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DESIGN AND OPERATION OF

THE CEILOMETER COMPUTER

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
/ SLZflOZ

THE NATIONAL BUREAU OF STANDARDS

ions and Activities

The functions of the National Bureau of Standards are set forth in the Act of Congress, March.

3, 1901, as amended by Congress in Public Law 619, 1950. These include the development and

maintenance of the national standards of measurement and the provision of means and methods for making measurements consistent with these standards; the determination of physical constants and

properties of materials; the development of methods and instruments for testing materials, devices,

and structures; advisory services to government agencies on scientific and technical problems; in- vention and development of devices to serve special needs of the Government; and the development

of standard practices, codes, and specifications. The work includes basic and applied research,

development, engineering, instrumentation, testing, evaluation, calibration services, and various

consultation and information services. Research projects are also performed for other government

agencies when the work relates to and supplements the basic program of' the Bureau or when the Bureau's unique competence is required. The scope of activities is suggested by the fisting of

divisions and sections on the inside of the back cover.

Publications

The results of the Bureau's work take the form of either actual equipment and devices or published papers. These papers appear either in the Bureau's own series of publications or in the journals

of professional and scientific societies. The Bureau itself publishes three periodicals available from

the Government Printing Office: The Journal of Research, published in four separate sections,

presents complete scientific and technical papers; the Technical News Bulletin presents summary

and preliminary reports on work in progress; and Basic Radio Propagation Predictions provides

data for determining the best frequencies to use for radio communications throughout the world.

There are also five series of nonperiodical publications: Monographs, Applied Mathematics Series,

Handbooks, Miscellaneous Publications, and Technical Notes. Information on the Bureau's publications can be found in NBS Circular 460, Publications of the

National Bureau of Standards ($1.25) and its Supplement ($1.50), available from the Superintendent

of Documents, Government Printing Office, Washington 25, D.C.
NATIONAL BUREAU OF STANDARDS

64

OCTOBER 1960

DESIGN AND OPERATION OF

THE CEILOMETER COMPUTER

Paul Meissner

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature. They are for sale by the Office of Technical Services, U. S. Department of Commerce, Washington 25, D. C.

DISTRIBUTED BY

UNITED STATES DEPARTMENT OF COMMERCE

OFFICE OF TECHNICAL SERVICES

WASHINGTON 25, D. C.

Price $2.00

TABLE OF CONTENTS

Page

1. INTRODUCTION 1

1. 1 Brief Description of Operation 2

2. DETAILED DISCUSSION OF COMPUTER OPERATION 4

2. 1 Analog -Digital Conversion 4

2. 1. 1 Sampling Circuit 10

2. 1. 2 Pulse Stretcher 12

2.1.3 Analog -Digital Converter 13

2. 2 Peak-Value Detector 16

2. 3 Altitude Number Counter 19

2. 4 Synchronizing Pulse Generator 25

2. 5 Address Counter 26

2. 6 Record Circuit 31

2. 6. 1 Record Package 31

2. 6. 2 Operation of Record Circuit 34

2. 7 Read Circuitry and Comparator 36

2. 7. 1 Comparator Circuit 37

2. 7. 2 Parity Check Circuit 38

2. 7. 3 Read Package 41

2. 7. 4 Storage Register 43

2. 7. 5 Max-Min Circuit 43

2. 7. 6 Circuit for Counts Up to Critical Altitude 44

2. 7, 7 Analysis Control 47

2. 8 Altitude Interval Generator and Control Circuit 49

2. 8. 1 Detailed Operation of Altitude Interval Circuit 49

2. 8. 2 Control Circuit 50

2. 8. 3 Synchronizing Pulse from Ceilometer 53

-ii-
TABLE OF CONTENTS (Cont'd)

Page

2. 9 Output Decoding and Display 56

2. 9* 1 Altitude Number Stepping Switches 56

2.9.2 Critical Counts Stepping Switches 58

2.9.3 Stepping -Switch Drive Circuit 58

3. MANUALLY OPERATED TEST FUNCTIONS 64

3. 1 Single Word Operation 66

3.2 One -Revolution 67

-in-
LIST OF ILLUSTRATIONS

Figure No* Page

1 Block Diagram of Ceilometer Computer 3

2 Front View of Computer 5

3 Interior View Showing Packages 6

4 Interior View Showing Wiring 7

5 Rear View of Interior 8

6 Photograph of Typical Packages 9

7 Sampling Process 11

8 Pulse Stretcher 14

9 Analog -Digital Converter 15

10 Peak-Value Detector 18

1

1

Table of Altitude Intervals 20

12 Altitude Number Counter 21

13 Synchronizing Pulse Pattern 27

14 Synchronizing Pulse Generator 28

15 Address Counter and Record Circuit 29

16 Record Package 32

17 Comparator Circuit 39

18 Read Package 42

19 Maximum- Minimum Circuit 45

20 Circuit for Counts up to Critical Altitude 46

21 Altitude Interval Generator 51

22 Control Circuit 54

23 Arrangement of Displays 57

24 Wiring of 2 6- Point Stepping Switches 59

25 Wiring of 11- Point Stepping Switches 60

26 Stepping Switch Drive Circuit 62

27 Stepping Switch Chassis Wiring 63

28 Front Panel 65

29 Switch and Pushbutton Wiring 69

30 Input Amplifier 70

31 Developed View of Cabinet Interior 71

-IV-
ABSTRACT

The ceilometer computer has been developed to provide a

display and electrical readout of cloud-height information for use

with an automatic weather station. The computer receives an

analog signal from the detector of a rotating-beam ceilometer,

and determines the height at which cloud indications occur. A

small magnetic storage drum contains ten minutes of cloudheight information which is continuously updated, and these data

are analyzed for the following factors:

1) Predominant cloud height over the past

ten minutes,

2) Maximum and minimum height at which

significant cloud occurences were observed,

3) Number of cloud observations up to a selected critical altitude.

