Logic terms and Basic logic symbols and truth tables.

Logic terms

Astable Type of multivibrator circuit, producing a square wave oscillation.

Asynchroa- 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.

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

nip-llop 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. Multivlbrator 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 Mil Std-8068 specification, as they are in more general use than those of B.S.3939. Positive logic convention used. i.e. I= high. 0 =low.

Medium scale integrated logic symbols and terminology

Medium scale integrated (MSI) logic elements are represented by rectangular blocks with appropriate external AND/0 R gates when necessary. A small circle at an external input means that the specific input is active Low, i.e., it produces the desired function. in conjunction with other inputs, if its voltage is the lower of the two logic levels in the system. A circle at the output indicates that when the function designated is True, the output is Low. Generally, inputs are at the top and left and outputs appear at the bottom and right of the logic symbol. An exception is the asynchronous Master

Reset in some sequential circuits which is always at the left-hand bottom corner.

Inputs and outputs are generally labelled with mnemonic letters. Those used in this book are listed below. Note that an active Low function labelled outside of tbe logic symbol is given a bar over tbe label, while the same function inside the symbol is labelled without the bar. When several inputs or outputs use the same letter, subscript numbers starting with zero are used in an order natural for device operation.