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LOGIC CIRCUIT
WE KNOW THEL UNIDIRECTIONAL CHARACTERISTICE OF A SOLID-STATE
SEMICONDUCTOR DIODE WHEN FORWARD BIASED THIS DIODE ACTED LIKE A COLSED
SWITCH :THAT IS IT PERMITTED CURRENT TO FLOW IN A COMPLETE CIRUCIT WHEN
REVERSE BIASED IT ACTED LIKE AN OPEN SWITCH AND PERMITTED LITTLE IF ANY
CURRENT FOTFLOW THIS UNIQUE CHARACTERISTIC OF DIODE EMPLYED IN THE
DESIGN F BASIC LOGIC CIRCUIT. IN DIGITAL ELECTRONICS A GATE IS A LOGIC CIRCUIT
WITH ONE OUTPUT AND ONE OR MORE INPUTS AN OUTPOUT SIGNAL OCCURS FOR
CERTAIN COMBINATIONS OF INPUT SIGNALS.
MODERN COMPUTER DO MANY JOBS FROM SCIENTIFIC COMPUTATION TO
BUSINESS ACCOUNTING COMPUTER ARE ALSO USED IN THE AUTOMATIC CONTROL OF
MAUFACTURING PROCESS. THESE DEVICE PERFORM HIGHLY COMPLEX
JOB. THEY
CONSIST OF ARRANGEMENT OF SIMPLE LOGIC CIRCUITS.
TO SOLVE PROBLEMS A COMPUTER HAS TO MAKE DECISIONS AS IT PROGRESSES
THROUGH THE STEPS IN THE PROBLEM SOLVING. COMPUTER CIRCUIT THAT MAKE
DECISIONS ARE OF THE YES/NO VARIETY. ALSO KNOWN AS TWO-STATE LOGIC. LOGIC
CIRCUITS CAN BE IN ONE OF TWO STATES SUCH AS ON OF OFF HIGH OR LOW
MAGNETIZED OR UNMAGENETIZED ETC.
A TOGGLE SWITCH IS A SIMPLE EXAMPLE OF A TWO-STATE SERVICE FIGURE I SHOWN
A DIODE CIRCUIT WITH A SWITCH INPUT AND A LOAD RESISTOR OF 100 K ohm. THE
SUPPLY VOLTAGE IS + 5V. WHEN THE SWITCH IS IN THE GROUND POSITION THE DIODE
IS FORWARD BIASED AND APPROXIMATELY 0.7 V APPEARS ACROSS THE DIODE.
THEREFORE THE OUTPUT VOLTGAGE IS LOW WHEN THE IMUT IS LOW.
ON THE OTHER HAND WHEN THE SWITCH IS AT + 5 V. THE NET VOLTAGE ACROSS
THE DIODE – RESISTOR COMBINATION IS 0. AS A RESULT THE DIODE IS NON
CONDUCTION. SINCE THERE IS NO CURRENT THROUGH THE LOAD RESISTOR THE
OUTPUT IS PULLED UP TO THE SUPPLY VOLTAGE IN OTHER WORDS THE OUTPUT IS
HIGH (+5V) WHEN THE INPUT IS HIGH.
A FORWARD BIASED DIODE ACTS LIKE A CLOSED SWITCH
BINARY MEANS “TWO” COMPUTERS USE THE BINARY NUMBER SYSTEM. RATHER
THAT HAVING DIGITS 0 TO 9. A BINARY NUMBER SYSTEM, HAS ONLY DIGITS 0 AND 1.
THIS IS BETTER SUITED TO DIGITAL ELECTRONICS WHERE THE SIGNALS ARE LOW OR
HIGH SWITCHES ARE OPEN OR CLOSED. LIGHTS ARE OFF OR ON. ETC. IN OUR
EXPERIMETNS WE WILL USE POSITIVE LOGIC, ; THIS MEANS BINARY O REPRESNTS THE
LOW STATE AND BINARY I THE HIGH STATE WITH THIS IS MIND TABLE – 2 IS THE
TRUTH TABLE OF A TWO INPUT AND GATE AS IT IS USUALLY SHOWN. THIS GIVES THE
SAME INFORMATION AS TABLE I EXCEPT IT USES A BINARY CODE WHERE O IS LOW
AND I IS HIGH.