The computer is a wired-program machine constructed

of transistorized plug-in packages. Several modes of manual

operation have been incorporated for testing and maintenance

purposes.

-v-
DESIGN AND OPERATION OF THE CEILOMETER COMPUTER.

by Paul Meissner

1. INTRODUCTION

The ceilometer computer has been designed to provide a

numerical display of cloud height information, utilizing data received

from a rotating beam ceilometer. The computer has been developed

to fulfill a requirement of the U. S. Weather Bureau in connection

with the establishment of automatic weather stations. The output

of the computer can be transmitted by teletype, together with the

outputs from a variety of other weather -indicating instruments.

The rotating beam ceilometer is a device which measures the

height of clouds from the ground up to about 10, 000 feet.

A light beam from a rotating searchlight is reflected by the

clouds and is received by a photocell placed at a known horizontal

distance from the searchlight, as shown in Fig. 1. The cloud height

is calculated by simple triangulation, using the angle at which the

cloud signal is received and the length of the baseline. The light

source is modulated by means of a rotating shutter, and the photocell operates in conjunction with a filter and a tuned amplifier to

discriminate against ambient light. The present ceilometer provides

10 scans per minute.

At present the ceilometer indications are observed by an

operator who interprets a cathode -ray display and obtains answers to

a variety of questions. Typical questions might be the following: At

what height did most of the clouds occur over the past ten minutes?

(or one minute?) What was the lowest level at which a significant

number of clouds occurred? The highest level? How many clouds

occurred below a certain critical level?

In order to obtain answers to these questions the cloud height

equipment samples the ceilometer output and records the height at

which the greatest signal is received during each scan. These data

are stored in a small 100 -word magnetic drum memory which retains

ten minutes worth of information. After each scan there is a blank

period during which no information is being received, and this time

is used to perform an analysis of the stored data. The output of the

equipment is a set of answers pertaining to the last ten minutes and

this is then updated each scan.
1. 1 Brief Description of Operation

Several portions of the equipment are time -shared, as indicated in the block diagram, Fig. 1. The input to the equipment

consists of two signals. One is a shaft -position signal from the

rotating searchlight, and consists of a pulse at the start of each

revolution. The other is the analog signal from the photocell. This

is an a-c signal at the modulation frequency of the light source. Its - amplitude is determined by the reflecting cloud layer. The shaft

position pulse synchronizes an altitude interval counter which counts

at the power line frequency, and whose count is related to the angle of

the searchlight. The angle is in turn related to cloud height. The

cloud heights have been divided into intervals for convenience. The

range from to 10,000 feet is divided into 23 intervals, the intervals

being small near the ground and becoming larger at higher altitudes.

The photocell signal is passed through an analog -to -digital converter

which samples the signal in synchronism with the modulated light

source. The sampled analog values become pulse trains of varying

length, and are compared in the peak -value detector. This determines

which portion of the signal has the greatest amplitude, and provides an

output signal whenever the input exceeds its previous value. This output

is used to transfer the current altitude number from the counter to a

buffer. Each new number replaces the previous number in the buffer.

Therefore, at the end of each scan the buffer contains the altitude number

corresponding to the maximum cloud signal. This number is recorded

on the magnetic drum memory in one of 100 spaces. The numbers from

successive scans occupy consecutive spaces on the drum. The 101'st

number will replace the original number in space number one, which

was recorded ten minutes earlier. Thus, the data is continually being

updated.

Between scans the data is summarized in the following fashion: The

altitude numbers are generated one by one with each number remaining

for one revolution of the drum. During the revolution the number is

compared with each of the 100 numbers on the drum. Each time this

number occurs on the drum a pulse is sent to the peak -value detector.

