The fact that all arithmetic and data-processing operations in a digital computer are accomplished by switching and storing electrical signals has been men-tioned several times. Now, the switching circuits that perform the logic operations have been examined in detail, alone and in simple combinations satisfying va-rious conditions. It is easy to see that if signals temporary storage circuit, capable of storing or hold-ing one pulse-type signal for one-quarter or one-half of a bit-time.
This type of storage device (usually some form of electromagnetic delay line) works nicely for pulse sig-nals and for brief storage periods of a few bit-times.
But what if voltage levels are used and what if the storage period must be 50 bit-times, or 10,000, or in-definite? It is certainly impractical to send signals from all parts of the computer to the main memory or storage element. The resulting circuitry would be an impossible maze, for one thing, and the memory would have to be and requires extra circuitry. Another, more commonly used, method will be described.
2.2.1 Bistable Circuits
With the possible exception of semiconductor di-odes, all the switching devices used in logic circuits can
Figure 3-41. Basic Bistable Storage Circuit
71
Bistable Circuits 2.2.1
UNCLASSIFIED T .0. 31 P2-2FSQ7-2
PART 3 CH 2
-~
..
...J
SET ~
...
OR2..
12 ~~ --
OR 3I I -I
I 3I I -
-] OUTPUTS
CLEAR
INPUTS
.. I I
ANDl~ - I, J
INH• I
I
COMPLEMENT
Fioure 3-42. Complete Logic Circuit Flip-Flop
• ~ I
0 I
FF
1 0
FF o
FF
I
•
I~ I IC LEAR SET CLEAR SET COMPo
COMPo
Figure 3-43. Flip-Flop Circuit Symbols disappeared. A 1 appearing on input B, called the
clear or reset input, causes 13 to produce a 0, switching the circuit back to the 0 state.
An arrangement of this sort is called a flip-flop (abbreviated FF), and various types are widely used in digital computers for bit storage. The inputs are nor-mally pulses, although it is possible to use levels; the outputs are voltage levels.
The flip-flop output shown in figure 3-41 is al-ways called the 1 output, whether it is indicating 1 or
o.
Notice that a second output from the circuit can be taken from 12 and that this is opposite in state to the 1 output. In other words, when the 1 output carries a down level (indicating that the flip-flop contains 0), the output from 12 is up, and vice versa. This output from 12 is called the 0 output. In this manner, ·it is possible to indicate not only the state of the flip-flop but its complement (opposite) as well, which is often useful.
The use of only one output line (either one) is called single line transfer; using both is double line
transfer. It is also possible-and common practice-to add a third input, so arranged that a pulse on this input switches the flip-flop, regardless of its state. This complement input can be added as shown in figure 3-42. The extra circuits, the AND and the INH, are used to switch the complement input to the proper OR circuit to either set or clear the flip-flop, depending upon its state at the time the pulse is received. (It is assumed that there is sufficient delay in the logic circuits to pre-vent switching 12 before the input pulse has dis-appeared.) For some purposes, flip-flops do not require separate set and clear inputs and, therefore, have only the complement input.
Flip-flops have their own circuit symbols, which may resemble any of the three shown in figure 3-43.
The designation, FF, is often omitted, but the 0 and 1 output sides are always shown. The input leads are not usually labeled. Intead, for ease of understanding, the set input is always drawn on the same side of the block as the 1 output, the clear input on the same side as the o output. Thus, a pulse entering on the 1 side sets the
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Vacuum Tube Flip-Flops 2.2.1-2.2.1.2
PT
IR
1 1
HS-
-Figure 3-44. Relay Storage Arrangement flip-flop to 1. The complement input enters at the cen-ter of the block.
Constructing a flip-flop of logic circuits is instruc-tive but not very practical, of course, since bistable storage circuit such a multivibrators can be made di-rectly from vacuum tubes, transist'Ors, etc.
2.2.1.1 Relay Storage
Even the lowly relay can be made to remain in the operated (1) state after the passing of the operating signal. One of the simplest methods of accomplishing this (there are several) is shown in figure 3-44. This is essentially the same circuit as that in figure 3-14, but with the addition of a hold path to make the PT relay act as a storage device. The PT relay operates when either the IR or the HS relay is operated, closing the corresponding set of contacts and completing the circuit to the PT coil. As s'Oon as PT operates, however, one pair of its own contacts closes the hold path to ground through the normally-dosed contacts of RL. If IR or HS now releases, removing the original operating
SETo---e---~
INPUTS COMPo
0---.
signal, relay PT nevertheless remains operated through its hold path. When the time comes to release PT (return it to the unoperated state), relay RL is oper-ated and its contacts open the h'Old path of PT. By this means or others, relays used in logic circuitry can be made to store information indefinitely.
2.2.1.2 Vacuum Tube Flip-Flops
The basic bistable multivibrator circuit appears in figure 3-45. Actually, this circuit is bistable only if the component arrangement and values are correct; this, however, is a design problem. In operation, the cross-coupling between either plate and the opposite grid means that one triode at a time can c'Onduct, but not both. The decrease in plate potential that occurs when one tube conducts is coupled to the grid of the opposite tube, driving it to cutoff. With VI conducting and V2 cut off, for example, the circuit is in one of its two stable states. The plate potential of VI is relatively low, placing a down level voltage on the 1 output. The plate potential of V2 is high because the tube is not conducting, so an up level voltage is on the 0 output and the circuit is said to contain a O.
