Digital Logic
Objective
The experimental objective of this lab is to design a combinational logic circuit for a given problem statement, and to activate it under specific conditions and test it using LabVIEW. After testing, it will be built on an NIELVIS II+ prototyping board.
Overview
The first step in understanding the digital circuits that control the function of electronic devices is the mastery of Boolean logic. George Boole, an English mathematician, established modern symbolic logic in 1854 with the publication of his paper, "Laws of Thought." Boolean logic is the foundation of digital circuitry. Boole's method of logical inference allows conclusions to be drawn from any proposition involving any number of terms. Boolean logic demonstrates that the conclusions found are logically contained in the original premises (Encyclopedia Britannica, 2003).
In Boolean logic, there are only two values, true and false, represented by the numbers 1 and 0, respectively. These values are combined in equations to yield results that also have these two values. The equations are represented by truth tables that show the inputs to the equation and the outputs produced for those inputs. The rows of the table contain all the possible combinations of 1s and 0s for the inputs. The number of rows is determined by the number of possible combinations.
To illustrate a truth table and additional concepts that will be introduced, an example problem statement will be given. Think of an ATM that has three options: print a statement, withdraw money, or deposit money. The ATM will charge a fee to (1) withdraw money or (2) print a statement without depositing money. The intent of the problem is to develop a Boolean equation and logic circuit that will determine after which possible combination of actions will someone get charged a fee. First, a truth table should be made for all the possible combinations of inputs. The inputs are the ATM's three functions. Let variable stand for printing a statement, for withdrawing money, and for depositing money. There is one output, which is whether or not the ATM will charge a fee. The output will be denoted by . The truth table in Table 1 shows all the possible combinations of the inputs and their corresponding outputs.
Table 1: Truth Table for the ATM Example
Inputs  Output  

0  0  0  0 
0  0  1  0 
0  1  0  1 
0  1  1  1 
1  0  0  1 
1  0  1  0 
1  1  0  1 
1  1  1  1 
Boolean logic is applied to digital circuitry through the use of simple logic gates. There are symbols for each of these gates, and the connections between them are represented by lines running from the output of one gate to the input of another. A line can connect only one output to each input. There are seven of these gates: the NOT, AND, OR, NAND, NOR, XOR, and XNOR gates. Only the first three will be used in this lab (Figure 1).
The NOT gate is the simplest of these three. It is an inverter. It has one input and produces its opposite as the output. For example, if a 1 value is put into a NOT gate, a 0 value is outputted, as seen in Table 2. The symbol for the operation is a horizontal bar over the variable. The truth table for a NOT gate is shown in Table 2.
0  1 
1  0 
The AND gate performs a multiplication operation on its inputs. If all the inputs are true, the output is also true. But if either of the inputs is false, the output is also false. An AND gate can have two or more inputs, but for this lab, it will have two inputs (denoted by A and B in Table 3). The symbol for the AND operation is a dot (·) or the two inputs one after the other with nothing between them. The truth table for an AND gate is shown in Table 3.
0  0  0 
0  1  0 
1  0  0 
1  1  1 
An OR gate performs an addition operation on its inputs. If either of the inputs is true, the output is also true. But if all the inputs are false, the output is also false. An OR gate can have two or more inputs, but for this lab, it will have two inputs (denoted by A and B in Table 4). The symbol for the OR operation is a plus (+). The truth table for an OR gate is shown in Table 4.
0  0  0 
0  1  1 
1  0  1 
1  1  1 
A truth table is used to write a Boolean equation for a problem. All the combinations that yield an output of 1 are kept, and the equation is written. This is called a Sum of Products solution. Only the combinations that yield an output of 1 are kept because the Boolean equation intends to represent a quantitative function for when the result will have a value of true (when a fee is charged in the example of the ATM machine). The combinations that yield an output of 0 are essentially discarded because there is no interest in when the result has a false value. The Boolean equation for the ATM example can be seen in Figure 2.
Figure 2: Boolean Equation
For each of the combinations (rows) resulting in true in Table 1, the terms of the Boolean equation are determined by multiplying the variables together (the AND operation). If an input is true, the variable can be used as is; for an input that is false, the variable is inverted using the horizontal bar (the NOT operation). Since the output is true if any of the combinations are true, the equation is formed by using addition operations (the OR operation) on all the terms formed by the product of each combination.