Thus, a pulse train is created for each number and these trains are

compared by the peak-value detector to determine which number occurred

most often. At the end of this summation the buffer contains the altitude

which occurred most frequently over the past ten minutes. The other

questions are answered by using counters and additional buffers. If it is

desired to know the lowest altitude at which the maximum cloud signal was

observed 3 or more times, a counter is pre -set to give an output at the

count of 3. This counts the pulse trains and transfers the altitude number

into a buffer when 3 or more counts are received, stopping when it has

transferred in a number. Similarly, the highest altitude with three or

-2-
BASE LINE

TUNED

AMPLIFIER

PREAMP

SAMPLING

CIRCUIT

120 PULSES

PER SECOND

FROM POWER LINE

ALTITUDE INTERVAL GENERATOR

SYNC. PULSE GENERATOR

ALTITUDE NUMBER

COUNTER

PULSE

STRETCHER

ANALOG - DIGITAL CONVERTER

COMPARATOR

PEAR -VALUE

DETECTOR

BUFFER

REGISTER

DISPLAY REGISTER

MINIMUM ALTITUDE

BUFFER

PRE-SET

COUNTER &+ MAXIMUM

ALTITUDE BUFFER

COUNTS UP TO

CRITICAL ALTITUDE

O

ADDRESS AND

-f\ RECORD

CIRCUIT

Q

(00 RPN

MAGNETIC DRUM

Figure 1. Block Diagram of Ceilometer Computer

-3-
more occurrences is obtained by counting the pulse trains and

transferring all the altitudes with 3 or more counts into a buffer.

Since the numbers are generated in ascending order, the number

remaining in the buffer will be the highest altitude which occurred

more than 3 times.

The various outputs from the computer are decoded to decimal

numbers which are set up as switch contacts. These can be used to

actuate remote displays and teletype equipment. Except for the

output switches and magnetic drum, the equipment is entirely

transistorized, and almost exclusively digital in operation. Thus,

a high degree of reliability should be assured. The basic transistor

"building blocks" are a dual flip-flop package, a package with 4 NOR

circuits each having 4 inputs, a diode gating package with 8 gates, a

power driver for operating larger numbers of other circuits, a timer

package for accurate, stable delays, an analog switch package, and

an indicator package for driving incandescent bulbs. About 145 of

these packages are used. The design is flexible enough to permit

modifications and additions.

Several manual modes of operation have been incorporated to

facilitate testing.

A photograph of the computer appears in Fig. 2. The arrangement

of packages on a recessed inner frame can be. seen in Fig. 3. This

frame is hinged to permit access to the wiring and the inside of the

cabinet, as shown in Fig. 4. A rear view of the computer appears in

Fig. 5. Several of the packages used in the computer are shown in

Fig. 6.

2. DETAILED DISCUSSION OF COMPUTER OPERATION

2. 1 Analog -Digital Conversion

The Analog -Digital Converter receives a modulated 120-cycle

signal from a photocell in the ceilometer detector. The amplitude

of this signal is proportional to the amount of light reflected from

clouds passing over the ceilometer. The detector includes an

amplifier which is broadly tuned around 120 cycles, in order to

discriminate against ambient light. The signal received from a

cloud may be very weak, depending on the density, thickness, and

height of the cloud, and the presence of rain, fog, or other obstructing

layers. These will attenuate both the light projected on the cloud and

the reflected light. For these reasons the cloud signals may be so

small as to approach the noise level of the photocell. In order to

amplify these small signals to a useful value, the amplifier has sufficient

-4-
Figure 2. Front View of Computer

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Figure 3. Interior View Showing Packages

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Figure 4. Interior View Showing Wiring

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Figure 5. Rear View of Interior

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gain so that the noise voltage is always noticeable in the output.

Because of the restricted bandwidth of the amplifier, the noise

signals are similar in appearance to the 120 -cycle cloud signals.

However, there is an important distinction in that the 120 -cycle

signals are coherent from one to another, whereas the noise

signals have their phases randomly displaced with respect to one

another. The light source in the ceilometer is modulated by a

full synchronous motor, so that the 120 -cycle light signal 'maintains

a constant phase relationship with the power line. Actually, there

are two searchlights in the projector, with the light beam coming

from first one and then the other. Each of these has its own lamp

and rotating shutter, and it is necessary that the shutters be

properly adjusted with respect to their motors so that the modulation

produced by each shutter has the same phase relationship with the

power line. When the shutters are properly adjusted, the cloud

signals will always have the same phase relationship with the power

line, whereas noise signals will occur randomly. Thus, by sampling

the photocell synchronously with the power line it is possible to

obtain a substantial degree of discrimination against noise. The

sampling circuit is adjusted to sample the photocell signal at those

instants at which the negative peaks of the 120 -cycle signal occur.

Since the noise is random, the peak values of the noise will seldom

occur at these instants; the sampled voltages will generally be less

than the peak values. The sampling process is illustrated in Fig. 7. Actually, if the sampled values were allowed to be both positive and

negative, and were averaged over a sufficient time, the noise would

average zero, whereas real signals would be preserved. However,

with the 120-cycle signal and a 3-second scan time, there are not

enough cycles over each altitude interval to permit averaging of the

sampled values. Nevertheless, there is still a worthwhile amount

of discrimination. First, real signals are always sampled at

their peak values. Second, only the negative peaks are sampled,

and for real signals this will always yield a value. However, the

sampling circuit has zero output for positive voltages. Since the

noise is just as likely to be positive as negative at the sampling

instant, the sampled values for noise will be zero half the time.

Thus, if a real signal is present, even though quite small, it has a

greatly increased chance of being recognized above the noise.

