Fan-in is a term that defines the maximum number of digital inputs that a single logic gate can 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.
Compare fan-out .