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Applications of the 7400 family are truly in?nite, because the various ICs representbasic building blocks rather than complete solutions. Up through the early 1980s,it was common to see computer systems constructed mainly from interconnected7400-series ICs along with a few LSI components such as a microprocessorand a few memory chips. These days, most commercial digital systems aredesigned using some form of higher-density logic IC, either fully custom oruser programmable. However, the engineer or hobbyist who has a relativelysmall-scale logic problem to solve, and who may not have access to moreexpensive custom or programmable logic ICs, may be able to utilize only 7400logic in an ef?cient and cost-effective solution. Two examples follow to provideinsight into how 7400 building blocks can be assembled to solve logic designproblems. A hypothetical example is a logic circuit to examine three switchesand turn on an LED if two and only two of the three switches are turned on.The truth table for such a circuit is as follows in

After consulting a list of available 7400 logic ICs, three become attractive for ourapplication: the 7404 inverter, 7408 AND, and 7432 OR. The LED driver logicequation requires four inverters, six two-input AND gates, and two 2-input ORgates. Four ICs are required, because a 7404 provides six inverters, a 7408provides four AND gates, and a 7432 contains four OR gates. These four ICscan be connected according to a schematic diagram as shown in Fig. 2.12.A schematic diagram illus- trates the electrical connectivity scheme of variouscomponents. Each component is identi?ed by a reference designator consistingof a letter followed by a number. ICs are commonly identi?ed by referencedesignators beginning with the letter U. Additionally, each component hasnumerous pins that are numbered on the diagram. These pin numbers conformto the IC manufacturers num- bering scheme. Each of these 7400-series ICs has14 pins. Another convention that remains from bi- polar logic days is the use ofthe label VCC to indicate the positive voltage supply node. GND representsground the common, or return, voltage supply node.
All ICs require connections to a power source. In this circuit, +5 V serves as thepower supply, be- cause the 7400 family is commonly manufactured in a bipolarsemiconductor process requiring a +5-V supply. The four rectangular blocks atthe top of the diagram represent this power connection information. Becausethis schematic diagram shows individual gates, the gates reference designatorscontain an alphabetic suf?x to identify unique instances of gates within the same IC.Not all gates in each IC are actually used. Those that are unused are tied inactiveby connecting their inputs to a valid logic level in this case, ground. It would beequally valid to connect the inputs of unused gates to the positive supply voltage,+5 V.
This logic circuit would work, but a more ef?cient solution is available to thosewho are familiar with the capabilities of the 7400 family. The 7411 provides three3-input AND gates, which is per- fect for this application, allowing a reduction inthe part count to three ICs instead of four. This circuit is shown in Fig. 2.13 withalternative notation to illustrate varying circuit presentation styles. Rather thandrawing gates as separate elements, the complete 7400-series ICs are shown asmonolithic blocks. Either notation is commonly accepted and depends on theengineers preference.