2. 1. 1 Sampling Circuit

A signal of 120 pps (pulses per second) is derived from the

60 -cycle power line, through the use of a full-wave rectifier driving

a flip-flop. See Fig. 7. This circuit consists of a pair of diodes

from the secondary of a 6 -volt filament transformer, the centertap

of which is grounded. The cathode leads of the diodes are joined

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together and fastened by means of a 51 -ohm resistor to the base

of the flip-flop. This point is connected to -12 volts through a

5K resistor, so that this side of the flip-flop normally tries to

conduct. However, the diodes ordinarily keep the base positive,

and cut off, except during the brief interval during each halfcycle when the transformer output voltage passes through zero.

The output of the flip-flop is a pulse about 200 microseconds in

length at a rate of 120 pps. This signal is used to trigger a oneshot (Fd) which serves as a phase adjustment, in order to permit

the sampling process to be centered at the negative peaks of the

120 -cycle photocell signals. This one -shot can be adjusted up to

a few milliseconds. This, in turn, drives a second one-shot (He)

which furnishes a reset pulse. This one-shot drives a third oneshot (Fe) which is adjusted for the duration of the sampling interval.

The sampling one -shot (Fe) is set so- that, the width orf the sample is

as long as possible in order to permit charging of a large storage

capacitor. This capacitor is used in a pulse -stretching circuit

which preserves the amplitude of the sampled pulse long enough

for the operation of the analog -digital converter. It is desired

that the amplitude of the sampled pulse be within 1% of the peak

value of the input signal. Examination of a sine wave, as shown in

Fig. 7, indicates that the value of a 120 -cycle signal is within 1%

of the peak value for about 200 microseconds. This is the setting

used for this one -shot (Fe). This one-shot operates an analog

switch (Fc) which is turned on for the duration of the sampling

interval. The incoming signal is applied to this switch, and the

output of the switch is a sampled pulse 200 microseconds long,

having an amplitude equal to the incoming signal.

The sampled pulse is applied to a pulse -stretching circuit (Fj)

which preserves its value for several milliseconds in order to

allow time for the value to be digitalized.

2. 1. 2 Pulse Stretcher

This circuit consists of an emitter -follower with a capacitor'

from emitter to ground (Fig. 8). Initially the capacitor is

discharged by means of another transistor. When a sampled

pulse is applied to the base of the emitter -follower, the capacitor

is charged to a voltage which closely approaches the pulse amplitude.

When the applied voltage goes to zero the transistor is turned off

and the capacitor remains charged. The voltage on the capacitor is

applied to the base of a second emitter -follower which acts as a

high -impedance load, and which in turn provides the input to the

analog -to -digital converter. The charge on the capacitor leaks off

slowly through the back resistance of the charging transistor and the

-12-
input resistance of the output transistor. However, it remains

within a few per cent of its initial value for 2000 microseconds or

so, which is the longest time that would be required for the analog - to-digital converter to reach a balance. Any error introduced by

decay of the charge does not affect the relative accuracy of the final

results, since the amount of decay would be the same for two signals

of the same amplitude.

2. 1. 3 Analog -Digital Converter

The pulse stretcher output is applied to a comparator (Fh) which

controls the analog -digital converter (Fig. 9). The converter is of

the ramp type, which means that an accurate stairstep voltage is

produced which stops increasing when it reaches the value of the

unknown voltage. This stairstep is produced by means of a 7-bit

binary counter which operates a set of analog switches. These in

turn connect a set of carefully adjusted networks to an accurate

reference voltage. The counter is driven by a blocking oscillator

(Fa), which triggers a one -shot (Gp) at a rate of about 35 kc. As the

counter advances, the various networks are switched in and out, in

such a fashion that the output voltage increases in small, accurate

increments. The converter divides the range from to 10 volts into

128 steps, giving a resolution slightly greater than 1%. This stairstep voltage is applied to the comparator, together with the unknown

voltage, and the stairstep is terminated when the two voltages are

equal. At this point the number in the counter represents the value

of the unknown voltage. The sequence of operation is as follows:

The 120 pps timing pulses which trigger the sampling process

also reset the pulse stretcher and the 7 -bit counter. When the phase

adjusting one -shot (Fd) goes off it triggers a reset one -shot (He)

which resets the pulse stretcher (Fg) and the counter. The trailing

edge of this pulse triggers the sampling one -shot (Fe). This causes

the input voltage to be sampled via an analog -switch (Fc) and the

voltage is applied to the pulse stretcher. The "stretched pulse" is

applied to the comparator (Fh). When the' sampling one -shot goes off

it triggers a control flip-flop (Ff) which opens a gate (OR-inverter Fb)

allowing pulses from a 35-kc blocking oscillator (Fa) to drive the

counter. The counter then advances until its output just exceeds the

unknown voltage, at which time the comparator puts out a pulse which

resets the control flip-flop (Ff) gating off the input to the counter.

Nothing further occurs until the next 120 pps timing pulse. The blocking

oscillator (Fa) drives a one -shot (Gp) which serves two purposes.

First, the pulse from the blocking oscillator is too narrow (about 1 microsecond) to pass through an OR-inverter. The one -shot increases

this width to 20 microseconds. Also, the comparator must be triggered

in order to perform a comparison, and must be held off during the

switching transients which occur as the counter is advanced. The pulse

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from the one -shot inhibits the comparator at the instant the

counter is advanced, and holds it off until the transients have

disappeared and the'output of the converter has stabilized.

