15 The number of iCs used in a computer, I.E. The chip count, can be reduced by connecting nand gates together to form other types of gates (see Fig. 3).

_-C

para

The number of components in an 1С is indicated by its scale of 5

Integration as shown in Table 1. The 1с shown in Fig. 2 is an ssi 20 device but microprocessors used in computers are vlsi or slsi devices.

Table 1

Scale of integration	Abbreviation	No. of active components
Small-scale integration	SSI	1 to 10
Medium-scale integration	MSI	10 to 102
Large-scale integration	LSI	102to 103
Very large-scale integration	VLSI	103to 104
Super large-scale integration	SLSI	104to 105

There are two common families of logic ICs used in computers. TTL (transistor-transistor logic) ICs use bipolar transistors to form each gate whereas CMOS (complementary metal oxide semiconductor) 25 ICs use field effect transistors (FETs). The different characteristics of each family determine which will be used in a particular computer (see Table 2). For example, TTL ICs are used in large, high-speed computers and CMOS ICs are better for battery-powered portable computers.

Table 2

Properties TTL CMOS

Supply voltage +5V±0.25% +3Vto+15V

Supply current mA цА

Power dissipation mW [xW

Switching speed fast relatively slow

Input impedance low high

Complete these statements with the help of the truth tables in Fig. 1 on page 125. For example, in the case of an AND gate:

a When A and В are low, the output is low.

b When A is low and В is high, the output is low.

When A is high and В is low,

Task 8

1	AND
2	NOT
3	OR
4	NOT
5	NAND
6	NAND
7	AND
8	NAND

When A and В are high, When A is high,

, the output is high.

the output is low.

, the output is high.

Task 9

Writing Explaining a process control system

Study this diagram. It shows how an industrial process is controlled using logic gates. With the help of the diagram, complete the blanks in the explanation which follows. Each blank may be one or more words.

motor

power

_У

supply

correct

sufficient

store

memory

reset

hopper

_V

correct

_Y

off

high

empty

pressure

A motor controlling the flow of aluminium blanks to a hydraulic press is switched on only under these conditions:

There are: The-

1 2 3 4

The power is on.

The supply voltage is i.

aluminium blanks in the hopper (store).

in the hydraulic press is correct.

Information on these four conditions is fed into an'

conditions must be satisfied for the motor to run. When:

as all four , the

output from the AND gate is high. This is fed into the store input of the unit to provide a continuous signal to operate the motor.

The motor must stop if any one of these conditions occurs:

1 2 3 4

The power is The I- -

The hopper is: The^

rises.

. drops.

Information on each of these conditions is fed through a

ii

When

the input is low,

12

The output from each NOT gate is fed to an

13

14

When any of the four inputs to the OR gate is high, the output

When this is fed to the memory reset, it interrupts the

continuous signal to the motor. The motor is switched ~ and the

flow of aluminium blanks to the press is thus

i6

Digital watch

Taskl

Tuning-in

Study this diagram of a watch. How many functions can you list?

/ ALM	DUAL	/ n. in\ 1 U -CI -
TMR	ST.W
/ п. ALARM III'		UU-n 1 l Л I \

Buttons enable watch to display time and date, buzz several daily alarms, count down from 24 hours, and act as a stopwatch with an accuracy of 1/100th second.

Digital watch