Fanout is a measure of the ability of a logic gate output, implemented electronically, to drive a number of inputs of other logic gates of the same type. In most designs, logic gates are connected together to form more complex circuits, and it is common for one logic gate output to be connected to several logic gate inputs. The technology used to implement logic gates usually allows gate inputs to be wired directly together with no additional interfacing circuitry required. A logic gate performs a logical operation on one or more logic inputs and produces a single logic output. ... The field of electronics comprises the study and use of systems that operate by controlling the flow of electrons (or other charge carriers) in devices such as thermionic valves (vacuum tubes) and semiconductors. ...

A perfect logic gate would have infinite input impedance and zero output impedance, allowing a gate output to drive any number of gate inputs. However, since real-world fabrication technologies exhibit less than perfect characteristics, in reality a limit will be reached where a gate output cannot drive any more current into subsequent gate inputs - attempting to do so causes the voltage to fall below the level defined for the logic level on that wire, causing errors. The input impedance or sometimes loading impedance of a circuit or electronic device is the impedance actually experienced by a signal which is connected to its input. ... The output impedance, source impedance, or internal impedance of an electronic device is the opposition exhibited by its output terminals to the flow of an alternating current (AC) of a particular frequency as a result of resistance, induction and capacitance. ... In electricity, current refers to electric current, which is the flow of electric charge. ... now. ...

The fanout is simply the number of inputs can be connected to an output before the currents required by the inputs exceeds the current that can be delivered by the output while still maintaining correct logic levels.

( is the floor function). The floor and fractional part functions In mathematics, the floor function of a real number x, denoted or floor(x), is the largest integer less than or equal to x (formally, ). For example, floor(2. ...

Going on these figures alone TTL logic gates are limited to perhaps 2 to 10, depending on the type of gate, while CMOS gates have fanouts that are generally far higher than is likely to occour in practical circuits (e.g. using NXP Semicondutors specs for their HEF4000 series cmos chips at 25°C and 15V gives a fanout of 34 thousand). A Motorola 68000-based computer with various TTL chips. ... Static CMOS Inverter Complementary-symmetry/metal-oxide semiconductor (CMOS) (see-moss, IPA:), is a major class of integrated circuits. ...

However inputs of real gates have capacitance as well as resistance to the power supply rails. This capacitance will slow the output transition of the previous gate and hence increase its propogation delay. As a result rather than a fixed fanout the designer is faced with a tradeoff between fanout and propogation delay (which affects the maximum speed of the overall system). This effect is less marked for TTL systems, which is one reason why they maintained a speed advantage over CMOS for many years.

See also

• FO4 - Fanout of 4
• Fan-In - A related term reffering to the number of inputs of a logic gate.
• Reconvergent fanout