FIGURE 17. Basic input and output parameters of a
Standard TTL logic element.
FIGURE 18. Fan-in and fan-out values of the major TTL sub-families.
margins for both logic-1 (NM-H) and logic-0 (NM-L)
have defined worst-case values of 400 mV. With LS
TTL, the noise margins are 700 mV for logic-1, and
300 mV for logic-0. With CMOS, both margins have
values of VDD/3. Figure 15 illustrates the values of
these three sets of threshold and margin values.
Figure 16 expands the above information and
shows actual defined threshold voltage and noise
margin values, together with typical propagation
and power dissipation values for single 00-type
two-input NAND gates, for the seven major
sub-families of TTL (FAST TTL is regarded here as
simply a minor variation of AS TTL).
Fan-In and Fan-Out
In TTL circuitry, an element’s input drive
requirements are known as its fan-in values, and its
output driving capability limits are known as its fan-out values. Figure 17 illustrates the meanings and
worst-case values of these items when applied to a
Standard TTL element. Thus, (a) shows that when
the TTL element is driven from a Standard TTL output stage, it draws a worst-case input current (IIH)
of 40µA when fed with a 2.4 V logic-1 input, but —
as shown in (b) — feeds 1.6 mA (IIL) into the driver
when it provides a 0.4 V logic-0 input. Diagram (c)
shows that the TTL element’s output can, when in
the logic-1 state, provide up to 400µA (IOH) before
its output voltage falls below 2.4 V; it is thus
capable of feeding up to 10 Standard inputs, and is
said to have a logic-1 fan-out (= IOH/IIH) of 10.
Similarly, (d) shows that the output stage can —
when in the logic-0 state — absorb up to 16 mA
before its output voltage falls below 0.4 V; it is thus
capable of driving up to 10 Standard inputs, and is
said to have a logic-0 fan-out (= IOL/IIL) of 10. Thus,
the element has a worst-case fan-out of 10, and it
58 August 2006
FIGURE 19. TTL fan-in and fan-out in terms of Standard TTL units.
FIGURE 20. Maximum number of TTL inputs that may be driven from
any TTL sub-family output.
FIGURE 21. IC2 is defective; it is a Standard TTL device. Can it be replaced
directly by a 74LSXXX IC? Figure 20 shows that the answer is yes.