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PLC Ladder Logic Functions for Electrical Engineers - Beginners
PLC Ladder Logic Functions for Electrical Engineers - Beginners (on photo: SIEMENS PLC Simatic S7-300; credit: dbicontrol.co.uk)

TRUE or FALSE condition //

To understand programmable logic controllers (PLCs) and their applications, you must first understand the logic concepts behind them. We’ll will explain the relationship between Boolean algebra and logic contact symbology, so that you will be ready to learn about PLC processors and ladder logic functions and diagrams.

The binary concept shows how physical quantities (binary variables) that can exist in one of two states can be represented as 1 or 0.

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.

Programmable logic controllers (PLCs) make decisions based on the results of these kinds of logical statements.

Operations performed by digital equipment, such as programmable controllers, are based on three fundamental ladder logic functions – AND, OR, and NOT. These functions combine binary variables to form statements. Each function has a rule that determines the statement outcome (TRUE or FALSE) and a symbol that represents it.


Ladder Logic Functions //

There are many control situations requiring actions to be initiated when a certain combination of conditions is realized. 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.

Such a situation involves the AND logic function, condition A and condition B having both to be realized for an output to occur. This section is a consideration of such logic functions.

Make sure you read the first part of this article “PLC Ladder Diagrams for Electrical Engineers (Beginners)”. Now let’s talk about the six most used logic functions in PLC ladder programming //

  1. AND function
  2. OR function
  3. NOT function
  4. NAND function
  5. NOR function
  6. Exclusive OR (XOR) function

1. AND logic function

Figure 1a shows a situation where an output is not energized unless two, normally open, switches are both closed. Switch A and switch B have both to be closed, which thus gives an AND logic situation. We can think of this as representing a control system with two inputs A and B (Figure 1b). Only when A and B are both on is there an output.

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.

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.

Thus for the AND gate we have //

InputsOutput
AB
000
010
100
111

 (a) AND circuit (b) AND logic gate
Figure 1 – (a) AND circuit (b) AND logic gate

An 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.

Figure 2a shows an AND gate system on a ladder diagram.

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. The line then terminates with O to represent the output. 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).

In general // On a ladder diagram contacts in a horizontal rung, i.e., contacts in series, represent the logical AND operations.

AND gate with a ladder diagram rung
Figure 2 – AND gate with a ladder diagram rung

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2. OR logic function

Figure 3a shows an electrical circuit where an output is energized when switch A or B, both normally open, are closed. This describes an OR logic gate (Figure 3b) in that input A or input B must be on for there to be an output.

The truth table is //

InputsOutput
AB
000
011
101
111

(a) OR electrical circuit (b) OR logic gate
Figure 3 – (a) OR electrical circuit (b) OR logic gate

Figure 11.10a shows an OR logic gate system on a ladder diagram, Figure 4b showing an equivalent alternative way of drawing the same diagram.

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. Either input A or input B have to be closed for the output to be energized (Figure 4c). The line then terminates with O to represent the output.

In general // Alternative paths provided by vertical paths from the main rung of a ladder diagram, i.e., paths in parallel represent logical OR operations.

An example of an OR gate control system is a conveyor belt transporting bottled products to packaging where a deflector plate is activated to deflect bottles into a reject bin if either the weight is not within certain tolerances or there is no cap on the bottle.

OR gate
Figure 4 – OR gate

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3. NOT logic function

Figure 5a shows an electrical circuit controlled by a switch that is normally closed. When there is an input to the switch, it opens and there is then no current in the circuit. This illustrates a NOT gate in that there is an output when there is no input and no output when there is an input (Figure 5c). The gate is sometimes referred to as an inverter.

The truth table is //

Input AOutput
01
10

Figure 5b shows a NOT gate system on a ladder diagram. The input A contacts are shown as being normally closed. This is in series with the output ( ). With no input to input A, the contacts are closed and so there is an output. When there is an input to input A, it opens and there is then no output.

An example of a NOT gate control system is a light that comes on when it becomes dark, i.e., when there is no light input to the light sensor there is an output.

(a) NOT circuit (b) NOT logic with a ladder rung (c) high output when no input to A
Figure 5 – (a) NOT circuit (b) NOT logic with a ladder rung (c) high output when no input to A

4. NAND logic function

Suppose we follow an AND gate with a NOT gate (Figure 6a). The consequence of having the NOT gate is to invert all the outputs from the AND gate. An alternative, which gives exactly the same results, is to put a NOT gate on each input and then follow that with OR (Figure 6b).

The same truth table occurs, namely //

InputsOutput
AB
001
011
101
110

NAND gate
Figure 6 – NAND gate

Both the inputs A and B have to be 0 for there to be a 1 output. There is an output when input A and input B are not 1. The combination of these gates is termed a NAND gate (Figure 7).

A NAND gate
Figure 7 – A NAND gate

An example of a NAND gate control system is a warning light that comes on if, with a machine tool, the safety guard switch has not been activated and the limit switch signalling the presence of the workpiece has not been activated.

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5. NOR logic function

Suppose we follow an OR gate by a NOT gate (Figure 8a). The consequence of having the NOT gate is to invert the outputs of the OR gate. An alternative, which gives exactly the same results, is to put a NOT gate on each input and then an AND gate for the resulting inverted inputs (Figure 8b).

The following is the resulting truth table //

InputsOutput
AB
001
010
100
110

The combination of OR and NOT gates is termed a NOR gate. There is an output when neither input A or input B is 1.
Figure 9 shows a ladder diagram of a NOR system. When input A and input B are both not activated, there is a 1 output. When either X400 or X401 are 1 there is a 0 output.

NOR gate
Figure 8 – NOR gate

NOR gate
Figure 9 – NOR gate

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6. Exclusive OR (XOR) logic function

The OR gate gives an output when either or both of the inputs are 1. Sometimes there is, however, a need for a gate that gives an output when either of the inputs is 1 but not when both are 1, i.e., has the truth table:

InputsOutput
AB
000
011
101
110

Such a gate is called an Exclusive OR or XOR gate. One way of obtaining such a gate is by using NOT, AND and OR gates as shown in Figure 10.

XOR gate
Figure 10 – XOR gate

Figure 11 shows a ladder diagram for an XOR gate system.

When input A and input B are not activated then there is 0 output. When just input A is activated, then the upper branch results in the output being 1. When just input B is activated, then the lower branch results in the output being 1. When both input A and input B are activated, there is no output.

In this example of a logic gate, input A and input B have two sets of contacts in the circuits, one set being normally open and the other normally closed. With PLC programming, each input may have as many sets of contacts as necessary.

XOR gate
Figure 11 – XOR gate

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Programmable Logic Controller Basics (VIDEO)

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References //

  • Ladder and Functional Block Programming – W. Bolton (Purchase at Amazon)
  • Programmable controllers: theory and implementation by L.A. Bryan and E.A. Bryan (Purchase at Amazon)

About Author //

author-pic

Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on

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