The one -shot then unclamps the comparator, allowing a

comparison to be made. It will be noted that the input to the

counter is a series of pulse trains, at a 35-kc rate, the number

of pulses being proportional to the unknown voltages being

digitalized. These pulse trains also constitute the output from

the converter, and are sent to the peak-value detector, during

the input cycle. A separate gate (OR-inverter Fb) is used for

controlling these signals, so that the output may be turned on

and off without affecting the operation of the converter. The

converter thus functions continuously. There is an additional

gating circuit in series with these output pulses which is used

to establish an amplitude threshold for incoming signals. This

circuit is designed so that unless the digitalized value of a

signal exceeds a certain preset count the pulses will not be

gated to the peak -value detector. This circuit operates in the

following way: The output pulse trains from OR-inverter Fb

are inverted (Gm) and sent to an additional OR-inverter (Fb)

which is controlled by a flip-flop (Hn). This flip-flop is reset

so as to close the gate each time that the analog -digital converter

is reset. An OR-inverter (Hi) has inputs from several of the

counting stages of the analog -digital converter. When the preselected count is reached during a pulse train, this OR-inverter

puts out a pulse which triggers the gating flip-flop (Hn), thus

allowing the remainder of the pulse train to be transmitted to

the peak-value detector. A preselected count of 6 is presently

being used, with the inputs to Hi being obtained from the second

and third counting stages (Fj and Fk). One additional output

signal is required from the converter. The peak-value detector

requires a signal before the start of each new pulse train, in

order to permit the number in the adder register to be transferred

into the subtractor register. (This is explained in detail in the

section on the peak-value detector.) This "transfer up" pulse is

obtained from the reset one -shot (He) since this pulse precedes

each pulse train and does not extend into the period where counts

are taking place. This pulse is gated through an OR-inverter

(Fb) which is open only during the input cycle.

2. 2 Peak-Value Detector

This portion of the computer receives a series of pulse trains

of varying length. Its purpose is to put out a signal whenever an

incoming pulse train exceeds the largest previous train. It is time

shared during both the input and analysis cycles. It functions in

-16-
exactly the same way during both cycles, and its output is used

in the same way; namely, to transfer a number (the altitude

number) from a counter to a buffer. During the input cycle it

determines the point at which the maximum cloud signal occurs;

during analysis it detects the most frequent altitude number.

The peak-value detector consists of two registers: an adder

and a subtractor. (Fig. 10.) There is a control flip-flop (Bj)

which allows incoming pulses to go into either the adder (lower

halves of flip-flops ( Ba-Bi ) or the subtractor (upper halves).

Operation of this device is as follows:

Initially both registers are at zero. The control flip-flop

allows the first pulse to go into the subtractor, causing the flipflops to change to all one's. The last "borrow" pulse flips the

control flip-flop, allowing the trailing edge of this first input

pulse to be counted by the adder. Succeeding pulses go only into

the adder, so that at the end of this first pulse train it has a count

equal to the number of pulses in the train. At the end of the train

this number is transferred from the adder into the subtractor, and

the control flip-flop is reset. The same number is now in both the

adder and subtractor. Pulses from the next train start into the

subtractor. If there are fewer pulses in this train than in the

previous one, the subtractor will not go through zero; hence, the

control flip-flop will not change. At the end of this train the

previous number is again transferred into the subtractor. If, now,

a pulse train is received which is longer than the original train,

counts will be subtracted from the subtractor until it goes through

zero; the balance of the pulses will then be added to the number in

the adder. Thus, the number in the adder is brought up to the value

of the new number. At the end of a series of such comparisons the

adder has stored in it a number equal to the longest pulse train

that was received. The significant fact, however, is that the

control flip-flop changed state each time a new pulse train exceeded

the largest previous one. These changes of state constitute the output signals from the detector, and the last one will occur during

the largest pulse train that is received. One special condition should

be noted. When no cloud signal is received during an input scan,

the highest altitude number is recorded, namely, the binary number

"23" (11101). During analysis the number of occurrences of this

number will be counted along with the rest of the altitude numbers.

However, it is not desirable to read out an all-clear indication unless

the all-clear value occurs at least 80% of the time (or some similarly

large preselected percentage). In order to achieve this result a

special gate is attached to the adder register. An OR-inverter (Al)

receives signals from the "16" and "64" stages of the adder register

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and also from the control flip-flop. The output from this gate

is used when reading number "23", and will cause this number

to be transferred into the buffer register if it occurs 80 or more

times.

During the input cycle, pulse trains are supplied to the peakvalue detector from the analog -to -digital converter. During the

analysis cycle the detector receives coincidence pulses from the

comparator circuit. These pulses are OR'd through an ORinverter (Ac) and are fed to the input of either the subtractor register

or the adder, as determined by the control flip-flop (Bj) and four ORinverters (Ad). Prior to each pulse train, the contents of the adder

register are transferred up into the subtractor, and the control flipflop is reset. This is done by means of a power driver (Ab) operated

from OR -inverters Ac and Al. The input for this purpose is from

either .the comparator circuit or the analog -to -digital converter.

