Microsoft Word - Digital Logic Design v_4_6a

 Fan-out and Fan-in

1. Fan-out: The number of gate inputs that can be driven by a single output (for LS chips. the
   fan-out limit is 20)


2. Fan-in: The number of gate inputs that may be connected together. Typically, fan-out is the
   limitation factor for LS.

 Using NAND or NOR gates

NAND and NOR gates can be substituted for each other. Basically, NOR implements POS and

NAND implements SOP. A function can equally be written as POS or SOP.

We will explore NAND gates here and similar concepts also apply to NOR gates.

 NAND is one the most desirable basic gates for three reasons:

1) NAND gates are faster than other gates such as AND.

2) NAND gates can be used to make all other gates such as inverters, ANDs and ORs

(Demorgan’s Theorem is the basis of this statement.)

Therefore NAND is said to be “functionally complete” since it can be used to build any other

function. Below are examples of Inverter and OR function implementation.

3) NANDs are available in more variety (#of input) than AND, OR gates

INVERTER OR