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308 Fundamentals of Computers NPP
Combinational Logic Circuits H$m°på~ZoeZb bm°{OH$ g{H©$Q>
Combinational Logic Circuits are those in H$m°på~ZoeZb Vm{H©$H$ n[anW do hmoVo h¢ {OZHo$ dV©_mZ
which the present output depends upon the AmCQ>nwQ>, dV©_mZ BZnwQ> na {Z^©a H$aVo h¢Ÿ& n[anW H$s nwamZr
present inputs only. It has nothing to do with
previous output. Various combinational logic AdñWmAm| go `o à^m{dV Zht hmoVo h¢Ÿ& `o n[anW H§$ß`yQ>a Ho$
circuits work as building blocks of computer _yc^yV Ad`d hmoVo h¢Ÿ& `hm± BÝh| g_Pm`m J`m h¡Ÿ&
circuit. These are explained in this chapters.
3.42 Half Adder 3.42 hm\$ ES>a
Definition: “Half Adder may be defined n[a^mfm… “Eogm H$m§{~ZoeZb Vm{H©$H$ n[anW Omo Xmo {~Q>m|
as the combinational logic circuit which H$m A§H$J{UVr` `moJ àXmZ H$aVm h¡, hm\$ ES>a H$hbmVm h¡Ÿ&”
performs arithmetic addition over two bits.”
Block Diagram: A block diagram of Half ãbm°H$ S>m`J«m_… {H$gr hm\$ ES>a Ho$ ãbm°H$ S>m`J«m_
Adder can be drawn to show its different inputs _|§ h_ BZnwQ> VWm AmCQ>nwQ> Xem©Vo h¢Ÿ& My±{H$ `h Xmo {~Q>m|
and outputs. Since it adds two bits, therefore it H$mo Omo‹S>Vm h¡, AV… `hr BgHo$ BZnwQ> hm|JoŸ& {H$Ýht Xmo
must have two inputs. The arithmetic addition
of two bits gives two bits at the output, one for {~Q>m| H$mo Omo‹S>Zo na AmCQ>nwQ> _| ^r Xmo {~Q>| àmßV hmoVr
sum and one for carry. This is shown in the h¡, EH$ `moJ H$s VWm Xygar hm{gb H$sŸ& Bgr H$mo {ZåZ
block diagram: ãbm°H$ S>m`J«m_ _| Xem©`m J`m h¡…
A Carry
Input Half
Bits Adder
B Sum
Truth Table of Half Adder hm\$ ES>a H$s gË`-Vm{bH$m
A truth table for Half Adder gives EH$ hm\$ ES>a H$s gË` Vm{bH$m ~ZmB© Om gH$Vr
relationship between inputs and outputs. It can h¡, Omo BgHo$ {d{^ÝZ BZnwQ>m| VWm AmCQ>nwQ>m| _| g§~§Y
be drawn as:
~VmE Ÿ&
A B Carry Sum
0 0 0 0
0 1 0 1
1 0 0 1
1 1 1 0
Logic Circuit of Half Adder hm\$ ES>a H$m Vm{H©$H$ n[anW
A Half Adder can be implemented using EH$ hm\$ ES>a H$m n[anW JoQ>m| H$s ghm`Vm go ~Zm`m
logic gates. A visual inspection of truth table Om gH$Vm h¡Ÿ& gË`-Vm{bH$m H$mo XoIH$a JoQ>m| H$s nhMmZ
