With the advent of ICs in the early 1960s, engineers needed ready access to a library of basic logic gates so that these gates could be wired together on circuit boards and turned into useful products. Rather than having to design a custom microchip for each new project, semiconductor companies began to recognize a market for standard, off-the-shelf logic ICs.

In 1963 and 1964, Sylvania and Texas Instruments began shipment of the 7400-series discrete logic family and unknowingly started a de facto industry standard that lasts to this day and shows no signs of disappearing anytime soon.

Using the 7400 family, an engineer can select logic gates, flip-flops, counters, and buffers in individual packages and wire them together as desired to solve a specific problem. Some of the most common members of the 7400 family are listed in Table 2.1.

TABLE 2.1 Common 7400 ICs
Part Number Function Number of Pins
7400 Quad two-input NAND gates 14
7402 Quad two-input NOR gates 14
7404 Hex inverters 14
7408 Quad two-input AND gates 14
7432 Quad two-input OR gates 14
7447 BCD to seven-segment display decoder/driver 16
7474 Dual D-type positive edge triggered flip-flops 14
7490 Four-bit decade counter 14
74138 Three-to-eight decoder 16
74153 Dual 4-to-1 multiplexer 16
74157 Quad 2-to-1 multiplexers 16
74160 Four-bit binary synchronous counter 16
74164 Eight-bit parallel out serial shift registers 16
74174 Quad D-type flip-flops with complementary outputs 16
74193 Four-bit synchronous up/down binary counter 16
74245 Octal bus transceivers with tri-state outputs 20
74373 Octal D-type transparent latch 20
74374 Octal D-type flip-flops 20

These are just a few of the full set of 7400 family members. Many 7400 parts are no longer used, because their specific function is rarely required as a separate chip in modern digital electronics designs.

However, the parts listed above, and many others that are not listed, are still readily available today and are commonly found in a broad range of digital designs ranging from low-end to hightech devices. 7400-series logic has been available in DIPs for a long time, as well as (more recently) SOICs and other high-density surface mount packages.

All flavors of basic logic gates are available with varying numbers of inputs. For example, there are 2-, 3-, and 4-input AND gates and 2-, 3-, 4-,8-, 12-, and 13-input NAND gates. There are numerous varieties of flip-flops, counters, multiplexers, shift registers, and bus transceivers.

Flip-flops exist with and without complementary outputs, preset/ clear inputs, and independent clocks. Counters are available in 4-bit blocks that can both increment and decrement and count to either 15 (binary counter) or 9 (decade counter) before restarting the count at 0.

Shift registers exist in all permutations of serial and parallel inputs and outputs. Bus transceivers in 4- and 8 bit increments exist with different types of output enables and capabilities to function in unidirectional or bidirectional modes. Bus transceivers enable the creation and expansion of tri-state buses on which multiple devices can communicate.

One interesting IC is the 7447 seven-segment display driver. This component allows the creation of graphical numeric displays in applications such as counters and timers. Seven-segment displays are commonly seen in automobiles, microwave ovens, watches, and consumer electronics.

Seven independent on/off elements can represent all ten digits.. The 7447 is able to drive an LED-based seven-segment display when given a binary coded decimal (BCD) input. BCD is a four-bit binary number that has valid values from 0 through 9.

Hexadecimal values from 0xA through 0xF are not considered legal BCD values. Familiarity with the 7400 series proves very useful no matter what type of digital system you are designing.

For low-end systems, 7400-series logic may be the only type of IC at your disposal to solve a wide range of problems. At the high end, many people are often surprised to see a small 14- pin 7400-series IC soldered to a circuit board alongside a fancy 32-bit microprocessor running at 100 MHz.

The fact is that the basic logic functions that the 7400 series offers are staples that have direct applications at all levels of digital systems design. It is time well spent to become familiar with the extensive capabilities of the simple yet powerful 7400 family.

Manufacturers’ logic data books, either in print or on line, are invaluable references. It can be difficult to know ahead of time if a design may call for one more gate to function properly; that is when a 40-year old logic family can save the day.

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