Experiment 19: Learning Logic
192 Chapter 4
BAckground
The confusing world of TTL and CMOS (continued)
Bottom line: I suggest you use the 4000B chips only if you want to replicate an
old circuit, or if a modern equivalent is unavailable (which is why I specified the
4026B chip for the reaction timer—I could not find a modern equivalent that
will drive seven-segment numeric displays directly, and I didn’t want you to
have to deal with more parts than necessary).
If you check online suppliers such as Mouser.com you’ll find that the HC family is
by far the most popular right now. They are all available in through-hole format
(to fit your breadboard and perforated board). They have the high input imped-
ance of CMOS (which is useful) and they have the same pin identities as the old
74LSxx series.Abbreviations
When looking at data sheets, you are likely to encounter some or all of these
abbreviations:- VOH min: Minimum output voltage in high state
- VOL max: Maximum output voltage in low state
- VIH min: Minimum input voltage to be recognized as high
- VIL max: Maximum input voltage recognized as low
BAckground
Logic gate origins
The 7400 family of integrated circuits was introduced by
Texas Instruments, beginning with the 7400 NAND gate in- Other companies had sold logic chips previously, but
the 7400 series came to dominate the market. The Apollo
lunar missions used a computer built with 7400 chips, and
they were a mainstay of minicomputers during the 1970s.
RCA introduced its 4000 series of logic chips in 1968, built
around CMOS transistors; Texas Instruments had chosen
TTL. The CMOS chips used less power, thus generating much
less heat and enabling flexible circuit design, as each chip
could power many others. CMOS was also tolerant of wide
voltage ranges (from 3 to 15 volts) but prohibited switching
speeds faster than around 1MHz. TTL was 10 times faster.
Design tweaks gradually eradicated the speed penalty for
CMOS, and TTL chips have become relatively rare. Still, some
people retain a special nostalgic loyalty to “the logic gates
that went to the moon.” A hardcore enthusiast named Bill
Buzbee has built an entire web server from TTL-type 7400
chips, currently online at http://magic-1.org. Figure 4-65
shows just one of the handmade circuit boards that Bill as-
sembled to run his computer.Figure 4-65. Hobbyist Bill Buzbee built himself a web server
entirely from 7400 series logic chips, the oldest of which was
fabricated back in 1969. The web server can be found online at
http://magic-1.org, displaying pictures of itself and details of
its construction. The picture here that Bill took shows just one
of the circuit boards of this remarkable project.