rl LONG DELAY

-

^{AND 3 I}

^{I ..}

^{WORD OUT}

Figure 3-52. Circulating Register for Serial Words

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Storage Registers

2.2.3.1-2.2.3.2

WORD IN

READOUT OR SHIFT PULSES

WORD CUT

Figure 3-53. Core Register for Serial Words

PARALLEL WORD IN

SERIAL _WORD

~

IN

(

SHIFT PULSES

.. "

:-0 ^SERIAL r----, ~

~l~r4, ~~;D

~~---~---~)

PARALLEL WORD OUT

Figure 3-54. Core Shifting Register used in serial m.ode c.omputers. An.other useful type .of

register f.or serial w.ords can be made with magnetic c.ores, as sh.own in figure 3-53. T.o follow the .opera-tion .of this register, consider the effect .of applying a single pulse (1) to the input c.oil .of the c.ore at the left and then applying a series .of readout pulses, .one each bit-time. The input pulse sets the first c.ore t.o 1. The first read.out pulse immediately resets this c.ore t.o 0, pr.oducing an .output pulse which is applied t.o the input .of the next c.ore. The delay circuit between c.ores (which includes the necessary di.ode t.o c.ontr.ol p.olarity) prevents the pulse fr.om setting the second c.ore t.o 1 un-til the read.out pulse has died away. So, the applicati.on .of .one read.out pulse shifts the 1 fr.om the first c.ore t.o the sec.ond. The next read.out pulse resets the sec.ond c.ore t.o 0 and shifts the 1 t.o the third c.ore. This effect c.ontinues with the third and f.ourth read.out pulses, shifting the 1 finally t.o the c.ore .on the right. If the read.out pulses (which may in this use be called shift pulses) are stopped at this point, the 1 remains st.ored in the right-hand c.ore. With a c.omplete, 5-bit w.ord fed into the register, instead .of a single bit, the same shifting acti.on takes place up.on applicati.on .of the read.out .or shift pulses. The shift pulses are applied at intervals .of .one bit-time s.o the first bit that enters the left-hand c.ore is shifted .out in time to clear it f.or the sec.ond bit .of the input w.ord, etc. When it is desired t.o

read the w.ord .out .of the c.ore register, all that is nec-essary is t.o apply a string .of five shift pulses, at I-bit intervals. This shifts the bits .of the w.ord t.o the .output line in the pr.oper relati.onship. The principles .of shift-ing in c.ore registers can be seen clearly in this example.

Shifting is simply a matter .of m.oving all the bits .of a w.ord in step, .one .or m.ore places t.o the left .or right.

The register .of figure 3-53 w.ould seem t.o qualify as a shifting register, but actually d.oes n.ot. It perf.orms the shifting .operati.on .only as a means .of getting serial w.ords into' and .out .of it; its functi.on is simple st.orage.

2.2.3.2 Shifting Registers

A shifting register is built with the intenti.on .of shifting any numbers st.ored in it f.or a purp.ose .other than that .of .ordinary st.orage. The purp.ose may be t.o c.onvert w.ords fr.om serial t.o parallel f.orm, fr.om paral-lel t.o serial, .or it may be t.o multiply .or divide the num-bers by s.ome p.ower .of 2. (Remember fr.om Part 2 that shifting a binary number .one place t.o the left multiplies it by the radix, 2; .one place t.o the right divides it by 2, etc.)

One means .of changing the register .of figure 3-53 t.o a true shifting register is t.o add a sec.ond input c.oil t.o each c.ore, as sh.own in figure 3-54. If a w.ord in parallel f.orm is applied t.o these inputs, the c.ores are set acc.ording t.o the pattern .of l's in the w.ord. Then, 79

Shifting Registers 2.2.3.2

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PART 3 CH 2 applying a string of five shift pulses shifts the bits of

the word to the output line in serial form.

By taking outputs in parallel off the capacitors in the delay networks between cores (plus that in the out-put line), a serial word can be written into the register and then taken out in parallel form upon the applica-tion of a single readout pulse. With parallel outputs available, the use of shifting for arithmetic purposes can easily be shown. The serial input and output can be ignored for the moment (consider them disconnected).

A word is written into the regiser on the parallel inputs and stored momentarily. The word might be, for example:

0.1110 (which equals decimal 14/16).

If a single shift pulse is now applied, each bit of the word is shifted one core to the right. The bit in the right-hand core, however, has no place to go, with the serial output line disconnected, and thus is lost. The left-hand core shifts the bit it contained to the core on its right, but there is no incoming bit to replace it, so this core is reset to 0 by the shift pulse and remains in the 0 state. Therefore, the register now contains:

0.0111 (which equals decimal 7/16).

The shift of one place to the right has thus divided the number in the register by 2. Notice that during the shift the original number appeared on the parallel output lines taken from the delay networks. The core switching can be so arranged, however, that these out-puts can be ignored at this time. They will be used only when it is desired to take the shifted number out of the register, by applying one more readout or shift pulse.

Notice, also, that any remainder left by the division is lost because the right-hand (least significant) bit is lost. If the sample number shown above is shifted once more, the number left in the register is:

0.0011

(which equals 3/16), instead of 0.00111, the exact an-swer. Since the lost remainder in division by shifting is always smaller than the least significant bit, however, dropping it has only a minor effect on the accuracy of computations.

Multiplication by shifting requires a shift to the left, instead of the right. To accomplish this, the

series-Figure 3-55. Flip-Flop Shifting Register

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and logic circuits, there are many possible arrange-ments. Which one will be used in a given case depends upon the exact operations to be handled, plus such fac-tors as the requirements of associated circuitry.

One of the simplest of such shifting registers operation time. In an arithmetic operation involving carries, for example, a ripple shift can be started while in constructing shifting registers but they differ little from the basic types described here.

