FIGURE 1. Basic coding system used on the
FIGURE 2. The basic 74-series code is often elaborated with a
manufacturer’s prefix and/or suffix.
FIGURE 3. Circuit
of a Standard TTL
or 24-pin dual-in-line package (DIP), can
be used with supplies within the limit
+ 4. 75 V to + 5. 25 V, and can be operated over the temperature range 0° C to
+ 70° C. The 54 identifies the IC as a
high-quality military-grade member of
the family. These devices are encapsulated in exotic packages, can use
supplies within the limit + 4. 5 V to + 5. 5
V, and can operate over the temperature range - 55° C to +125° C. The 75
identifies the IC as a commercial-grade
interface device that is designed to
support the 74 range of devices.
The second (central) sub-code
consists of up to three letters, and
identifies the precise technology or
sub-family used in the construction of
the device, as shown in the diagram.
Note that standard TTL devices carry
either no central code at all, or an N.
Each of the other seven major TTL
sub-family devices carry a central
identifying code, and the five major
CMOS 74 sub-families carry a central
code that includes the letter C.
The last (right hand) sub-code
usually consists of two to five digits
(but occasionally includes a letter A or
a star), and identifies the precise function of the IC (e.g., quad two-input
NAND gate, decade counter, four-bit
shift register, etc.). The precise
relationship between this sub-code
and the device function can be ascertained from the manufacturer’s lists.
Thus, a 74 type of IC may carry a
code that, in its simplest form, reads
something like 7400, 74N00, or 7414,
etc., if it is a standard TTL device, or
74L14, 74LS38, or 74HC03, etc., if it is
some other sub-member of the 74 family. Note that in practice 74-series ICs
often carry an elaborated form of the
basic code that includes a two-letter
pre-fix that identifies the manufacturer,
plus a lettered manufacturer’s suffix that
indicates the packaging style, etc., as
shown in Figure 2. Hence, a device
marked SN74LS90N is a normal
74LS90 IC, manufactured by Texas
Instruments and housed in a plastic DIP.
Eight major sub-families of TTL
have been used in the 74-series
throughout its lifetime, as follows:
Standard TTL — Standard TTL is similar
to the basic type already described,
except that each of its inputs are
provided with a protection diode that
helps suppress transients and speeds
up its switching action. Figure 3 shows
the actual circuit of a 7400 two-input
NAND gate. Its power consumption is
10 m W, and its propagation delay is 9
nS when driving a 15pF/400Ω load.
Low-Power (L) TTL (now obsolete) —
Low-power TTL is a modified version
of the standard type, with its resistance values greatly increased to give
a dramatic reduction in power consumption at the expense of reduced
speed. Figure 4 shows the circuit of a
74L00 two-input NAND gate. Its
power consumption is 1 m W, and its
propagation delay is typically 33 nS.
High-Speed (H) TTL (now obsolete) —
High-speed TTL is a modified version
of the standard type, with its
resistance values reduced to give an
increase in speed at the expense
of increased power consumption.
Figure 5 shows the circuit of a 74H00
two-input NAND gate. Its power
consumption is 22 m W, and its propagation delay is typically 6 nS.
Schottky (S) TTL (now obsolete) — A
transistor switch can be designed to
give either a saturated or an unsaturated switching action. Saturated
August 2006 53