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D I G I T A L   L O G I C
Logic Functions using Doors   XOR
Logic Functions using Switches  Flip-Flops
Positive & Negative Logic Explained  Binary : Decimal Explained
Logic Element Symbols  Binary Codes
Positive & Negative Logic In Action  Glossary
Real World DOs & DON'Ts   !  Various Logic Family Specs
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AND Logic Function
using Doors
OR Logic Function 
 Explanation
     
AND Logic Function
 using Switches
OR Logic Function
 Explanation
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Positive Logic, Negative Logic Explained using Doors & Switches 
In this animated AND Logic example of Doors Opening and Closing, you can see that in order for the "Light" to get through the house, the front door AND the back door must be Open. 
Same example: if either the front door OR the back door is Closed the light does NOT get through. 
In this animated OR Logic example, you can see that in order to get light through the house: the left front door OR the right front door (or both) must be Open. 
Same example: in order to block the light through the house: the left front door AND the right front door must be Closed. 
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In order for the Output of an AND Logical Function to be TRUE: input (1) AND input (2) must both be TRUE.  This is Positive Logic. Hint: TRUE is when the switch is closed,  applying power to the LED.  FALSE is when the switch is open,  NOT applying power to the LED)
     
      Truth Table->
    A & B are the Input switches
    C is the Output LED
Using the Same Function --It is also correct to say: If either input (1) OR input (2) (or both) is NOT TRUE the Output Will be FALSE. This is Negative Logic.
In order for the Output of an OR Logical Function to be TRUE: either 
input (1) OR input (2) (or both) must be TRUE.  This is Positive Logic.
     
      Truth Table->
    A & B are the Input switches
    C is the Output LED
Using the Same Function --It is also correct to say: In order for the Output to be FALSE: input (1) AND input (2) must both be FALSE. This is Negative Logic.
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    Remember: 
    1) A Positive AND gate is a Negative OR gate 
    2) A Positive OR gate is a Negative AND gate 
Logic Element Symbols
 
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Positive Logic Gates    Animate Me                Negative Logic Gates      TOP
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Exclusive OR (XOR)
In order for the Output of an XOR Logical Function to be TRUE: input (1) & input (2) must both Different  01, 10.  This is Positive Logic.
     
    Truth Table->
    A & B are the Input switches
    C is the Output LED
Using the Same Function --It is also correct to say: In order for the Output to be FALSE: input (1) & input (2) must both be Identical  11, 00. This is Negative Logic.
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  Flip-Flop in--   Flip-Flop in--    Flip-Flop in-- 
Set Reset Mode
 Toggle Mode
 Data Mode
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Binary-Decimal-Real Values
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Binary Codes

Number Decimal
fraction
Two's 
complement
One's
complement
Sign +
magnitude
Offset
binary
+7
+6
+5
+4
+3
+2
+1
+7/8
+6/8
+5/8
+4/8
+3/8
+2/8
+1/8
0111
0110
0101
0100
0011
0010
0001
0111
0110
0101
0100
0011
0010
0001
0111
0110
0101
0100
0011
0010
0001
1111
1110
1101
1100
1011
1010
1001
0
0
0+
0-
0000
0000)
0000
1111
0000
1000
1000
1000)
-1
-2
-3
-4
-5
-6
-7
-8
-1/8
-2/8
-3/8
-4/8
-5/8
-6/8
-7/8
-8/8
1111
1110
1101
1100
1011
1010
1001
1000)
1110
1101
1100
1011
1010
1001
1000
1001
1010
1011
1100
1101
1110
1111
0111
0110
0101
0100
0011
0010
0001
0000)

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Glossary          TOP
TRUE   = 1 = YES = POSITIVE = MARK 

FALSE = 0 = NO = NEGATIVE = SPACE 

NOT = INVERSION 
That is: if a  FALSE is Inverted (NOT), it becomes a TRUE 
Likewise: if a TRUE is Inverted (NOT), it becomes a FALSE. 

A NOT or Inversion is usually indicated by a Bubble, on either the input or the output of a logic gate symbol. 

The convention (which is not always adhered to) is that a POSITIVE Gate Inversion is indicated by a bubble on the output, and with Negative logic the bubble is at the input. 
See Symbol Examples

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Logic Families' Propagation Delay Specs       TOP
by Charles Toth
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"Real World"   DOs & DON'Ts
What should I do with the UNUSED INPUTS?
TTL devices normally default to a logic ONE if the input is allowed to FLOAT. 

However, depending on many factors, these inputs can act as an antenna, and pick up NOISE which can cause mischief to the circuit. The worst case is, the device will act as an uncontrolled intermittent OSCILLATOR.   --This is BAD!

Therefore:
ALL unused inputs of TTL logic devices IDEALLY should be returned to either a logic ONE or logic ZERO. Certain devices--74XX and earlier 74LS--the logic ONE (Vcc) should be a ~ 1k resistor to Vcc; or alternatively, connected directly to ground (ZERO). Most/All other logic devices, the resistor is UNNECESSARY.

In the case of CMOS devices, there is NO input DEFAULT; CMOS input impedance is on the order of thousands of MEGOHMS and will cause EXTREME FAILURES if allowed to FLOAT. 

To make this point: if you put your finger or a scope probe on or NEAR an un-terminated CMOS input, it can cause a logic CHANGE!!  If allowed to float, some CMOS devices can oscillate so fast, that it exceeds the device power rating and will literally destroy itself.

Unused Outputs: tri-state[1], and open collector devices, etc., DO NOT require any attention.
NOTE: In the case of the tri-state control pin, it should be treated as an INPUT.

  Notes  [1]  Tri-state: Zero, One, Open. 

WARNING:
NEVER EVER USE the original generic 7400 DEVICE.
INSTEAD: Use anything else, e.g., 74LS, 74ALS, etc.
The older devices require five to ten times the drive power and force very LARGE amounts of NOISE on to the Vcc rail!  --This is NOT GOOD!

Remember, the newer the technology the happier the circuit will be--well almost!
 

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