Write short notes on (any two):

  1. State diagram
  2. De-Morgan’s theorem
  3. TTL

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a) State Diagram

In addition to graphical symbols, tables or equations, flip-flops can also be represented graphically by a state diagram. In this diagram, a state is represented by a circle, and the transition between states is indicated by directed lines (or arcs) connecting the circles. An example of a state diagram is shown in Figure below.

User Loaded Image | CSIT Guide

The binary number inside each circle identifies the state the circle represents. The directed lines are labeled with two binary numbers separated by a slash (/). The input value that causes the state transition is labeled first. The number after the slash symbol / gives the value of the output. For example, the directed line from state 00 to 01 is labeled 1/0, meaning that, if the sequential circuit is in a present state and the input is 1, then the next state is 01 and the output is 0. If it is in a present state 00 and the input is 0, it will remain in that state. A directed line connecting a circle with itself indicates that no change of state occurs. The state diagram provides exactly the same information as the state table and is obtained directly from the state table.

b) De-Morgan’s Theorem

The De-Morgan’s first law states that, “The negation of conjunction is the disjunction of the negations” i.e.

Boolean Expression:

(A.B)’ = A’ + B’

(A.B.C)’ + A’ + B’ + C’

The De-Morgan’s second law states that, “The negation of disjunction is the conjunction of the negations” i.e.

Boolean Expression:

(A + B)’ = A’ . B’

(A + B + C)’ + A’ . B’ . C’

De-Morgan's Theorem | CSIT Guide

c) TTL

The Transistor-Transistor Logic (TTL) is a logic family made up of BJTs (bipolar junction transistors). As the name suggests, the transistor performs two functions like logic as well as amplifying. The best examples of TTL are logic gates namely the 7402 NOR Gate & the 7400 NAND gate.

TTL logic includes several transistors that have several emitters as well as several inputs.

The characteristics of TTL include the following.

  1. Fan Out
  2. Power Dissipation
  3. Propagation Delay
  4. Noise Margin

The advantage of TTL are

  1. TTL provides very high speed.
  2. TTL is more reliable.
  3. TTL needs lowe power for any operation.
  4. TTL can stack the transistors to make for fast pull up and pull down.
  5. TTL circuits are not so complicated.

The disadvantage of TTL are

  1. Noise immunity is not very high; so TTL gates cannot be used in applications where large noise voltages exist.
  2. Because of isolation problems, which require more chip space, TTL VLSI circuits are not possible in its conventional form.
  3. Power dissipation of TTL gates is much higher than that of MOS gates.
  4. Cost of TTL gates is higher than that of NMOS/CMOS gates, when MSI and LSI gates are considered.
  5. TTL gates generate transient voltages at switching instants.
  6. Wired-OR capability is not possible for the conventional TTL gates; open-collector gates are required for this application.
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