Digital Circuits for High School Students (Part 2)

Prepared by Charles S. Tritt, Ph.D.
July 18, 1997

Equipment (for each pair of students working together)

1 Digi Designer
1 Set of wires (these should come with the Digi Designer)
1 Set of alligator clip patch cords
1 74x85 Mono 4-bit Magnitude Comparator chip
1 74x283 Mono 4-bit Binary Adder chip

Introduction

In this exercise you will perform binary mathematics in hardware. You will use a 7485 comparator chip and a 75283 adder chip. The pin outs for these chips are shown in Figure 1.

7485 chip 74283 chip

Figure 1 -- Pin outs for the 7485 comparator and 75283 adder chips.

Safety Considerations

Dangerous line voltages are present at the outlets on your work bench. Do not inert anything other than approved power plugs into these outlets.

The voltages and currents produced by the Digi Designer are generally safe. However, you should not short +5V (Vcc) or logic High outputs directly to ground or intentionally make yourself part of a logic circuit.

Chip pins and the tips of the small wires may be sharp. Handle with care. Chips can become very hot if wired incorrectly. Be careful.

Procedures

4-bit Comparator

A comparator compares the magnitude of two binary values. The 7485 is a 4-bit comparator meaning that it compares two 4-bit values. You will use the Digi Designer switches to enter one of these values (called A). You will "hard wire" the other value (called B) into your circuit. You will use the value of 0101₂ (= 5₁₀) for B. You will use three Monitor Lights on your Digi Designer to display the result of the comparison. Possible results are A < B, A = B and A > B.

Insert your 7485 into the bread board on your Digi Designer. Connect power and ground to pins 16 and 8, respectively. Make the following connections from left to right across the lower edge of the chip:

Pin Function Connection

1 B₃ input Ground, logic Zero

2 A < B cascade input Ground, logic False

3 A = B cascade input +5V, logic True

4 A > B cascade input Ground, logic False

5 A > B output J1, lamp monitor

6 A = B output J2, lamp monitor

7 A < B output J3, lamp monitor

Pin	Function	Connection
1	B₃ input	Ground, logic Zero
2	A < B cascade input	Ground, logic False
3	A = B cascade input	+5V, logic True
4	A > B cascade input	Ground, logic False
5	A > B output	J1, lamp monitor
6	A = B output	J2, lamp monitor
7	A < B output	J3, lamp monitor

Make the following connections from right to left across the upper edge of the chip:

Pin Function Connection

9 B₀ input +5V, logic One

10 A₀ input J18, switch

11 B₁ input Ground, logic Zero

12 A₁ input J17, switch

13 A₂ input J16, switch

14 B₂ input +5V, logic One

15 A₃ input J15, switch

Pin	Function	Connection
9	B₀ input	+5V, logic One
10	A₀ input	J18, switch
11	B₁ input	Ground, logic Zero
12	A₁ input	J17, switch
13	A₂ input	J16, switch
14	B₂ input	+5V, logic One
15	A₃ input	J15, switch

Test your circuit for all possible values of A (i.e., all possible combinations of A₀ through A₃) and complete the table in your work sheet. Remove all the wires and the 7485 from your Digi Designer.

4-bit Adder with Carry In and Out

Next you'll use a 74283 adder chip to add two 4-bit binary numbers. The two values to be added will be called A and B and consist of bits A₀ through A₃ and B₀ through B₃, respectively. You will use the switches on your Digi Designer to generate A and will hard wire B = 0101₂ = 5₁₀. The output of the adder will be called Sigma and consist of bits Sigma₀ to Sigma₃. The 74283 also has a carry input called C₀ and a carry output called C₄. You will ground the carry input and use the DMM to monitor the carry output.

Insert your 74283 into the bread board on your Digi Designer. Connect power and ground to pins 16 and 8, respectively. Turn on the DMM and connect the black lead to the GND terminal on your Digi Designer with an alligator clip. Make the following connections from left to right across the lower edge of the chip:

Pin Function Connection

1 Sigma₂ output J3, lamp monitor

2 B₂ input Ground, logic Zero

3 A₂ input J17, switch

4 Sigma₁ output J4, lamp monitor

5 A₁ input J18, switch

6 B₁ input +5V, logic One

7 C₀ input Ground, logic Zero

Pin	Function	Connection
1	Sigma₂ output	J3, lamp monitor
2	B₂ input	Ground, logic Zero
3	A₂ input	J17, switch
4	Sigma₁ output	J4, lamp monitor
5	A₁ input	J18, switch
6	B₁ input	+5V, logic One
7	C₀ input	Ground, logic Zero

Make the following connections from right to left across the upper edge of the chip:

Pin Function Connection

9 C₄ output Red DMM lead (via a wire to an alligator clip)

10 Sigma₄ output J1, lamp monitor

11 B₄ input Ground, logic Zero

12 A₄ input J15, switch

13 Sigma₃ output J2, lamp monitor

14 A₃ input J16, switch

15 B₃ input +5V, logic One

Pin	Function	Connection
9	C₄ output	Red DMM lead (via a wire to an alligator clip)
10	Sigma₄ output	J1, lamp monitor
11	B₄ input	Ground, logic Zero
12	A₄ input	J15, switch
13	Sigma₃ output	J2, lamp monitor
14	A₃ input	J16, switch
15	B₃ input	+5V, logic One

Test your circuit for all possible values of A (i.e., all possible combinations of A₁ through A₄) and complete the table in your work sheet. Remove all the wires and the 74283 from your Digi Designer. Turn off your DMM.

Chip pin out diagrams in this document were taken from Fairchild/National Instruments chip data sheets.


Figure 1 -- Pin outs for the 7485 comparator and 75283 adder chips.