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Input and output Structures A logic family should provide features for effective interfacing both at the input and output. Interfacing at the input requires facility to accept different voltage levels for the two logic states, and to accept signals with rise and fall times very different from those of the signals associated with that logic family. At the output we require larger current driving capability, facility to increase the voltages associated with the two logic levels, and the ability to tie the outputs of gates to have wired logic operations.

here what is the meaning of large current driving capability?

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In that context, 'large driving capacity' means sufficient output current to drive typical loads at something approaching the full speed of that logic family's gates.

Examples of typical loads are:

(1) A number of input stages of the same/similar logic family. The maximum number of loads ('fan-out') depends upon the logic family but generally improves with newer families. Guide numbers are only 5..10 loads for early families up to 1000's for later families (refer to family specs' for specifics). A further limit is the capacitance of an increasingly large number of longer interconnecting tracks with stubs at each gate, which will reduce signal speed and/or raw signal quality.

(2) Parallel buses of control and data signals with timing related o each other (e.g. memory bus, multi-board interface bus). Again, larger/longer interconnecting tracks will increasingly reduce signal speed and/or raw quality.

(3) Line driver/buffer circuits. For outputs at different signalling standards (e.g. communications, displays) or controlling higher-power loads (e.g. solenoids, motors, lights).

There's plenty more that could be written on the subject but this illustrates the point for your further reading.

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