Either input A or input B have to be closed for the output to be energized (Figure 4c). The ladder diagram starts with | |, normally open contacts labeled input A, to represent switch A and in parallel with it | |, normally open contacts labeled input B, to represent switch B. This describes an OR logic gate (Figure 3b) in that input A or input B must be on for there to be an output.įigure 3 – (a) OR electrical circuit (b) OR logic gateįigure 11.10a shows an OR logic gate system on a ladder diagram, Figure 4b showing an equivalent alternative way of drawing the same diagram. Figure 2 – AND gate with a ladder diagram rungįigure 3a shows an electrical circuit where an output is energized when switch A or B, both normally open, are closed. In general // On a ladder diagram contacts in a horizontal rung, i.e., contacts in series, represent the logical AND operations. For there to be an output, both input A and input B have to occur, i.e., input A and input B contacts have to be closed ( Figure 2b).
The line then terminates with O to represent the output.
#THE MAIN FUNCTION OF THE LADDER LOGIC PROGRAM IS TO CONTROL OUTPUTS BASED ON ______ CONDITIONS SERIES#
The ladder diagram starts with | |, a normally open set of contacts labeled input A, to represent switch A and in series with it | |, another normally open set of contacts labeled input B, to represent switch B. Such an operation is said to be controlled by a logic gate and the relationship between the inputs to a logic gate and the outputs is tabulated in a form known as a truth table.įigure 1 – (a) AND circuit (b) AND logic gateĪn example of an AND gate is an interlock control system for a machine tool so that it can only be operated when the safety guard is in position and the power switched on.įigure 2a shows an AND gate system on a ladder diagram. Thus if we use 1 to indicate an on signal and 0 to represent an off signal, then for there to be a 1 output we must have A and B both 1. Only when A and B are both on is there an output. We can think of this as representing a control system with two inputs A and B (Figure 1b). Switch A and switch B have both to be closed, which thus gives an AND logic situation.
Such a situation involves the AND logic function, condition A and condition B having both to be realized for an output to occur. Thus, for an automatic drilling machine, there might be the condition that the drill motor is to be activated when the limit switches are activated that indicate the presence of the workpiece and the drill position as being at the surface of the workpiece. There are many control situations requiring actions to be initiated when a certain combination of conditions is realized. Each function has a rule that determines the statement outcome (TRUE or FALSE) and a symbol that represents it. These functions combine binary variables to form statements. Operations performed by digital equipment, such as programmable controllers, are based on three fundamental ladder logic functions – AND, OR, and NOT. Programmable logic controllers (PLCs) make decisions based on the results of these kinds of logical statements. Now, you will see how statements that combine two or more of these binary variables can result in either a TRUE or FALSE condition, represented by 1 and 0, respectively. The binary concept shows how physical quantities (binary variables) that can exist in one of two states can be represented as 1 or 0. PLC Ladder Logic Functions for Electrical Engineers - Beginners (on photo: SIEMENS PLC Simatic S7-300 credit: .uk)
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