Make Electronics

Experiment 19: Learning Logic

190 Chapter 4

BAckground

The confusing world of TTL and CMOS

Back in the 1960s, the first logic gates were built with

Transistor-Transistor Logic, abbreviated TTL, meaning that tiny

bipolar transistors were etched into a single wafer of silicon.

Soon, these were followed by Complementary Metal Oxide

Semiconductors, abbreviated CMOS. Each of these chips was

a collection of metal-oxide field-effect transistors (known as

MOSFETs). The 4026 chip that you used earlier is an old CMOS.

You may remember that bipolar transistors amplify current.

TTL circuits are similar: they are sensitive to current, rather

than voltage. Thus they require a significant flow of electricity,

to function. But CMOS chips are like the programmable uni-

junction transistor that I featured previously. They are voltage-

sensitive, enabling them to draw hardly any current while they

are waiting for a signal, or pausing after emitting a signal.

The two families named TTL and CMOS still exist today. The

table in Figure 4-64 summarizes their basic advantages and

disadvantages. The CMOS series, with part numbers from

4000 upward, were easily damaged by static electricity but

were valuable because of their meager power consumption.

The TTL series, with part numbers from 7400 upward, used

much more power but were less sensitive and very fast. So,

if you wanted to build a computer, you used the TTL family,

but if you wanted to build a little gizmo that would run for

weeks on a small battery, you used the CMOS family.

From this point on everything became extremely confusing,

because CMOS manufacturers wanted to grab market share

by emulating the advantages of TTL chips. Newer genera-

tions of CMOS chips even changed their part numbers to

begin with “74” to emphasize their compatibility, and the

functions of pins on CMOS chips were swapped around to

match the functions of pins on TTL chips. Consequently, the

pinouts of CMOS and TTL chips are usually now identical,

but the meaning of “high” and “low” states changed in each

new generation, and the maximum supply voltages for CMOS

chips were revised downward. Note I have included question

marks beside two categories in the CMOS column, as modern

CMOS chips have overcome those disadvantages—at least to

some extent.

Here’s a quick summary, which will be useful to you if you

look at a circuit that you find online, and you wonder about

the chips that have been specified.

Where you see a letter “x,” it means that various numbers

may appear in that location. Thus “74xx” includes the 7400

NAND gate, the 7402 NOR gate, the 74150 16-bit data selec-

tor, and so on. A combination of letters preceding the “74”

identifies the chip manufacturer, while letters following the

part number may identify the style of package, may indicate

whether it contains heavy metals that are environmentally

toxic, and other details.

TTL CMOS

Speed Faster Slower?

More?

Vulnerable

to static

electricity

Less

Part

number

series

7400 4000

(Later adopted

7400 numbering)

????

Power

Supply Range

Narrow

5v

Wider

3v-6v

Power

Consumption Higher Very Low

Input

Impedance Low Very High

Figure 4-64. The basic differences between the two families of

logic chips. In successive generations, these differences have

gradually diminished.

TTL family:

74xx

The old original generation, now obsolete.

74Sxx

Higher speed “Schottky” series, now obsolete.

74LSxx

Lower power Schottky series, still used occasionally.

74ALSxx

Advanced low-power Schottky.

74Fxx

Faster than the ALS series.