Page 103 - Mechanical Engineers Reference Book

2/44 Electrical and electronics principles
The first example shows that the parentheses may be removed numbers as the standard series, but have ‘LS’ inserted before
by multiplying out, as in normal arithmetic. The second two the type code (e.g. SN74LS00). The operating speed is about
examples have no arithmetic counterpart. twice as high and the power consumption is about 20% of the
De Morgan’s theorem states that, in any logical expression, standard series. Schottky devices are, however, slightly more
AND can be replaced by OR and vice versa, provided that expensive.
each term is also replaced with its inverse complement. The
resulting expression is then the inverse of the original. 2.3.20.2 CMOS
Example 1 The problematic features of the power supply associated with
From ABC we negate to the TTL family of logic devices has been largely responsible
ABC = + B + c for the growth of its major competitor, CMOS. CMOS ICs are
based on the field effect transistor and can operate off a range
Hence of power supply voltages between k3 V to +18 V. CMOS
ABC = X+B +C devices dissipate very little power, are very cheap and are
simple in operation. The fan out is about 50 and they have a
Example 2 far greater immunity to power supply noise. The noise immun-
From F = AB + CD we negate to ity of CMOS devices means that there is no requirement for
F = (2 + B) + (C + a) smoothing capacitors to the extent that they are generally
found in TTL circuitry.
Applying De Morgan again, There are also some disadvantages associated with CMOS
F = (2 + B).(C + a) devices, the main one being that CMOS is slower than TTL,
roughly about one tenth of the equivalent TTL circuit. CMOS
The equivalence of the original and the final expressions in the ICs are also very sensitive to electrostatic voltages. Manufac-
above two examples may be checked by using a truth table. turers do build in some safety features to reduce the elec-
trostatic sensitivity, but CMOS devices must still be handled
with due care. Table 2.4 gives a brief comparison between
2.3.20 Digital electronic gates TTL and CMOS devices.
The principles of Boolean algebra have been considered with
respect to manually switched circuits. In modern digital Table 2.4 Comparison between TTL and CMOS devices
systems the switches are formed with transistors for speed of
operation, and they are generally referred to as ‘gates’. Over Property TTL CMOS
the years, various technologies have been developed in the
manufacture of logic gates. The earliest forms of electronic Power supply 5 V k 0.25 V 3 V to 18 V d.c.
gate were based on the unidirectional conduction properties of Current required Milliamps Microamps
diodes. Diode logic gates have now been superseded by Input impedance Low Very high
transistor-transistor logic gates (TTL) or the more recent Switching speed Fast - 10 ns Slow - 300 ns
CMOS family of logic gates. Fan out 10 50
The internal construction and operation of modern logic
gates may be quite complex, but this is of little interest to the
digital systems designer. Generally, all that the designer need
to know is the power supply voltages, the transient switching 2.3.21 Gate symbols
times, the ‘fan out’ and the ‘fan in’. Fan out refers to the Having defined a system output in terms of a Boolean
number of similar gates which can be driven from the output expression, the actual circuit can be constructed using the
of one gate. Fan in, on the other hand, denotes the number of required gates selected from the logic family chosen. Gen-
similar gate outputs which can be safely connected to the input erally, the design will be centred round the more readily
of one gate. available NAND and NOR logic gates. In laying out a gate
interconnection diagram, standard symbols are used to repre-
2.3.20.1 TTL sent the individual gates. Unfortunately, no universal set of
symbols has emerged, and several systems are in current use.
The TTL family is based on the bipolar junction transistor, Figure 2.90 summarizes the most common gate symbol
and was the first commonly available series of logic elements. systems.
TTL logic gates are rapid-switching devices (the SN7400, for
example, takes just 15 ns to change state). The standard
power supply is 5 V with a low tolerance band of f0.25 V. 2.3.22 Logic systems using simple gates
This, in turn, necessitates a reliable power supply regulation
which is reasonably facilitated through the great variety of A vending machine which dispenses either tea or coffee can
supply regulators which are now available in IC form. For the serve as an illustrative example. The logic circuit may be
SN74 series TTL ICs. the fan out is about 10. realized using AND gates as shown in Figure 2.91.
A TTL-based system can draw quite large instantaneous The money input is common to both gates, and the system,
loads on a power supply, and this can result in substantial although workable, has, a minor fault in that if both buttons
interference ‘spikes‘ in the power lines. Since the spikes can are pressed, after the money criterion is satisfied, then the
upset the normal operation of the system it is common practice output will be both tea and coffee. This fault can be designed
to connect small capacitors directly across the power lines, as out of the system by extending the logic circuit as shown in
close to the TTL ICs as possible. One capacitor, 0.1-10 pF, Figure 2.92.
per five ICs is sufficient in most instances. The extended system incorporates a NAND gate and an
TTL circuits are continually being improved and a major additional AND gate. If both buttons are now pressed then
recent advance has been the introduction of the low-power the output from G3 will be 0. With the output 1 from GI, the
‘Schottky’ TTL circuits. These use the same generic code output from G4 will be 0 and the machine will dispense tea. On