2.1 COMPUTER LOGIC CIRCUITS
What switching devices are commonly used in logic . . , T
cIrcuIts. he answer, today, is relays, semiconductor ( c~stal) diodes, vacuum tubes, transistors, and mag-netic cores. Tomorrow's answer may be different, for research is constantly seeking smaller, faster, more
effi-c~ent: and more reliable switching devices. Today's logic cIrcuIts are shown, briefly, below. The actual circuit coverage is brief because, except in relay computers (those composed principally of relays), a good working knowledge of a digital computer rarely requires know-ing exactly what is inside a logic circuit.
All the AND circuits in a given computer, for ex-ample, are usually identical and individually packaged.
The machine is ordinarily serviced by locating and cir-cuits interconnected.) The technician must know the types of electrical signals used and the inputs, outputs, and "rules" of each type of logic circuit. In other words, to understand and troubleshoot the computer, it is nec-essary to know the logic but not the individual circuits.
For a complete understanding of the computer, it is necessary to know how logic circuits are made up, using the various switching devices mentioned earlier.
2.1 • 1 Relay Logic Circuits normally-open contacts in parallel.
This relay computer OR circuit, then, is almost ex-actly similar to the simple doorbell OR circuit. Com-puters composed principally of relays are not often built today, due to the comparatively slow operate and release times of the relays, but the relay logic circuits are nevertheless still important. The automatic dial tele-phone exchanges, for example, the world's largest dig-ital data-processing machines, or computers, use thou-sand.s of relays. And some relay circuitry is often used
Relay Logic Circuits 2.1.1
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-:~
ALARMI IIII
I
~L-Jrtf
RANGE + INrfH:STlLE 1
FROM FROM
RANGE IDENTIFICATION COMPUTER CIRCUIT
Figure 3-15. Relay AND Circuit tile flies within 20,000 yards of the ship. The relay
com-puter AND circuit shown in figure 3-15 automatically sounds the alert. The coil of the HOSTILE relay is con-nected to an identification circuit that operates this re-lay whenever a hostile aircraft is detected. Operating this relay, in effect, signals that there is a binary 1 at this input. The hostile aircraft is automatically tracked and a range computer operates the IN RANGE relay if the aircraft comes within 20,000 yards of the ship. This signals a binary 1 on the second input. The normally-open contacts of the two relays are connected in series in the alarm circuit; thus, when a hostile aircraft is de-tected AND its range is less than 20,000 yards, both sets of contacts are closed and a d-c level is applied to the alarm device to sound the alert. Putting it in general terms, when binary l's are on both inputs, the circuit produces an output of 1. The 100gic is:
In Range AND Hostile
==
AlarmThe circuit is not limited to two inputs, of course. More sets of normally-open contacts can be added in the series path.
A very important requirement is that, regardless of the number of inputs, all must be signaling l' s at the same time to produce an output 1. In the AND circuit of figure 3-15, if only one set of contacts closed, or if one set closed and then opened again before the second set closed, the alarm device could not operate. In this example, it is likely that an aircraft would be identified as hostile before it came within the specified range.
FROM RANGE COMPUTER
FROM IDENTIFICATION
CIRCUIT
Therefore, the HOSTILE relay would be O'perated first.
The AND circuit cannot produce an output, however, unless both input conditions are satisfied simultane-ously, so no alert could be sounded (no output pro-duced) until the IN RANGE relay also operated.
Inversion - the NOT operation - is easily accom-plished with relays by using a normally-closed set of contacts. The above discussions have indicated that a binary 1 is inserted into a circuit by closing a pair of normally-open contacts. This is done by operating the relay on which the contacts are mounted. In other words, a binary 1 operates a relay, and the 1 is trans-ferred intO' another circuit by the closing of the relay's normally-open contacts in that circuit. Now, consider the relay that has normally-closed contacts. When this relay is not operated, its contacts are placing a binary 1
in some other circuit, so the relay is receiving binary 0 (at its coil) and its contacts are indicating NOT 0, or 1.
When a binary 1 operates the relay, its contacts open, indicating
o.
Thus, using normally-closed contacts ac-complishes the NOT operatiO'n. Either OR NOT orAND NOT circuits can be built. The AND NOT oper-ation can be illustrated by making a change in the ex-ample of figure 3-15. The HOSTILE relay is simply replaced by a FRIENDL Y relay with a set of closed contacts connected in series with the normally-open contacts of the IN RANGE relay, as shown in figure 3-16.
It must be assumed that the identification circuit operates the FRIENDLY relay only if an aircraft is
l
Figure 3-16. Relay AND NOT Circuit
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Diode Logic Circuits 2.1.1-2.1.2 identified as friendly. When the FRIENDLY relay is
operated~ its contacts open. If the aircraft comes within 20,000 yards, the contacts of the IN RANGE relay close, but no alarm is given because the alarm circuit is broken by the open contacts of the FRIENDLY relay.
However, if the incoming aircraft is hostile, the FRIENDLY relay is not operated, and its contacts re-main closed. When the RANGE relay now operates, the alarm circuit is completed, and the alert is given. Be-cause of the series connection, the network is still an AND circuit, but the normally-closed contacts introduce inversion. The logic of the circuit is now:
In Range AND (NOT Friendly) = Alarm An OR NOT circuit is constructed like an OR, but using a normally-closed set of contacts in one (or more) of the parallel paths.
Many different combinations of logic operations are possible in relay contact networks, of course, using various arrangements of series-parallel paths. A rela-tively simple example is shown in figure 3-17, with the logic written below the circuit. The relay coils are not shown, which is the normal practice in relay work. In the logic of figure 3-17, note that there are two paral-lel OR connections and three parts to the principal series AND connection. For experience in recognizing and working with logic, the reader might try drawing the logic block diagram of this circuit, using logic blocks like those of figure 3-5 through 3-12. Use as few blocks as possible to do the job correctly, just as a designer would attempt to reduce the number of logic circuits in a computer.