These signals are OR'd through an OR inverter (Ac) together with a

normally-open push-button to -12 volts. Thus, the transfer can be

performed manually, for test purposes. The detector is zeroed at

the beginning of the input and analysis cycles by means of a power

driver (Ab). This receives a reset pulse from the input-control

circuit, and may also be energized by a normally-open push-button

to -12 volts. Indicator lights are operated by the adder register,

and indicate the number stored in this register, which is the peak

value. A light on the control flip-flop comes on whenever the

subtractor register goes through zero, indicating an increase in the

peak value.

2. 3 Altitude Number Counter

The altitude number counter generates numbers which represent

the various altitude intervals of interest. There are presently 23

such intervals, as shown in the Table, Fig. 11. A five -bit binary

counter is used, (Fig. 12) and hence there are potentially 32 numbers

available, since (2)5 = 32. However, feedback is used to reset the

counter to zero at the count of 24. This counter is time -shared, and

functions during both the input and analysis cycles. The analysis

cycle is actually terminated when the count of 24 is reached.

During the input cycle the counter is advanced in accordance

with pulses which correspond to various angular positions of the rotating

searchlight, and hence represent various altitudes. At the same

time, the photocell signal from the detector is being converted into

pulse trains by the analog -to -digital converter, and these are being

compared in the peak-value detector. Each time the peak-value detector

detects an increase in the photocell signal, it develops a signal

-19-
Altitude

Number

BinaryForm

Nominal

Height,

Hundreds

of Feet

* Limits,

Hundreds

of Feet

Angle,

Degrees

Number of

Counts,

1248 16 120 pps

1 00 (00)

000

lOOuu

50 6 3/4 027

2 01000 01

150 16 3/4 067

3 11000 02

250 26 1/2 106

4 *t>100 03

350 35 140

5 10100 04

450 42 168

6 01100 05

550 47 3/4 191

7 11100 06

650 52 1/2 210

8 00010 07

750 56 1/4 225

9 10010 08

850 59 1/2 238

10 01010 09

950 62 1/4 249

11 11010 10

1100 64 1/2 258

12 00110 12

1350 69 3/4 279

13 10110 15

1750 74 296

14 OHIO 20

2250 77 1/2 310

15 11110 25

2750 79 3/4 319

16 00001 30

3500 81 3/4 327

17 10001 40

4500 83 3/4 335

18 01001 50

5500 84 3/4 339

19 11001 60

6500 85 1/2 342

20 00101 70

7500 86 1/4 345

21 10101 80

8500 86 3/4 347

22 01101 90

9500 87 348

23 11101 99

Figure 11. Table of Altitude Intervals

-20-
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-21-
(its control flip-flop changes state) which causes the current

altitude number to be transferred into a buffer register. Thus,

at the end of the input cycle the buffer contains the altitude

number for which the greatest cloud signal was received. This

number is then recorded on the magnetic drum in the next

consecutive address. In the event that no cloud signal is received

the number "23" corresponding to "all clear" is recorded.

During the analysis cycle the counter is advanced by pulses

from the comparator circuit, which occur at a rate of once per

revolution of the magnetic drum. Thus, all of the altitude

numbers are generated, one by one, with each remaining for one

revolution of the drum. A comparator circuit compares each

altitude number with all of the 100 numbers stored on the drum

and generates trains of coincidence pulses. These pulse trains

are fed to the peak-value detector which determines which

altitude number occurs most frequently. The output of the

detector accomplishes the same function as during the input cycle;

namely, whenever a new number occurs more frequently than a

previous number, this number is transferred into the buffer register.

Hence, at the end of the analysis cycle the buffer contains the altitude

number which occurs most frequently on the drum. The altitude

numbers are generated consecutively, in ascending order. If two

altitudes occur the same number of times, the lower one will remain

in the buffer, since an increase is required to produce a transfer

signal. The "all clear" signal must occur at least 80% of the time

in order to be transferred into the buffer. At the end of the analysis

cycle the contents of the buffer are transferred to a storage register,

from which they are decoded to the output display.

The above description applies to the mode, or most predominant

cloud height. Two other altitude numbers are transferred out during

the analysis cycle. These are the minimum and maximum altitudes

at which significant activity was observed. By "significant activity"

is meant a preset number of observations, such as 3 or more over

the observation period. These are described more fully under the maxmin circuitry. There is also a counter which counts the number of

observations up to the critical height. A gate opens at the beginning of

the analysis cycle and allows coincidence pulses to actuate this counter

until the critical altitude number occurs, at which time the gate closes.

A suitable gating structure is wired to the altitude number counter

for this purpose, as described under the critical height counter.

For purposes of the comparator, the altitude number is serialized

by means of commutating pulses on five lines from the sync, circuit.

These pulses correspond to the bit positions on the magnetic drum,

-22-
and , ach set of five correspond to one word on the drum. Thus,

the first bit of the counter is sampled during the first bit of a

word on the drum; then the second, third, and so on. The resulting

outputs appear on a pair of lines marked N, N. The N line will have

a positive spike if the flip-flop being sampled is in the "one" state.

The N line will have a positive spike if it has a "zero".

The output of the buffer goes either to the record circuit (end

of input cycle) or the output storage register (end of analysis).

