YILDIRIM BEYAZIT UNIVERSITY ECE 205 – DIGITAL DESIGN LABORATORY MANUAL – 6 – Construction of Simple Combinational Circuits Objectives:
Learn how to use the breadboard for prototyping
Develop good wiring practices
Improve circuit diagram drawing and annotating abilities
Equipment:
Breadboard, IC7408, IC7432, IC7400
Introduction:
The types of chips used in this experiment are known as the 7400 series. (Their code numbers begin with the two digits 74). They are encapsulated in black epoxy. This encapsulation form is called “Dual In-Line Package” or DIP in short. This name refers to the location of the pins. The pins of a DIP chip have a fixed, conventional numbering scheme: counterclockwise viewed from the top of the chip. Pin 1 is usually marked with a dimple or a notch in the package.
For each chip that you use in a logic design, you need to know what function the chip performs, and how it is wired internally. There is a four-digit or five-digit code printed on the chip. This code number is the identifier to use in locating the function of the chip. In the following figure (Fig. 1) the specific numbering scheme is provided for several chips, showing which pin numbers connect to which elements of each component inside the package. (Quadruple means that there are four gates in the chip). This numbering information is called the "pin-out" information for the chip.
(a)
(b)
Fig.1. Pin configuration and logic symbol for a.7408 chip (Quadruple 2-input AND gate) b. for 7432 chip (Quadruple 2-input OR gate)
The pin diagrams are extracted from the data sheet which is provided by the manufacturer. In the laboratory there will be two types of diagrams associated with the digital circuits you design. The circuit schematic shows the type of gates used, together with their logical interconnection. However, the specific code numbers for the chips and the selected pin numbers are not shown. This circuit schematic emphasizes the logical structure, without cluttering the flow with construction details.
An example of a simple circuit is shown in Fig.2. The logic function desired is: F = AB + CD. Counting from the logic diagram, two AND gates and one OR gate are needed. Therefore, construction will require two chip packages: one 7408 (Quad 2-input AND), and one 7432 (Quad 2-input OR). With pin-outs obtained from their datasheets, it is possible to construct the wiring diagram shown in Fig. 3. A B F C D
Fig. 2. Logic diagram for the sample circuit
The wiring diagram below shows the details of what chips and which pins are used. From the logic diagram, you can make a count of the number and types of gates you require. Then, using the datasheets, you can create and label the needed parts of the wiring diagram. +5V A to logic switche s
Fto logic indicators
B C D 14
13 12 11 10 9
8
14 13 12 11 10 9
8
U1 (7408 )
U2 (7432 ) 1
2
3
4
5
6
7
1
2
3
4
5
6
7 GND
Fig. 3. Wiring diagram for the sample circuit
Complete schematic diagrams indicate IC types, package identifiers, and pin numbers, as in Figure 4. The IC type is a part number identifying the integrated circuit that performs a given logic function. For example, a 2-input NAND gate might be identified as a 74HCT00 or a 74LS00. In addition to the logic function, the IC type identifies the electrical characteristics such as its speed.
A B
9 10
74X08
IC Type
8
Pin Numbe rs
U1
C D
12
74X32
13
Packag e Identif er
11
12
F
U2
74X08 11
13 U1
Packag e Identif er
Fig.4. Schematic diagram for the sample circuit
The package identifier (also called the reference designator) for an IC identifies a particular instance of that IC type installed in the system. A unique package identifier is assigned to each chip. In this example the 7408 chip is assigned the identifier U1, and 7432 chip is assigned the identifier U2. The package identifier allows a particular IC to be located during assembly, test, and maintenance of the system. Traditionally, package identifiers begin with the letter U (for “unit”).
Once a particular IC is located, pin numbers are used to locate individual logic signals on its pins. The pin numbers are written near the corresponding inputs and outputs of the standard logic symbol, as shown in Figure 4.
Laboratory Exercise: 1) Prepare a truth table for the circuit shown in Figure2. 2) Breadboards tend to wear out with use, especially if somebody tried to insert a fat wire into the holes. Excessive heating of the IC’s due to wrong connections may have melted the breadboard’s plastic material. Such defects make the connections less reliable. Identify a portion of the breadboard free of defects as much as possible. 3) If you are having problems with your circuit, use an ohmmeter to make sure there really is continuity where you think there is. 4) Start constructing the circuit shown in Fig. 3. 5) Identify the chips by checking their part numbers. Chips are typically labeled on the top with distinct codes as seen in the figure. The most important is the part number. Sometimes the date of manufacture and the lot code are also present.
Part number
6) Often, when you use a brand new chip, the pins will not be at right angles to the seating plane. This makes it difficult to insert the chip into the board. To bend the pins inward, it's best to bend a whole row at a time by laying the chip on its side against the table and gently pushing down with a rotating motion.
7) Avoid plugging the chip upside down. There should be a notch or dot at one end of the chip signifying pin 1.
notch
dot
8) Insert ICs into the breadboard. Make sure all the pins are in the holes correctly and the IC does not have any missing legs.

Use appropriate length wires for connections, i.e. use short wires for short distance connections. When necessary cut the wires to appropriate length using the cutter provided.
Wrong
Right
9) Do not run wires over the top of the chips. Always run the wire around the chips. You may have to replace a chip if you get a bad one.
10) Strip the wire ends using the cutter provided. Do not strip wire ends longer than 5mm and insert long bare ends into the breadboard holes. This will cause shorts.
5 mm
11) Each chip requires DC power before it will work. For each chip two connections are necessary: "+5 volts" and "Ground". Refer to the pin diagrams to identify power and ground pins of ICs. 12) For power and ground, use the rows of connected pins on the breadboard (see the figure below). Make all power and ground connections to these rows.
5V GND
GND 3.3V 13) Connect the inputs of the circuit to the logic switches on the breadboard to the regarding pins of the ICs. The input voltages of logic ‘1’ should be in between 3-5V. Choose 3.3V for logic ’1’. For logic ‘0’ choose ground. Make the necessary connections. 14) Inputs of unused gates should be connected to ground or the 5V supply. When left unconnected, some kinds of gates can oscillate and cause problems. (Caution: Never connect a gate output to ground or 5V supply!) 15)
Did you cut the wires to appropriate length? Show this to your instructor.
16)
Are the cable bare ends ~5mm? Show this to your instructor.
17) Have the unused inputs been properly connected? Show this to your instructor.
18) Verify the operation of the circuit you have constructed in Step 2. I.e. show experimentally that for each input combination in the truth table, the correct out is obtained. 19) If the circuit is not working properly: Work backwards. First check the gate which produces the output (check whether or not this gate works). If you discover that some of its inputs are incorrect, and then check the gates which produce these incorrect inputs, and so on. 20)
Show the circuit operation to your instructor.
21) Now you will construct the circuit in Figure2 with NAND and OR gates. In other words, you will replace AND gates with NAND gates. Note that after this replacement, your truth must stay the same. 22)
Draw the logic diagram of the constructed circuit and show it to your instructor.
23) 24)
Draw a wiring diagram for the circuit and show it to your instructor. Draw a circuit schematic (with pin numbers and unit numbers indicated) and show it to your instructor.
25) Construct the circuit by referring to the wiring diagram you have just drawn. Verify the circuit operation. 26)
Show the circuit operation to your instructor.