Editing Digital electronics (section)

==Logic families==
{{main|Logic family}}

Digital design started with [[relay logic]] which is slow. Occasionally a mechanical failure would occur. Fan-outs were typically about 10, limited by the resistance of the coils and arcing on the contacts from high voltages.

Later, [[vacuum tube]]s were used. These were very fast, but generated heat, and were unreliable because the filaments would burn out. Fan-outs were typically 5 to 7, limited by the heating from the tubes' current. In the 1950s, special computer tubes were developed with filaments that omitted volatile elements like silicon. These ran for hundreds of thousands of hours.

The first [[semiconductor]] logic family was [[resistor–transistor logic]]. This was a thousand times more reliable than tubes, ran cooler, and used less power, but had a very low fan-out of 3. [[Diode–transistor logic]] improved the fan-out up to about 7, and reduced the power. Some DTL designs used two power supplies with alternating layers of NPN and PNP transistors to increase the fan-out.

[[Transistor–transistor logic]] (TTL) was a great improvement over these. In early devices, fan-out improved to 10, and later variations reliably achieved 20. TTL was also fast, with some variations achieving switching times as low as 20&nbsp;ns. TTL is still used in some designs.

[[Emitter coupled logic]] is very fast but uses a lot of power. It was extensively used for high-performance computers, such as the [[Illiac IV]], made up of many medium-scale components.

By far, the most common digital integrated circuits built today use [[CMOS logic]], which is fast, offers high circuit density and low power per gate. This is used even in large, fast computers, such as the [[IBM System z]].