For the record circuit it is again desirable to serialize the number,

making use of the commutating lines from the sync, circuit. However,

since a NRZ (non-return-to-zero) recording is used, it is only

necessary to sent "one's" to the record circuit. Since the commutating

lines are always active, it is necessary to gate off the serialized

buffer contents, except for the single word during which it is recorded.

The differentiated spikes developed by the commutating circuit (R-Cdiode gates are used for this purpose) cannot be passed through an ORinverter. Hence, they are used to trigger a flip-flop (CI) which is

reset near the end of each bit by a one -shot in the comparator circuit

which is triggered for each bit. Thus, the flip-flop is always in the

zero state at the beginning of a bit; if the corresponding bit in the

buffer is a "one", the flip-flop (CI) will be set to the "one" state, then

reset to zero. If the bit in the buffer is a "zero", the flip-flop will

remain in the zero state. Thus, a square pulse is obtained from the

flip-flop for each "one" in the buffer, and this pulse can be passed

through an OR -inverter, as required.

Counting pulses to the altitude number counter are OR'd in an ORinverter (Ca) from the two different sources. The searchlight position

pulses are passed through an extra OR -inverter (Ca) which permits

them to be gated off during analysis and during manual operation. The

count of 24 is recognized by combining the outputs of the 4th and 5th bits in

an OR-inverter (Ca) together with the input pulses to the counter. The

output from this OR-inverter is a negative pulse at the count of 24 which

is equal in width to the input pulse. The trailing edge of this negative

"24" pulse is used to reset the altitude number counter at the count of 24,

during the analysis cycle. This is also the signal for the analysis to

terminate. Note that this "24" pulse is OR'd with a reset pulse from the

control circuit (OR-inverter Ca), requiring an additional OR-inverter

(Cn) to regain a negative pulse. During the input cycle the counter does

not reach 24, since there are only 23 altitude interval pulses. The

control circuit furnishes a reset pulse at the beginning of the analysis

cycle, and also at the beginning of the input cycle.

-23-
Transfer of the altitude number into the buffer is accomplished

by two OR -inverters (Ck) driving gating packages, with the gates

divided between them. Note that all four inputs to these ORinverters (Ck) are used. The rules for transferring numbers into

the buffer are complicated by the special way in which "all clear"

signals (number 23) must be handled.

During the input cycle it is desired to transfer "23" into the

buffer if there has been no cloud signal above the amplitude

threshold established in the analog -digital converter. Accordingly,

a control flip-flop (CI) is reset at the end of each analysis cycle and

this flip-flop controls a gate (Cn) which is strobed by an "end of input"

pulse. The control flip-flop (CI) has a "set" input from the peakvalue detector, so that if a signal is received by the detector during

the input cycle its output will trigger the control flip-flop and close

the gate (Cn). Thus, if no signal is received during an input cycle,

the "end of input" pulse will pass through the gate (Cn) and transfer

the number "23" into the buffer. If, however, a cloud signal occurs,

the flip-flop is triggered to its opposite state, and the "end of input"

pulse cannot pass through. The buffer would not receive number

"23", but would have the altitude at which the cloud was received.

During analysis it is desired to transfer down the "all clear"

number only if it occurs 80 or more times. In the section on the

peak -value detector it was explained that there is a special output

signal corresponding to a count of "80 or more". This signal

goes into the transfer gates (Ck) at all times. However, the normal

signal, which is the "greater than" pulse, is gated off (via ORinverter Al) during the analysis of number "23". The count of 23 is

recognized by an OR -inverter (Cn) with inputs from the appropriate

counter stages. Thus, it is not sufficient for the number of

occurrences of "23" to merely exceed the most frequent cloud

height. It must actually occur 80 or more times in order to be

transferred into the buffer.

For manual operation, altitude numbers can be transferred into

the buffer by means of a normally-open pushbutton to -12 volts at

the input to the OR -inverters (Ck).

Note : This push-button is OR'd with the output from the control flipflop (Bj) of the peak-value detector. If the control light is "on", the

input to the OR -inverters will be held negative and the push-button

will not operate. The peak -value detector must first be zeroed.

Indicator lights are operated by both the altitude number counter and

the buffer.

-24-
2. 4 Synchronizing Pulse Generator

Communication with the magnetic drum requires a knowledge

of where the information is placed on the drum. NRZ (non -returnto -zero) recording is used; hence, there is no indication as to

where one bit ends and the next bit begins if the second bit is a

zero. Nor is it ordinarily possible to tell where a word starts and

stops'. In order to establish word and bit positions on the drum,

therefore, a sync, channel is added, which is entirely independent

of the information channel. It feeds into a read amplifier only;

there is no provision for recording on the sync, channel.

Note: Care should be used not to disturb the sync, channel. Never

measure the sync, read head with an ohmmeter, as the meter

current could result in an erasure. Do not bring magnetized objects

near the drum. Use care not to short-circuit any instrument probe

to the read circuit input connections which might apply power to the

read head. Recording of the sync, channel takes only a moment with

the proper equipment, but special circuits must be constructed for

this purpose and they will not ordinarily be readily available.

