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Transcript
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