Ieee symbol

HPRI/BIN
10 11 12 13 1 2 3 4

APPENDIX
≥1

0/Z10 1/Z11 2/Z12 3/Z13 4/Z14 5/Z15 6/Z16 7/Z17

10 11 12 13 14 15 16 17

A

18 α

15

14

V18
5

ENα

1α 2α 4α

9 7 6

IEEE STANDARD SYMBOLS

Together with the American National Standards Institute (ANSI), the Institute of Electrical and Electronic Engineers (IEEE) has developed a standard set of logic symbols. The mostrecent revision of the standard is ANSI/IEEE Std 91-1984, IEEE Standard Graphic Symbols for Logic Functions. It is compatible with standard 617 of the International Electrotechnical Commission (IEC), and must be used in all logic diagrams drawn for the U.S. Department of Defense. A.1 GENERAL DEFINITIONS

ANSI IEEE

The IEEE standard supports the notion of bubble-to-bubble logic design with thefollowing definitions: • An internal logic state is a logic state assumed to exist inside a symbol outline at an input or an output. • An external logic state is a logic state assumed to exist outside a symbol outline either (1) on an input line prior to any external qualifying symbol at that input, or (2) on an output line beyond any external qualifying symbol at that output.
internal logic stateexternal logic state

791

792

IEEE STANDARD SYMBOLS

APP. A

qualifying symbol

internal 1-state internal 0-state

A qualifying symbol is graphics or text added to the basic outline of a device’s logic symbol to describe the physical or logical characteristics of the device. The “external qualifying symbol” mentioned above is typically an inversion bubble, which denotes a “negated”input or output, for which the external 0-state corresponds to the internal 1-state. This concept is illustrated in Figure A–1. When the standard says that a signal is in its internal 1-state, we would say that the signal is asserted. Likewise, when the standard says that a signal is in its internal 0-state, we would say that the signal is negated.
Figure A–1 Internal and external logic states.external logic states

internal logic states

external logic state

external logic states

internal logic states

external logic state

distinctive-shape symbols rectangular-shape symbols

The IEEE standard provides two different types of symbols for logic gates. One type, called distinctive-shape symbols, is what we’ve been using all along. The other type, called rectangular-shapesymbols, uses the same shape for all the gates, along with an internal label to identify the type of gate. Figure A–2 compares the two types. According to the IEEE standard, “the distinctiveshape symbol is not preferred.” Some people think this statement means that rectangular-shape symbols are preferred. However, all the standard really says is that it gives no preference to distinctive-shapesymbols compared to rectangularshape symbols. On the other hand, since most digital designers, authors, and computer-aided design systems prefer the distinctive-shape symbols, that’s what we use in this book. Before the promulgation of the IEEE standard, logic symbols for largerscale logic elements were drawn in an ad hoc manner; the only standard rule was to use rectangles with inputs on the leftand outputs on the right. Although the logic symbol might contain a short description of the element (e.g., “3–8

ANOTHER KIND OF BUBBLE

In addition to the familiar bubble, the IEEE standard also allows an external, triangular “polarity symbol” to be used to specify active-low inputs and outputs, for which the external LOW level corresponds to the internal 1-state. However, under apositive-logic convention, the bubble and the triangular polarity symbol are equivalent, so we use the more traditional bubble in this appendix.
Draft of July 6, 1999

Copyright

© 1994 by John F. Wakerly

SEC. A.2

DECODERS

793

Figure A–2 Distinctive- and rectangularshape logic symbols.

AND

&

NAND

&

BUFFER

1

OR

≥1

INVERTER

1

NOR

≥1

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