To change the state, a positive pulse is applied to TI through the set input. This pulse is inverted through the transformer action, and a negative pulse is placed on the grid of V1. This drives the tube toward cutoff; the plate potential rises as conduction decreases;
and the rising potential is coupled to the grid of V2.
As V2 begins conducting, the decrease of voltage at its plate is coupled to the grid of VI, helping the initial pulse to cut off VI completely. With VI cut off, its plate potential is high, so an up level voltage is now on the 1 output, while conduction through V2 places a down level on the 0 output.
+
.--_II_~
...- - - 0 ~ J
OUTPUTSCLEARo---~---~
Figure 3-45. 8as;c Vacuum Tube Flip-Flop
73
Fig. 3-46 UNCLASSIFIED T.O. 31P2-2FSQ7-2
PART 3 CH 2
(a)
I OUTPUT 2 o OUTPUT
SET INPUT CLEAR INPUT
(b)
Figure 3-46. Transistor Flip-Flop The circuit is now in its second stable state,
indi-cating a I on the I output line. If a second pulse is re-ceived on the set input, nothing happens because VI is already cut off. To dear the circuit and return it to the 0 state, it is necessary to apply a pulse on the dear input. This cuts off V2 and starts VI conducting again.
A pulse on the complement input changes the state of the circuit regardless of whether it contains 0
or 1. This puts pulses on both inputs simultaneously.
The one reaching the grid of the cutoff tube has no effect, but the other causes the switching action to take place by cutting off the conducting tube. The diodes are necessary to prevent pulses on the set or dear inputs from reaching both grids and complementing.
The vacuum tube flip-flop is comparatively simple and stable and can be made quite fast in its switching action.
2.2.1.3 Transistor Flip-Flops
Transistors can be used to replace vacuum tubes in multivibrator circuits similar to that shown in figure
3-45, resulting in savings in power and space. As in the logic circuits, however, they may also be used in the form of the grounded-emitter switch. The result-ing direct-coupled transistor flip-flop appears at (a) of figure 3-46. Here, again, either transistor conducts if its base is driven negative. When a transistor is turned on, its collector swings in a positive direction almost to ground potential. If Q2 is turned on, for ex-ample, node 2 is almost at ground, and this relatively positive voltage on the base of QI keeps QI shut off.
At the same time, node I is about 3 volts negative, supplying the currents to keep Q2 conducting. This can be called the 0 state of the circuit, Q I off and Q2 on.
To switch the flip-flop to the o'pposite state, it is neces-sary to drive more positive the base of the transistor that is turned on, Q2. As Q2 shuts off, its collector goes negative, turning on Q1. This causes node I to swing up toward ground, keeping Q2 shut off. Now, with QI on and Q2 off, the flip-flop is in the second of its stable states, the I state. To return it to the 0 state
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This direct-coupled transistor flip-flop provides an extremely small, reliable storage circuit.
2.2.1.4 Dynamic Flip-Flops
Figure 3-47. Dynamic Flip-Flop
) OUTPUT delay circuits and several amplifier-reshapers would be required to hold up or store one pulse until the other
Storage Registers
computer, hut the artificial transmission line of figure 3-49 offers similar characteristics lumped conveniently in the form of coils, capacitors, and resistors. A single section such as this offers only a very short delay, but by using special techniques of coil construction' and connecting several line sections in series, delays of up Since the two principal methods of information-transmission are parallel and serial, some registers are designed to take words in parallel form, some to take serial words. A third type, sometimes useful, is the serial-parallel register, which accepts a number (word) in parallel form and feeds it out in serial form, or vice present discussion is restricted to storage and shifting registers. flip-flop representing a different bit position.There are two methods of writing words into this
PART 3
CH 2
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Figs. 3-50 & 3-51
CLEAR PULSE
FF
(a)
FF
FF
,~---~/
INPUT WORD ( b)
figure 3-50. Parallel flip-flop Storage Register
OUTPUT WORD
0"-2
READOUT PULSE
'~---~I
INPUT WORD
Figure, 3-51. Parallel Core Register Instead of generating a separate pulse to clear each
flip-flop before storing a new word in the register, the practical thing to do is use a single pulse, as shown at (b) of figure 3-50. By connecting all the clear inputs in parallel, a single pulse can be made to clear the whole register just before the new word to be stored is due to arrive. A parallel storage register of magnetic cores can be built up in a manner similar to this. Such a register appears in figure 3-51. Each bit of the in-put word is applied to the inin-put winding of a separate core. The readout windings, connected either in series or in parallel, serve a dual purpose. When a readout
pulse is applied, it not only reads out (in parallel form) all the bits stored in the individual cores, but it also clears the register by resetting all cores to
o.
The regis-ter is then ready to store a new word. When cores are used for parallel word storage, like this, the readout pulse does not appear each bit-time but is more in the nature of a control pulse generated only when the stored information is needed.These are -the basic parallel storage registers. What about storage of words in serial form?
This appears rather difficult, at first thought, but, actually, it is quite simple. A clue to one often-used
Storage Registers method lies in the dynamic flip-flop, or circulating
mem-ory, shown in figure 3-47. There, a feedback loop with inci-dentally, is called nondestructive readout because the information is not lost from the storage circuit through
The circulating register is practical and widely
READOUT PULSES
.. -,
-
AND 3 II ..
WORD OUTFigure 3-52. Circulating Register for Serial Words