Download experiment no 4

Esc 102 Introduction to Electronics, 2004-2005/I
EXPERIMENT 10
BASIC COMBINATORIAL CIRCUITS
Introduction
Logic gates are the basic building blocks in digital circuits. In this experiment you will study the
characteristics of basic gates and also implement some simple combinatorial circuits. Logic gates
and several other digital functions are available in the form of ICs (Integrated Chips). These ICs
fall into several ‘Logic Families’, such as TTL, CMOS, ECL, etc, of which TTL (TransistorTransistor Logic) is the most popular one. The basic differences between these logic families
arise from their speed and power consumption. The TTL family itself has several sub-families,
such as, Standard TTL, Schottky TTL (S TTL) , Low-power Schottky TTL (LS TTL), FAST
TTL (F TTL), etc. Among the TTL families the ones which are most popular are LS TTL and F
TTL.
The commercial versions of the TTL ICs have numbers 74 YY XXXX, where YY (up to 2 two
characters) represent the sub-family and XXXX (up to 4 digits)a particular number. These
numbers were arrived at as and when various manufacturers designed different functions. This
number may be a 2-digit, a 3-digit or a 4-digit number. Thus 7404 is a Hex-Inverter of Standard
TTL Technology, whereas 74S04 is a Hex-Inverter using the Schottky Technology. All TTL
family ICs use +5V as Vcc. Logical HIGH output (VOH) is typically >2.4V, while Logical LOW
output (VOL) is <0.4V. The output signal levels can vary with temperature and load. However,
the manufacturer specifies VOH and VOL levels, for the worst case conditions, i.e for the
maximum load and for the entire temperature range (0-70C).
1. INVERTER CHARACTERISTICS
In this experiment we first study the transfer characteristics of a TTL inverter and the effect of
load on the VOH and VOL levels. The recommended load for an output is specified in terms of its
‘Fanout’, where fanout is defined as the number of identical inputs the specified output can
drive.
Experiment
A) Inverter Characteristics using DMM
(i) Wire the inverter circuit of Fig.1.1 Note that this is one of the gates of the 7404 Hex
Inverter IC. In addition to what is shown in Fig.1.1, the IC must have its Vcc (+5V) and
GND connections. Refer to the pin connections of 7404.
(ii) Switch ON the Vcc supply. Using the DMM obtain several input voltages (0.4V, 0.8V,
1V, 1.2V, 1.4V, 1.6V, 1.8V, 2V, 2.5V, 3V, 4V, and 5V) by varying the potentiometer.
Note down the corresponding output voltages. Tabulate your results.
(iii) Now connect the remaining 5 gates of the IC as a load to the inverter under test. You do
this by connecting the inverter output of Fig.1.1 to the inputs as indicated in Fig.1.2. Note
down the output voltages for input voltages of 0.4V, and 2.5V. Note the change in the
output voltages with load.
B) Inverter Characteristics using FG and CRO
(i) Wire the circuit of Fig.1.3. (Again make sure that you connect the Vcc and GND of 7404).
Keep the load connected to the inverter output as in (iii) above.
(ii) Switch ON Vcc and connect a Triangular waveform (1 KHz) with amplitude 0-5V to the
input of the inverter as indicated. Using XY mode of the CRO to display the characteristics.
Sketch the transfer characteristics of the inverter (VI vs VO).
2. FAMILIARIZATION WITH THE DIGITAL TEST BOARD
Refer to the details of the Digital Test Board given at the end. This board will be used in all the
experiments on digital circuits.
Experiment
(i) Connect Vcc and GND to the Test Board through the microboard.
(ii) Verify the working of all data switches by connecting the LP input to each of their
outputs. For each of the switches check that both logic ‘0’ and ‘1’ levels are obtained by
toggling.
(iii) Connect any 4 data switches to the 7-segment display driver inputs. Check the display for
all the 16 input combinations. Note that for binary inputs 0000 to 0111, the display should
show the corresponding decimal number from 0 to 7. For binary inputs 1000 to 1111, the
top left corner decimal point will also light up in addition to the numerals 0 to 7.
(iv) Connect the Logic Probe (LP) input to the CLOCK output and verify the clock functions.
Note that the Red LED glows whenever CLOCK output is at logic ‘0’.
3. REALIZATION OF HALF-ADDER
Half-Adder is one of the basic combinatorial circuits. The aim of the experiment is to design and
test a Half-Adder circuit.
Lab Preparation
Before coming to the Laboratory write the truth table for a Half-Adder and design a Half- Adder
using EX-OR and NAND gates. See the details of the ICs giving EX-OR and NAND functions
(7486 and 7400, respectively). Draw a neat circuit diagram (a functional diagram showing gates).
Indicate the pin numbers against all the input and output connections of the gates in the circuit.
This is very important and will save time while wiring the circuit and also during debugging.
Experiment
(i) Wire your Half-Adder circuit. Use two of the data switches as the inputs. Switch ON the Vcc
supply.
(ii) Using the Logic Probe(LP), test the SUM and CARRY outputs for various inputs. Verify that
your circuit is working properly.
4. REALIZATION OF LOGIC FUNCTIONS USING NAND & NOR GATES
NAND and NOR gates are called Universal gates since all logic functions can be realized using a
combination of either NAND gates or NOR gates. The aim of this experiment is to realize the
logic functions given below using the indicated gates.
(i)
2-input OR function using 2-input NAND gates
(ii)
2-input NOR function using 2-input NAND gates
(iii) 2-input AND function using 2-input NOR gates
(iv)
2-input NAND function using 2-input NOR gates
Lab Preparation
For each of the above logic functions design circuits using the indicated gates. Details of these
gates are given at the end. The ICs to be used are: 7400 (Quad 2-input NAND gates), and 7402
(Quad 2-input NOR gates). For each case draw a neat circuit diagram (a functional diagram
showing gates). Indicate the pin numbers against all the input and output connections of the gates
in the circuit.
Experiment
(i) One by one, wire the circuits you have designed using the detailed diagram you have drawn.
Use two of the data switches as the inputs. Switch ON the Vcc supply.
(ii) Using the Logic Probe(LP), test the output of each circuit. Write the Truth Table for the logic
function under consideration and verify that your logic circuit implementation is working
properly.