A truth table showing every possible combination of inputs and their corresponding outputs yields a Boolean equation that is the solution to a problem in digital logic. However, this equation can be simplified using a Karnaugh map (Kmap). A Kmap identifies and eliminates all the conditions that do not contribute to the solution. The resulting simplified Boolean equation is used to build the digital circuit and will be a combination of the logic gates described above.
A Kmap is a twodimensional representation of the truth table that shows the common characteristics of the inputs. For an equation with three inputs, usually all the combinations of the first two inputs are shown as four columns and the values for the third input are shown as two rows. For four inputs, all the combinations of the third and fourth inputs are shown as four rows. Only one value can change at a time in adjacent rows or columns. For example, in Table 5, the columns change from 00 to 01 to 11 to 10. Table 5 illustrates the Kmap for the ATM example.
0 0  0 1  1 1  1 0  

0  0  1  1  1  
1  0  1  1  0 
To develop the simplified Boolean equation, look for the cells that have the 1 value in common. Look for cells that can physically be boxed together, where the number of cells in the box is a power of 2 (1, 2, 4, or 8 and so on). The boxes are allowed to overlap so the same cells can be used more than once. Look for the biggest boxes first, and keep finding smaller boxes until all the cells with a 1 in them are used. This means that sometimes just single cells will remain at the end. The cells in the Kmap for the ATM example can be grouped like what is shown in Figure 3.
In this example, all the cells containing a 1 were covered with two boxes. This means is that if the conditions for either box are true, the output will be true. This also means that the simplified Boolean equation will only have two terms. The term for each box can be determined by observing which variables are constant throughout each cell of that box. For the red box, only remains constant through the four cells. switches from the top row to the bottom row, and switches from the left column to the right column. For the blue box, and remains constant through the two cells, while switches from the left cell to the right cell. The two boxes yield the simplified Boolean equation shown in Figure 4.
This equation meets the conditions in the truth table in Table 1 and achieves the desired result with the minimum number of logic elements.
Lastly, a combinational logic circuit can be created using the simplified equation. First, a NOT operation is performed on by inputting and outputting through a NOT gate. Then, the inverted and are inputted into an AND gate, as denoted by them being multiplied in Figure 4. Finally, the result of the AND operation between the inverted and and the are inputted into an OR gate, as denoted by the addition function in Figure 4. The combinational logic circuit can be seen in Figure 5.
Materials and Equipment
 A 7432 IC (4 dualinput OR gates)
 A 7408 IC (4 dualinput AND gates)
 A 7404 IC (6 singleinput NOT gates)
 A lab PC with LabVIEW software
 An NIELVIS II+ with prototyping board
 Wiring
Sample Problem
An ATM has three options: print a statement, withdraw money, or deposit money. The ATM will charge a $1 fee to:
 Withdraw money
 Print a statement without depositing money
Truth Table
The inputs are the ATM's three functions. Let variable P stand for printing a statement, W for withdrawing money, and D for depositing money. There is one output, which is whether or not the ATM session has a cost. The output is C. The truth table in Table 6 shows the combinations of the inputs and their corresponding outputs.
Inputs  Output  

P  W  D  C 
0  0  0  0 
0  0  1  0 
0  1  0  1 
0  1  1  1 
1  0  0  1 
1  0  1  0 
1  1  0  1 
1  1  1  1 
In Table 6, a 1=true and 0=false.
The inputs are P=print, W=withdraw, D=deposit. The output is 0=false (do not charge) or 1=true (charge $1.00).
Boolean Equation
The combinations that yield an output of 1 are kept. For each of these combinations, determine the input values and multiply those variables together. If an input is true, it can be used as is; for an input that is false, invert it using the horizontal bar NOT operation. Since the output is true if any of the combinations are true, the equation is formed by using addition operations on all the terms formed by the product of each combination. The Boolean equation in Figure 5 is created.
Figure 5: Boolean Equation
Karnaugh Map
A table like the one in Table 7 that maps out all the possible combinations is drawn. Only one variable can change at a time between adjacent columns. For this example, the columns will change from 00 to 01 to 11 to 10, as shown in Table 7. A TA will explain this further. The cells that are true are grouped in rectangles of two, four, or eight cells. This shows the values that these cells have in common. These common values yield the simplified Boolean equation. The Kmap in Table 7 corresponds to the Boolean equation created.
Compare this Kmap in Table 7 with the equation to see why some cells are 1 and the others are 0.
