Fan in & Fan out

In digital electronics, the fan-out of a logic gate output is the number of gate inputs it can feed or connect to. In most designs, logic gates are connected to form more complex circuits. While no more than one logic gate output is connected to any single

input, it is common for one output to be connected to several inputs. The technology used to implement logic gates usually allows a certain number of gate inputs to be wired directly together without additional interfacing circuitry. The maximum fan-out of an output measures its load-driving capability: it is the greatest number of inputs of gates of the same type to which the output can be safely connected.

Fan-in is a term that defines the maximum number of digital inputs that a single logic gatecan accept. Most transistor-transistor logic ( TTL ) gates have one or two inputs, although some have more than two. A typical logic gate has a fan-in of 1 or 2.

In some digital systems, it is necessary for a single TTL logic gate to drive several devices with fan-in numbers greater than 1. If the total number of inputs a transistor-transistor logic (TTL) device must drive is greater than 10, a device called a buffer can be used between the TTL gate output and the inputs of the devices it must drive. A logical inverter (also called a NOT gate) can serve this function in most digital circuits.