Digital Logic

"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, open collector, etc., DO NOT require any attention.
NOTE: In the case of the tristate control pin, it should be treated as an INPUT.

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!

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

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

1111
1110
1101
1100
1011
1010
1001

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)

Binary-Decimal-Real Values

Exclusive OR (XOR)

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.

Truth Table->A & B are the Input switches
C is the Output LED

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.

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.

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.

Truth Table->

A & B are the Input switches
C is the Output LED

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)

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.

Truth Table->

A & B are the Input switches
C is the Output LED

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.

Logic Element Symbols

Remember:
1) A Positive AND gate is a Negative OR gate
2) A Positive OR gate is a Negative AND gate

Binary Codes