Nomenclature :
Logic families are
named in the format “XYL” or X Y Logic.
A logic family means
a family of active or passive electronic devices used to build logic gates.within the Logic gate, usually
two functions have to be handled:
- The gati ng of the input signals, or for which a input network is required within the gate, and
- Switching/amplifying the logic signal levels.
In the nomenclature “XY Logic” , device X performs the
first function, while device Y performs the second function.
Some basic definitions:
(a)FAN IN : the maximum number of digital inputs that a single logic gate can accept
(b)FAN-OUT:maximum number of digital inputs that the output of a single logic gate
can feed
(c)Propagation Delay
: The time required for the output of a digital circuit to change states after
a change at one or more of its input
Comparison of primitive logic families:
The DRL, RTL, DTL, and TTL logic families are compared in the following table :
Logic
family
|
Year
Introduced
|
Variants
|
Characteristics
1
|
Characteristic
2
|
Advantage
|
Disadvantage
1
|
Disadvantage
2
|
Present
status
|
DRL
|
Before
1950
|
Uses
passive components, no active device used
|
Doesnot
provide amplification and hence no amplification is possible
|
Simple
and inexpensive
|
Does
not provide amplification, hence no inversion
is possible , i.e., a NOT gate cant be constructed
|
Lack
of amplification leads to signal degradation
|
Obsolete
due to its limitations
|
|
DTL
|
~
1950s
|
Complemented
Transistor –Diode Logic ,High Threshold Logic
|
Eliminates
signal degradation and and power dissipation
|
Propagation Delay is Larger
|
||||
RTL
|
~1956,
|
Direct
coupled Transistor logic,Resistor Capacitor transistor Logic
|
Very
in efficient, because it dissipates a
large amount of pawer as heat
|
Low
fan In
|
Obsolete
due to its limitations
|
|||
TTL
|
1961
|
Many
generations of it :L, H, S, LS, ALS, F
AS
|
Have
been in market for a log time , example 7400 series of ICs
|
