In addition, when it comes to modification of logic or process sequence you need to destroy all old wiring and start over rewiring according to the new requirements while you can do this by modifying the program in PLC without touching the wiring. To sum up, the number of wires is reduced significantly, the effort of wiring is immensely reduced. On the other hand, PLC-based control shown on the right, you can notice only the PLC and input devices and output actuators are connected to the PLC. you can imagine how many components and punch of wiring work for connecting sensors, timers, relays, and motors. Now on the left of the figure, you can see the relay logic control. In addition, sensors will be used as protection to emergency stop motors at any time. Let’s say we need to run motor 1 at the start and after a while, we plan to run motor 2 and 4, and at last run motor 3 considering one constraint that no more than two motors can run simultaneously. To image the difference between old fashion classic control and PLC, figure 1 shows the case of a very simple process that contains four motors, and four sensors that are connected via four relays and a timer for performing a very simple logic. When it comes to troubleshooting and maintenance, you should be very patient and generous for your time and efforts to pay to keep tracking hundreds of wires and checking up a bunch of components to figure out the problem. Furthermore, there are no chances to change the logic or the sequence of operation without destroying, rebuilding, and rewiring everything from scratch. In addition, it has a huge number of wires and components to achieve a simple logic. You cannot imagine how complicated that control was besides its limitation in functionalities.
Relay logic control was the old fashion classic control in which input switches and sensors are connected between the hot voltage and relays’ coils to energize these coils and in turn, activate their contacts and thereby connect or disconnect the actuators i.e., motors, lamps, valves etc.
STARTING YOUR FIRST LADDER LOGIC PROGRAM CODE
Consequently, you will have been able to read a ladder logic code and translate the logic in your mind or the electrical circuit between your hand into ladder rungs. By completing this article, you will have enjoyed understanding the basics of the LD programming language. Each rung represents a line of logic by connecting inputs logically to form a condition on which the output is determined to be on or off.
This language of PLC programming is called ladder logic or LD as it looks like a ladder of many rungs. Ladder Logic Programming has been derived from relay logic electrical circuits. The language we introduce here is a visualized language that connects and combines graphical symbols in logical flow same as the way we wire electrical circuits and that is the secret behind its simplicity not only in implementation but also in diagnosing problems.
After welcoming every one of engineers, technicians, students, and hobbyists who have come to read this article willing to learn PLC programming, I would like to introduce one of the most used programming languages of PLC. It’s our first tutorial in this series, so we are going to have a look at the detailed introduction to PLC and ladder logic.
STARTING YOUR FIRST LADDER LOGIC PROGRAM SERIES
Today, we are starting a new tutorials series on Ladder Logic Programming, used in PLC.