INPUT SERIES
In this manner, design engineers could create a series of action-reaction electrical networks that would create a program that enabled a skilled operator to control a machine in a basic manner. It would have several relays set aside for securing a part, some more relays to automate the machine closing and opening, still more for the conveyors and so on until your whole process was controllable through a series of foot switches, push buttons and various hand controls to allow the operator to control his operation in a manner not unlike the Wizard of Oz. Some relays on the relay rack had timers on them to delay an action, like waiting for 5 seconds for a motor to finish starting before it marked the motor as faulted (energizing the fault relay) and so forth. The contacts would send power out into the devices in the field which would go through a series of switches to check to see if things like the e-stop system is powered and the state of other sensors, and would then energize another relay on the relay rack. This would close all of the normally open contacts on the relay and open all of the normally closed contacts on that relay. Each relay had multiple “contacts” on it and one “coil”.Ī relay’s coil would be connected to an operator panel switch where an operator could energize the coil by pressing a button or turning a switch. PLC Basics: The History of PLCsĪ circa 1970s relay rack, usually found in automobile plants or other large scale industrial complexes, consisted of a large 2 dimensional array of relays placed in a grid pattern, often on multiple 4ft x 8ft sheets of plywood. These racks of relays were numbered in some fashion, usually top to bottom and left to right like spreadsheet cells. This was invented because a Programmable Logic Controller was designed to replace something called a relay rack. PLCs in North America are usually programmed in a graphical language called ladder logic. PLCs are unusual devices in the way that they operate. Following up “The Basics of a PLC”, read here PLC Basics: learn about the history of PLC’s, how PLC’s have traditionally worked, and how they work today.