machines first became useful when the MTBF for a switch increased above a few hundred hours. Even so, many of these machines had complex, well-rehearsed repair procedures, and would be nonfunctional for hours because a tube burned-out, or a moth got stuck in a relay. Modern transistorized integrated circuit logic gates have MTBFs greater than 82 billion hours (). This level of reliability is required because integrated circuits have so many logic gates. Fan-out Fan-out describes how many logic inputs can be controlled by a single logic output without exceeding the electrical current ratings of the gate outputs. The minimum practical fan-out is about five. Modern electronic logic gates using CMOS transistors for switches have higher fan-outs. Speed The switching speed describes how long it takes a logic output to change from true to false or vise versa. Faster logic can accomplish more operations in less time. Modern electronic digital logic routinely switches at , and some laboratory systems switch at more than .. Logic families 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 tubes 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 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