0 0  0 1  1 1  1 0  

0  0  1  1  1  
1  0  1  1  0 
Simplified Boolean Equation
Form the largest rectangles of two, four, and eight cells. There is a rectangle of four cells (2 rows by 2 columns) plus an overlapping rectangle of two cells (1 row by 2 columns), as shown in Table 8.
Always try to group the greatest number of common values in powers of 2.
To create a simplified Boolean equation, group all the combinations together and eliminate those that contain both values of a variable. For example, in the big rectangle in the middle, W is always true, but the rectangle also includes both the true and false values of P and D. This means that if W is true, the values of P and D do not matter since all their combinations are included. Similarly, for the small rectangle, if P is true and D is false, the value of W does not matter since both its true and false values are included in the rectangle. Putting this together, if W is true, or P is true and D is false, the output should be true, and this includes all the input conditions required to produce a true output. A simplified Boolean equation for this problem is shown in Figure 6.
Figure 6: Simplified Boolean equation
This equation in Figure 6 is much simpler than the original equation, but is fully equivalent to it.
Combinational Logic Circuit
Construct a combinational logic circuit using the simplified equation. If a NOT operation is performed on D, and an AND operation is performed on the result then P, the second term in the equation, is found. This result combined with performing an OR operation with W produces the result.
This is shown in Figure 7.
Problem Statement
A farmer owns a 350acre dairy farm in upstate New York. In addition to milk and butter, the farmer sells fresh eggs at the Union Square Greenmarket in Manhattan. It is imperative that the farmer protect the hen that lays his eggs.
He has two barns, one hen, and a supply of corn. A fox has been attempting to eat the hen by hiding in a barn. The hen can move freely from one barn to the other. Farmer Georgi sometimes stores corn in one barn and sometimes in the other, but he never stores it in both at the same time. The hen would like to eat the corn and the fox would like to eat the hen. Farmer Georgi needs an alarm system that uses digital logic circuits to protect the hen and the corn. The design should use the fewest gates and input variables possible. The alarm will sound if:
 The fox and the hen are in the same barn
 The hen and the corn are in the same barn
Procedure
Prior to Beginning the Procedure
Before performing the lab, the integrated circuits (IC) used must be tested to determine if they are functioning. If the procedure is completed without verifying that the ICs work, problems may occur in the circuit that require additional time and troubleshooting. The system for checking the ICs uses an Arduino to send and receive signals from the AND/OR/NOT chips and determines if they are functioning properly.
First, load the chip onto the IC tester (Figure 8) and start the Serial Monitor on the Arduino program. The code will cycle through the basic inputs (0, 1 for NOT; 00, 01, 10, 11 for AND/OR) and then send the outputs of each gate within the chip back into the Arduino. The code compares those results to the expected result. It displays each input/output pair and displays the final result showing if the chip is functional or defective. The goal is to determine if the gates are outputting the correct results. Remove the chip carefully from the IC tester.
Problem Statement Procedure
Note: The two barns must be assigned a numeric equivalent before the truth table is prepared for Step 1. Use 1 for Barn 1 and 0 for Barn 2. For the alarm output, use 0 to indicate the alarm should be off, and 1 to indicate that the alarm should be on.
 On a sheet of lined lab note paper from the EG Website, create a truth table that includes three inputs and one output. Assign the input variables. Include all possible scenarios for the hen, the corn, and the fox.
 Compute the output column. To do this, analyze the three inputs and determine if the alarm will sound in each scenario. Place a 1 in the output column if the alarm will sound and a 0 if it will not.
 Note all the combinations that produce a 1 in the output column.
 Create a Boolean equation from this table that includes each of the inputs that produced a true output.
 Create a Kmap. Draw a map that lists all the possible combinations. Use the Boolean equation to fill in the 1s on the Kmap.
 Circle the pairs of 1s. The 1s may only be circled in powers of 2 starting with the largest possible combination and working down to the smallest. The outcomes that do not contribute to the solution are discarded.
 This process yields a simplified Boolean equation. Write this simplified equation down.
 Draw a schematic diagram of the simplified Boolean equation.
 Have a TA sign the diagram.
 Test the circuit diagram. Build the circuit in LabVIEW. Use the simplified Boolean equation. Open LabVIEW and select New VI. Pull down the Window menu, select Tile Left and Right.
 Place three switches on the front panel that represent the hen, the corn, and the fox. Place one Boolean indicator on the front panel to represent the output.
