Digital Circuits for John Deere Program (Part 2)

Prepared by Charles S. Tritt, Ph.D.
November 2, 1999

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 insert 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:

PinFunctionConnection
1B3 inputGround, logic Zero
2A < B cascade inputGround, logic False
3A = B cascade input+5V, logic True
4A > B cascade inputGround, logic False
5A > B outputJ1, lamp monitor
6A = B outputJ2, lamp monitor
7A < B outputJ3, lamp monitor

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

PinFunctionConnection
9B0 input+5V, logic One
10A0 inputJ18, switch
11B1 inputGround, logic Zero
12A1 inputJ17, switch
13A2 inputJ16, switch
14B2 input+5V, logic One
15A3 inputJ15, 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:

PinFunctionConnection
1Sigma2 outputJ3, lamp monitor
2B2 inputGround, logic Zero
3A2 inputJ17, switch
4Sigma1 outputJ4, lamp monitor
5A1 inputJ18, switch
6B1 input+5V, logic One
7C0 inputGround, logic Zero

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

PinFunctionConnection
9C4 outputRed DMM lead (via a wire to an alligator clip)
10Sigma4 outputJ1, lamp monitor
11B4 inputGround, logic Zero
12A4 inputJ15, switch
13Sigma3 outputJ2, lamp monitor
14A3 inputJ16, switch
15B3 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.