The end rung might be indicated by a block with the word END or RET for return, since the program promptly returns to its beginning. This procedure of going through all the rungs of the program is termed a cycle. When the PLC is in its run mode, it goes through the entire ladder program to the end, the end rung of the program being clearly denoted, and then promptly resumes at the start. Then the second rung down is read from left to right and so on. Convention 3 //Ī ladder diagram is read from left to right and from top to bottom, Figure 3 showing the scanning motion employed by the PLC. Convention 2 //Įach rung on the ladder defines one operation in the control process. The power flow is taken to be from the left-hand vertical across a rung. The vertical lines of the diagram represent the power rails between which circuits are connected. In drawing a ladder diagram, certain seven conventions are adopted: Convention 1 // Circuits are connected as horizontal lines, i.e., the rungs of the ladder, between these two verticals. The ladder diagram consists of two vertical lines representing the power rails. Writing a program is then equivalent to drawing a switching circuit. Thus we have a motor which is started by pressing button 1 and stopped by pressing button 2.Ī very commonly used method of programming PLCs is based on the use of ladder diagrams. Thus when push button 2 is released, there is still no power to the motor. This disconnects the power to the motor and the holding contacts open. Thus when the push button 1 is released, the holding contacts maintain the circuit and hence the power to the motor. Also, the holding contacts wired in parallel with the motor close and remain closed as long as the motor is running. When button 1 is pressed, the motor circuit is completed and the motor starts. In the normal state, push button 1 is open and push button 2 closed.
With ladder diagrams no attempt is made to show the actual physical locations and the emphasis is on clearly showing how the control is exercised.įigure 2 (see below) shows an example of a ladder diagram for a circuit that is used to start and stop a motor using push buttons. Circuit diagrams often show the relative physical location of the circuit components and how they are actually wired.
#Develop a simple ladder logic program where pressing one momentary button series#
In the case of Figure 1 it is just the switch in series with the motor. The horizontal rungs show only the control portion of the circuit. The power lines, or rails as they are often termed, are like the vertical sides of a ladder with the horizontal circuit lines like the rungs of the ladder. With such a diagram the power supply for the circuits is always shown as two vertical lines with the rest of the circuit as horizontal lines. Figure 1 – Ways of drawing the same electrical circuit The circuit shown in Figure 1b is termed a ladder diagram. Both circuits have the switch in series with the motor and supplied with electrical power when the switch is closed. We can redraw this diagram in a different way, using two vertical lines to represent the input power rails and stringing the rest of the circuit between them.įigure 1b shows the result. PLC Ladder Diagrams for Electrical Engineers - Beginners (photo credit: )