 Open the Functions palette. Under Programming, select Boolean and choose the AND, NOT, and OR gates necessary for the circuit. A TA will assist in this process.
 Once the LabVIEW program is complete, have a TA check the circuit.
 Build the circuit on a prototyping board.
 On the board, identify each of the three black chips as an AND, OR, or NOT gate. To do this, read the number on the chip and match it with the list in the Materials and Equipment section of this lab. Use the pinouts in Figure 9 to wire the gates. Look for the notch at one end of each chip to orient the chip to diagrams in Figure 9 to match the pinouts.
 Power the chips. Insert one end of the wire into a hole on Row 54 (marked +5V) of the side breadboards. Insert the other end of the wire into the hole on the breadboard nearest Pin 14 on the chip. Repeat this process for all three chips.
 The breadboard is set up in the following manner:
 The leftmost and rightmost columns of slots (the ones in between the red and blue lines) are connected vertically.
 The rest of the breadboard is connected horizontally, however, there is a break that runs down the center of the breadboard, separating Columns AE and FJ, as shown in Figure 10.
 Ground the chips using the same method. Insert one end of a wire into a hole on Row 53 (marked Ground) of the small breadboards and the other end into Pin 7 of the chip. Repeat this process for all three chips.
 Look at the top right corner of the breadboard to find the Digital Input/Output (DIO) holes. Select one of the DIO row of holes to use as one of the variables and begin wiring the circuit based on the simplified Boolean equation. Assign the other DIO rows for the remaining variables.
 Continue this process until the entire circuit is wired. Insert one end of the wire into the final output of the simplified Boolean equation. Attach the other end to any hole marked LED on the board.
 Plug in the NIELVIS II+ and connect the USB cable to the lab PC.
 Power on the NIELVIS II+ by toggling the switch on the back of the device. When the popup window appears, select NIELVISmx Instrument Launcher and click OK (Figure 11).
 Click on the DigOut Icon on the Instrument Launcher window (Figure 12).
 Select the appropriate range for the Lines to Write setting in the Digital Writer dialog window that corresponds to the DIO rows chosen. For example, if fox, hen, and corn are on DIO 21, DIO 22, and DIO 23, select 16–23.
 Click Run and toggle the switches corresponding to fox, hen, and corn.
 Have a TA check the circuit.
The lab work is now complete. Please clean up the workstation. Return all unused materials to a TA. Refer to Assignment for the instructions to prepare the lab report.
Assignment
Team Lab Report
Note: You will be writing a team lab report rather than an individual one. See the Team Authoring Strategies page in the Technical Communication of this online manual for guidance of how to do this.
Follow the lab report guidelines laid out in the page called Specifications for Writing Your Lab Reports in the Technical Communication section of this manual. The following points should be addressed in the appropriate section of the lab report:
 Describe the problem that was solved in the Introduction
 Describe how AND, OR, and NOT gates work
 What is a truth table? How does it contribute to obtaining a Boolean equation?
 What is a Kmap? How does it contribute to obtaining a simplified Boolean equation? To demonstrate the simplification, show what the equation would look like if terms were written for each of the true values and the simplified equation by applying the Kmap. Discuss how much simpler the circuitry is as a result of applying the Kmap
 Describe how to construct a digital logic circuit using Boolean equations and Kmaps
 Explain why the alarm system did or did not work
 Why is there a need for the minimization of a logic design and what are the advantages?
 How does the use of a combinational logic circuit contribute to advances in technology?
 Discuss other possible design/procedural improvements
 Describe how the final combinational logic circuit was obtained
 Insert a screenshot of the front and back panels in the report; include a picture of the NI ELVIS wiring, truth table/unsimplified boolean equation and kmap/simplified eqn
Remember: Lab notes must be taken. Experimental details are easily forgotten unless written down. EG1004 Lab Notes Paper can be downloaded and printed from the EG1004 Website. Use the lab notes to write the Procedure section of the lab report. At the end of each lab, a TA will scan the lab notes and upload them to the Lab Documents section of the EG1004 Website. One point of extra credit is awarded if the lab notes are attached at the end of the lab report. Keeping careful notes is an essential component of all scientific practice.
Team PowerPoint Presentation
Follow the presentation guidelines laid out in the page called EG1003 Lab Presentation Format in the Introduction to Technical Presentations section of this manual. When preparing the presentation, consider the following points:
 How does digital logic impact the world today?
 Look into the future, how will digital logic change the world?
Footnotes
