Digital Components – Basic GatesAlthough digital logic circuits are active devices they form a separate family

to linear devices such as transistors and operational amplifiers since they have only two states – ‘on or off’ known as 1 or 0, true or false.

TTL

Transistor-Transistor Logic uses bipolar transistor technology7400 series. LS is a low power fast variant, ie 74LS00.

TTL devices operate from a nominal 5 volt dc supply (4.75 to 5.25V)Very fast but require considerable power, not suitable for portable battery

operated equipment, not static sensitive.

CMOSComplementary Metal Oxide Semiconductor using MOSFET technology, 4000

series. Operating voltage for CMOS is 3 to 15 volts.

Very low power consumption making them ideal for portable equipment. Handling precautions must be taken when working with CMOS as they

are sensitive to static electricity.

Both families are available as 14 and 16 pin Dual In Line (DIL) and surface mount packages.

Sequential Logic Output depends on the current state of the inputs plus the conditions

previously applied.Sequential logic comprises memory devices, shift registers, flip-flops,

counters, multiplexors etc.

Combinational Logic Output depends on the current state of the inputs.Combinational logic is made up from logic gates

(AND, OR, NAND, NOR, NOT).

A

B

C

Q

D Q

CLK

QA

D0 D Q

CLK

QB

D1 D Q

CLK

QC

D2 D Q

CLK

QD

D3

CLOCK

Logic Functions

The AND function.

A

BQ

The output (Q) is true (1) providing both inputs (A AND B) are true.

A B Q

0 0 0

0 1 0

1 0 0

1 1 1

Q = A.B

Logic FunctionsActivity - Verifying the AND function

1. Connect the power supply to the device (pin 14 to +5V , pin 7 to 0V)

2. Connect a LED to the output (Q pin 3)

3. Write the truth table below

4. By connecting each input to 5v (1) or 0v (0) complete the output section (Q) for the LED conditions

5. From your results state in words the input conditions for the LED to be ‘ON’.

A(1)

B(2)Q(3)

LED condition

Logic FunctionsActivity - Verifying Logic Functions

1. Connect the power supply to the device (pin 14 to +5V , pin 7 to 0V)

2. Connect a LED to the output (Q pin 3)

3. Write the truth table below

4. By connecting each input to 5v (1) or 0v (0) complete the output section (Q) for the LED conditions

5. From your results state in words the conditions for the LED to be ‘ON’.

LED condition

A(1)

B(2)Q(3)

?

Primary Logic Functions

A B Q

0 0 0

0 1 1

1 0 1

1 1 1

The OR function

Q = A + B

QA

B

A B Q

0 0 0

0 1 0

1 0 0

1 1 1

The AND function

A

BQ

Q = A.B

A Q

0 1

1 0

The NOT function.

QAA

Q = A

Secondary Logic Functions

A B Q

0 0 1

0 1 1

1 0 1

1 1 0

The NAND function

A

BQ

Q = A.B

A B Q

0 0 1

0 1 0

1 0 0

1 1 0

The NOR function

Q = A + B

QA

B

Combinational Logic Functions

74LS08

Using the devices stated, connect the circuit as shown

74LS04

Activity – Logic Circuit

74LS32

Q

A

B

3

2

1

3

2

1

6

5

4

1 2

3 4

Test your circuit and produce the truth table.

State in words the input conditions to produce a ‘1’ at the output.

Logic Functions

The XOR function (Exclusive OR).

A B Q

0 0 0

0 1 1

1 0 1

1 1 0

The output (Q) is true (1) when each input is different.

Q = A.B + A.B

QA

B

Q = A + B

Logic Functions

The OR function.

A B Q

0 0 0

0 1 1

1 0 1

1 1 1

The output (Q) is true (1) when either or both inputs (A OR B) are true (1).

Q = A + B

QA

B

Logic Functions

The NOT function.

The output (Q) is the inverse of the input (A).

A Q

0 1

1 0

QAA

By adding the NOT function to the AND/OR functions we obtain the following two functions.

Q = A

Logic Functions

The NAND function.

A B Q

0 0 1

0 1 1

1 0 1

1 1 0

A

BQ

The output (Q) is true (1) if any input is false (0).

Q = A.B

Logic Functions

The NOR function.

A B Q

0 0 1

0 1 0

1 0 0

1 1 0

The output (Q) is false (0) when either or both inputs (A OR B) are true (1).

Q = A + B

QA

B

Logic Functions

The XOR function (Exclusive OR).

A B Q

0 0 0

0 1 1

1 0 1

1 1 0

The output (Q) is true (1) when each input is different.

Q = A.B + A.B

QA

B

Q = A + B

Logic Functions

The X-NOR function (Exclusive NOR).

A B Q

0 0 1

0 1 0

1 0 0

1 1 1

The output (Q) is true (1) when both inputs are the same.

Q = A + B

QA

B

Q = A.B + A.B

Combinational Logic

AB Q

Self Assessment

1. What is the function of a Hex inverter and how many devices does the package contain.

2. What is the function of the circuit shown below,

3. Write the truth table for the circuit shown and derive its function.

4. You are required to develop circuit that will operate from a 9 volt battery supply. What logic family would you use, give reasons.

A

BQ

Combinational LogicGenerating a Truth Table

To write a truth table for a combinational logic circuit we must follow the steps below

• Write up all the possible input combinations

• Label each of the gate functions in terms of its inputs

• Provide a column to represent each of the gate functions in the circuit

• Complete the table in terms of the input variables to each gate function

A B Q

0 0 1 1 1 0

0 1 1 1 0 1

1 0 1 0 1 1

1 1 0 1 1 0

AB (A) (AB) (B) (AB)

AB

A

B

Q

(A) (AB)

(B) (AB)

What function is this ?

Combinational LogicActivity

Construct the truth tables for the circuits shown

AB

QC

QA

B

Basic digital gates are available in both 8 and 16 pin dual-in-line (DIL) packages. In each case pin 1 is always on the top left.

Marking and PackagingManufactures use an alphanumeric code printed on the device as identification e.g.

DM7406, NE7490, CD4001.

It is necessary to refer to manufacturers data sheets to obtain details of individual components.

7404 Hex inverter

1 2 3 4 5 6 7

14 13 12 11 10 9 8

VCC

Gnd

7408 Quad two input AND gate

1 2 3 4 5 6 7

14 13 12 11 10 9 8

VCC

Gnd

7400 Quad two input NAND gate

1 2 3 4 5 6 7

14 13 12 11 10 9 8

VCC

Gnd

7404 Hex inverter

1 2 3 4 5 6 7

14 13 12 11 10 9 8

VCC

Gnd

7408 Quad two input AND gate

1 2 3 4 5 6 7

14 13 12 11 10 9 8

VCC

Gnd

7432 Quad two input OR gate

1 2 3 4 5 6 7

14 13 12 11 10 9 8

VCC

Gnd