TRUTH TABLE FOR TWO – INPUT AND GATE
THE ACTION OF LOGIC CIRCUIT IS USUALLY IS SUMMARIZED IN THE FORM
TRUTH TABLES THESE ARE TABLES THAT SHOW THE OUTPUT FOR COMBINATIONS OF
THE INPUT SIGNALS. TABLES I SHOW THE TRUTH FOR A TWO INPUT AND GATE.
A TOGGLE SWITCH IS A SIMPLE EXAMPLE OF A TWO-STATE SERVICE FIGURE I
SHOWN A DIODE CIRCUIT WITH A SWIRCH INPUT AND A LOAD RESISTOR OF 100 K ohm.
THE SUPPLY VOLTAGE IS + 5V. WHEN THE SWITCH IS IN THE GROUND POSITION THE
DIODE IS FORWARD BIASED AND APPROXIMATELY 0.7 V APPEARS ACROSS THE DIODE.
THERFORE THE OUTPUT VOLTGE IS LOW WHEN THE INPUT IS LOW.
ON THE OTHER HAND WHEN THE SWITCH IS AT + 5 V. THE NET VOLTAGE ACROSS
THE
DIODE-RESISTOR
COMBINATION
IS
0.
AS
A
RESULT
THE
DIODE
IS
NONCONDUCTION. SINCE THERE IS NO CURRENT THROUGH THE LOAD RESISTOR THE
OUTPUT IS PULLED UP TO THE SUPPLY VOLTAGE IN OTHER WORDS THE OUTPUT IS
HIGH (+5V) WHEN THE INPUT IS HIGH.
A FORWARD BIASED DIODE ACTS LIKE A CLOSED SWITCH
BINARY MEANS “TWO” COMPUTERS USE THE BINARY NUMBER SYSTEM. RATHER
THAT HAVING DIGITS 0 TO 9. A BINARY NUMBER SYSTEM, HAS ONLY DIGITS 0 AND 1.
THIS IS BETTER SUITED TO DIGITAL ELECTRONICS WHERE THE SIGNALS ARE LOW OR
HIGH SWITCHES AREOPEN OR CLOSED. LIGHTS ARE OFF OR ON. ETC. IN OUR
EXPERIMENTS WE WILL USE POSITIVE LOGIC, THIS MEANS BINARY O REPRESENTS THE
LOW STATED AND BINARY I THE HIGH STATE WITH HIS IS MIND TABLE – 2 IS THE
TRUTH TABLE OF A TWO INPUT AND GATE AS IT IS USUALLY SHOWN. THIS GIVES THE
SAME INFORMATION AS TABLE I EXCEPT IT USES A BINARY CODE WHERE O IS LOW
AND I IS HIGH.
OR GATE
FIGURE .A. SHOWS A TWO – INPUT OR GATE WHEN BOTH SWITCHES ARE IN THE
GROUND POSITION. THE DIODE ARE NON CONDUCTING AND THE OUTPUT IS LOW IT
EITHER SWITCH IS SET TO +5V THEN ITS DIODE CONDUCTS AND THE OUTPUT IS
APPROIMATELY + 4.3 V. IN FACT BOTH SWITCHES CAN BE AT + 5 V AND THE OUTPUT
WILL BE AROUND + 4.3 V. ( THE DIODES ARE PARELLEL.)
THEREFORE IF EITHER INPUT IS HIGH OF IF BOTH ARE HIGH THE OUTPUT IS HIGH
TABLE SUMMARIZES THE OPERATION OF A TWO INPUT OR GATE IN TERMS OF BINARY
OS AND IS
.AS YOU SEE. IF BOTH INPUTSARE LOW THE OUTPUT IOS LOW. IF EITHER
INPUT IN HIGH THE OUTPUT IS HIGH IF BOTH INPUT ARE HIGH THERE OUTPUT IS HIGH.
UNLIKE THE AND GATE WHERE ALL INPUT MUST BE HIGH TOP GET A HIGH
OUPTOU THE OR GATE HAS A HIGH OUTPUT IF ANY INPUT IS HIGH FIGURE B SHOWS
THE SYMBOL COR A TWO – INPUT OR GATE. BY ADDING MORE DIODES TO THE GATE
WE CAN PRODUCE THREE- INPUT OR GATES. FOUR – INPUT OR GATES. AND SO ON
FIGURE C. AND D/ SHOW THE SCHEMIATIC SYMBOLS FOR THREE – AND FOUR – INPUT
OF GATES OF ANY DESIGN.
Truth Table of Or Gate
INPUT
OUTPUT
A
B
Y
0
0
0
0
1
1
1
0
1
1
1
1