Logic terms and Basic logic symbols and truth tables

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Logic terms

Astable Type of multivibrator circuit producing a square wave oscillation.

Asynchronous Operation not dependent on clock pulses. Bistable Type of multivibrator circuit, having two stable states. Buffered Capable of driving external circuits, isolated from previous stage.

Clock Source of regular voltage pulses, used to synchronise systems.

Decoder Device capable of translating a BCD input to separate control line inputs (also known as a demultiplexer).

Dual Two, twin.

Edge triggered Operation of device takes place on rising (or falling) part of input pulse.

Enable Over-ride input.

Fan-out Number of devices that can be placed in parallel on output.

Flip-flop Two-state device, changes state when clocked.

Hex Six.

Latch Retains previous input state until over-ridden.

Monostable Type of multivibrator circuit, with one stable state. Multiplexer Samples many inputs in sequence, gives one output. Multivibrator Circuit having two output states, each of which may or may not be stable. Oscillators of astable, bistable, or monostable types can be built with multivibrators.

Octal Eight.

One-shot Gives single output pulse of defined duration from variable input pulse.

Open collector TTL output which needs external pull-up resistor, can be used to wire OR outputs.

Parity Check bit added to data, can be odd or even parity. In odd parity sum of data Is + parity I is odd.

Propagation delay Time taken for signal to pass through a device, limits highest frequency of operation.

Quad Four.

Quiescent Stable state not driving a load.

Schmitt trigger Circuit with hysteresis.

Synchronous Operation dependent on clock pulses.

Basic logic symbols and truth tables

Logic symbols are to the North American Mil Std-806B specifica­ tion, as they are in more general use than those of BS 3939, Positive logic convention used i.e. I= high, 0 =low.

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