2.3 ELECTRICAL CONSIDERATIONS AND NONLOGIC CIRCUITS

The information signals that have been illustrated thus far in this part (figs. 3-1, 3-3, and 3-4) have stood tall and square - as, in theory, they should. In actual circuitry, however, as mentioned earlier, there are many factors that act to attenuate or knock down the digital computer and displayed on an oscilloscope might look almost as good as those in figure 3-4, but instantaneous and therefore impossible).

None of these seeming drawbacks of the actual

Nonlogic Circuits 2.3

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PART 3 CH 2

~---'~+---r---+---~~~~s

Figure 3-56. Register Using Ripple Shift

plitude within a certain time. There may also be a limit on the amount of overshoot permitted.

Whenever a pulse or a level must travel through a number of circuits or devices without amplification, loss of amplitude must be expected. Such attenuation oc-curs in diode logic circuitry, for example. Circuits or signal paths with poor high-frequency response lengthen the rise times of pulses or levels, rounding the leading edges. Excessive capacitance in signal lines or circuits causes attenuation, distorts pulses, and may

IDEAL P U L L

r

-l

I I

I

+ o -I I I I

Figure 3-57. Typical Pulse in Computer Circuitry

upset timing because of undesired phase shifts. And im-pedance mismatches are particularly serious and diffi-cult to locate. They may result in misshapen pulses, severe attenuation or complete loss of signals, or phase shifts affecting signal timing. Mismatches of this sort can be caused by improper resistive values terminating coaxial signal lines (setting up reflections or standing waves on the lines); changing values of circuit compo-nents, often due to aging; leakage or partial shorts, and other factors. These are some of the electrical or electronic problems encountered in digital computers by both designers and maintenance technicians. The de-signer must include circuits to amplify and reshape pulses or reset levels wherever it appears possible that the pulses or levels may be forced outside limits. He may also have to provide impedance-matching and power-amplifying circuits to drive loads too large for the ordinary logic circuit to handle. Circuits such as these, included because of electrical necessity but not performing operations necessary to the logic of the com-puter, are called nonlogic circuits. Examples are cathode followers for both impedance-matching and power amplification, pulse amplifiers, level setters, register

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Circuit Packaging 2.3-2.4 drivers, etc. Pulse generators may produce an output of

one pulse or a series of pulses and may be used either for reshaping or for logic.

Nonlogic circuits of a particular type are usually identical throughout a given computer; therefore, they can be drawn as circuit blocks on the machine sche-matics, like logic circuits. Since they contribute nothing to the logic operations, they are normally omitted from the simplified or "pure" logic block diagrams used to describe the theory of operation, but must be included, of course, in the complete diagrams of the equipment.

2.4 CIRCUIT PACKAGING

It has been mentioned that digital computer cir-cuits are usually packaged, either individually or in small groups. The circuit connections are made by some sort of plug-in arrangement, so once a trouble has been localized to a given circuit, replacement is a simple matter of pulling out the defective circuit package and plugging in a good one from a supply of spares. The computer is then ready to run again, much sooner than it would have been if the circuit had to be repaired in the machine. Business expense or military necessity makes it important to keep almost every computer (commercial or military) running and solving prob-lems as continuously as possible. Thus, the mainte-nance time saved by using pluggable packaged circuits is important.

It would not be practicable to attempt to describe all of the many different packaging methods used.

Among the principal aims in all designs are making the

circuit packages as small as possible while maintalmng efficient cooling for reliable operation, making wiring simple and uniform, and simplifying repair (except in types designed to be thrown away if they fail). Printed circuits and miniaturized circuit components are widely used.

In the AN/FSQ-7, -8 computer, for example, circuits are constructed in rectangular metal forms of uniform size, illustrated in figure 3-58. These pluggable units, as they are called, are designed to be plugged into rack assemblies, one above the other, like drawers in a bu-reau. The circuit components are mounted on etched cards, one or more to a circuit, which are then inserted in vertical slots in the pluggable unit. The vacuum tubes are mounted horizontally on the front of the unit, and standard wiring is used to connect the cards and the tube socket pins. When all the units are in place in a stack, conditioned air is blown up from the base and escapes through openings around. the tube sockets in each unit, cooling the cards and the tubes.

These pluggable units are fairly sizable, but conve-nient for one man to handle. By way of contrast, one transistorized digital computer for airborne use has its circuits individually packaged in small plastic cubes, each less than half the size of a cigarette package. The circuit packages are interconnected by printed wiring on the cards upon which groups of circuits are mounted, and the cards, in turn, are plugged into air-conditioned cabinets. The entire computer occupies less space than the average office desk.

83

Fig. 3-58

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Figure 3-58. Circuits Packaged in Pluggable Unit

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PART 3 CH 2

PART 3 combinations or sequences of logic operations. The dig-ital computer is built by assembling networks of these basic circuits to perform the operations of arithmetic, to handle the input and output of information, and to control the internal working of the machine.

This chapter will examine some of the networks that can be used to accomplish the functions of arithme-tic and internal control in computers working with the they are often needed for amplification, level setting, etc.

Some types of the basic circuits described in Chapter 1 could be used to construct these networks without change, but others, for electrical reasons, would require modifications of the networks to make them work. This, however, would not change the overall principles of operation. It can be assumed, for most of the networks to be described, that either voltage level or pulse signals, or both, could be used in different parts of the network.

There are usually several different network ar-rangements that will perform a given arithmetic opera-tion and it is not possible or practical to treat all of by counting the individual signals.

The cycling of the bits in a binary number can be ca-pacitor in each output line represents a differentiating

85

Im Dokument COMBAT DIRECTION CENTRAL (Seite 100-107)