2.1.2 Diode Logic Circuits
Many computers use what is called diode logic, performing most or all of the logic operations in cir-cuits made up of semiconductor diodes, and using vac-uum tube or transistor circuits primarily for building up attenuated pulses or levels, where necessary. The basic diode OR circuit appears in figure 3-18. The crystal diode, of course, is like the vacuum tube diode. It has an anode and a cathode, identified as shown, and offers
A D
OUTPUT
(A OR B) AND C AND [DOR(E
ANDNOTF~=OUTPUT
Figure 3-17. Sample Relay Logic Combination
INPUTS [
AB
NEGATIVE VOLTAGE
SOURCE
R
CRYSTAL DIODE
A OR BJ OUTPUT
Figure 3-J,8. Diode OR Circuit
practically no forward resistance to the flow of elec-trons from cathode to anode; in other words, it con-ducts easily when the anode is made more positive than the cathode. However, when the cathode is more posi-tive than the anode, the diode offers a very high back resistance, and practically no current can flow.
The input lines to the diode OR are connected in parallel, each through a separate diode, to the output.
(Only two inputs are shown, but it is possible to add more.) As in the doorbell and relay circuits, it is the parallel connection itself that makes it an OR circuit.
The diodes are required to isolate the inputs from each other to prevent interaction between the circuits sup-plying the input signals. The junction is tied through resistor R to a source of voltage more negative than the level used to represent binary
o.
Therefore, when 0' s are on both input lines, both diodes conduct because the anodes are more positive than the cathodes. Since the diodes offer practically no resistance to current flow in this direction, nearly all the voltage drop in the cir-cuit is across the relatively large resistance of R. Thus, the output, tied to the more positive end of R, is at approximately the same voltage level as the input lines, indicating binaryo.
When a positive-going voltage level, representing a binary 1, appears on either input line, there is a greater difference of potential between the negative source and that input. Again, nearly all the increased voltage drop appears across R, so the volt-age at the output end of R rises to approximately the binary 1 level. The same effect occurs if l' s appear simultaneously on both inputs.59
Diode Logic Circuits oper-ates somewhat better with-levels. For pulse signals, load resistor R is often connected to ground instead of a other type of memory and information-transfer device because of loading and other electrical problems. The therefore, they may become available at slightly differ-ent times. For this reason, it is often necessary to of the compensating delay.
One other important matter must be mentioned.
PART 3 opera-tion. Thus, reversing the signal polarities interchanges the diode circuit functions. In practice, a circuit is polar-ity-inverting transformer Tl. There is a sizable induct-ance but practically no resistinduct-ance in this branch.
When a positive-going pulse, representing a binary 1, appears at input A (with a 0 at input B), the differ-ence in potential from the cathode of CR1 to the posi-tive source is decreased, and the voltage on the output line rises to approximately the binary 1 value. The in-ductance of the transformer secondary opposes any sud-den change in current, so there is little change in inhibition rather than straight inversion, more AND inputs and more inhibit inputs can be added in parallel with those shown in figure 3-21. The basic OR circuit can be modified in a fashion similar to this to develop an inhibiting OR.
All three types of diode logic are used in a digital computer by interconnecting them in various combina-tions and sequences to perform the proper logic
Vacuum Tube Circuits
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A p
OR2
B
---- -
AND-c OUTPUT
U
OR 3
D
~
INH
E
(0) F 1"
(A OR BlAND C AND [D OR (E AND NOT
FlJ
= OUTPUT• •
..J
OR 2.. 14
DI I
A B
.. I I
~ 2 D II --
AND ~J I Y4
D• I
:] OR 3..
C
D
OUTPUT
I
I.. J
a
~
IN HI
E F (b)
Figure 3-22. Sample Logic Circuit Combination is not. (This arrangement at (a), incidentally, is the
correct answer to the problem of drawing the logic of fig. 3-17.) A pulse appearing on input E, if it gets through the inhibit circuit, reaches the input to the AND one-half bit-time later than a pulse entering at the same instant on input C and encountering no de-lays. Pulses arriving on A, B, or D are each delayed one-quarter bit-time before reaching the AND. To make it possible for the desired input combinations to produce an output from the AND circuit, the designer must de-lay all the input signals by equal amounts. This is done by adding delay circuits in the proper places, as shown at (b) of figure 3-22. Now, the pulses of any of the desired combin~tions appearing at the input ter-minals reach the AND at the same instant and fire it, producing the binary 1 output pulse.
This illustrates the manner in which logic circuits are put together to perform the desired combinations or sequences of logic operations in a digital computer.
2.1.3 Vacuum Tube Logic Circuits
Vacuum tubes are excellent switching devices, of-fering the advantage of high speed and the possibility of amplifying signals as they are switched. They nave comparatively large space, power, and cooling require-ments, however, and these disadvantages indicate that the vacuum tube will see less and less use in the future, as newer devices are perfected.
One of the basic vacuum tube lcircuits is the NOT, or inverter, shown in figure 3-23. This is a simple triode amplifier in which driving the grid more posi-tive makes the plate more negaposi-tive. Thus, feeding the grid with a relatively positive signal representing bi-nary 1 produces a less positive (relatively negative) plate voltage representing binary O. In other terms, an input of A produces an output of NOT A. It is also true that an input of 0, or NOT A, yields an output of 1, or A. Although the NOT circuit is quite straight-forward, there are many possible variations of the OR and AND circuits. One type of vacuum tube OR circuit appears in figure 3-24. A twin-triode tube is used, and the output is taken from the common cathodes which
+
INPUT