The sync, channel has 100 six-bit words recorded on it, with

one's in every bit position. The one's are uniformly spaced with

a gap between words which is equivalent to two one's. This pattern

is shown in Fig. 13. At a drum speed of 600 RPM, or 10 revolutions

per second, the bit spacing is about 120 microseconds. The 100

words require about 96, 000 microseconds, leaving a gap of about

4, 000 microseconds (since each revolution takes l/lO second or

100, 000 microseconds) where no information is recorded.

The sync, circuit produces three outputs - revolution pulses,

word pulses, and bit pulses.' This circuit is shown in Fig. 14, The

revolution pulse is produced by means of a special circuit using

a one -shot (Ek) operating in conjunction with a pair of OR-inverters

(Dj, Dl), which acts as a gap sensor. The one -shot is set for about

2 milliseconds and is constantly restarted by the bit pulses. It does

not run through its normal time period until an interval of about 2

milliseconds elapses without any bit pulses. Then it completes it

normal period and generates a step corresponding to one revolution

of the drum. This step exists until the bit pulses occur again. This

one -shot triggers a second one -shot (Ep) which gives a revolution

pulse of about 200 microseconds, the width being not at all critical.

As an indication that this circuit is running, the revolution pulses

cause a flip-flop (Ea) to count at a rate of 10 pulses per second.

This , in turn, operates the Revolution Pulse Light which blinks on

and off, producing five flashes per second.

-25-
The remainder of the sync, circuit is a six-bit ring counter,

in which each stage represents one bit "position in the words. The

altitude numbers which are recorded on the drum are five-bit

words. However, space has been allotted at the end of each word

for a sixth bit; this bit serves as the end-of-word signal, and also

allows a parity bit to be added to the numbers being recorded.

This is discussed in the section on the record circuit. Every

revolution the ring counter is reset to its zero condition, which

consists of the first five bits being zeroes. At the first bit after

the revolution pulse the first stage becomes- a "one", and the sixth

stage goes to zero. At the second bit, the "one" advances to the

second stage, and so on. The sixth bit signifies the end of a word,

and when the sixth stage comes on it triggers the word one -shot

(Eb). This one -shot is in the record circuit and its timing is

critical. It is discussed further in that section. The five stages

corresponding to information bits actuate five commutating lines

which are used to serialize numbers from the altitude number

counter and the buffer register. The bit pulses perform two

functions. They cause the record circuit to record one's in the

proper locations, and they indicate the bit locations for purposes

of the read circuit.

2. 5 Address Counter

The Address Counter keeps track of the 100 word locations

on the drum and causes each successive number to be recorded in

the next consecutive position. In normal operation the address

circuit is triggered at the end of the input cycle, signifying that a

number is ready to be recorded. The address counter is then

activated and produces a pulse when the appropriate drum location

occurs. This pulse activates the record circuit, causing the number

to be recorded. The actual circuitry is made more complex by the

addition of several control functions which are needed to obtain the

correct timing, and to provide several manual modes of operation

for test purposes.

Basically, the address circuit consists of a two -decade counter

and a gate circuit which can be opened for one revolution of the

drum (Fig. 15). When this gate is triggered it admits a train of

100 word pulses into the counter, plus one additional pulse at the

start. Thus, the counter cycles through its full capacity, passing

through zero in the process, and returning to its original number,

plus one. When the counter reaches zero the trigger pulse for the

record circuit is produced. As successive numbers are recorded,

the counter keeps advancing by one each time, with the 100 word

positions on the drum being used consecutively. Normally, this

-26-
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-29-
would represent ten minutes of data, with each, number replacing

one that was ten minutes old.

The one -revolution gate operates in the following manner:

The gate is triggered by means of a "set" pulse applied to flipflop Eh, causing pin 1 of OR -inverter Ei to go positive. This

permits the next revolution pulse to reach the "T" input of a

second flip-flop (Eh). Triggering of this flip-flop causes pin 13

of Ei to go positive, permitting pulses from the word one -shot

(Eb 4) to enter the counter. This gate remains open for one

revolution, at which time the next revolution pulse reaches the "T"

input of the second flip-flop. When this flip-flop reverts to its

original state it gates off the word pulses and resets the first flipflop, gating off the revolution pulses. Polarities in this circuit

have been carefully selected in order to achieve the proper timing

sequences for the analysis cycle and manual operation. This will

be referred to in. the appropriate sections. Note that the word

pulse applied to pin 15 of Ei is negative -going, rather than positive,

as would normally be the case for an AND circuit. This means that

an output occurs as soon as the gate is opened by means of a positive

signal on pin 13. It is this signal which furnishes one extra count

to the 100-counter, along with the 100 word pulses. Also, the

counter will count the trailing edge of the word one -shot pulse.

The significance of this timing will be discussed under recording.

In normal operation, the one -revolution gate is triggered at the

end of the input cycle, causing one altitude number to be recorded

on the drum. Closing of the one -revolution gate indicates that the

recording process has been completed, and is the signal for the

analysis cycle to begin.

One- Minute Mode

In order to obtain a one -minute mode of operation it is necessary

to carry out the computer operation on the basis of one -minute of

stored information, instead of ten minute's worth. This is

accomplished in a very simple way, merely by recording each new

altitude number in ten locations on the drum, rather than in a single

location. Thus, the 100 locations on the drum will contain only 10

different words, corresponding to one minute of data. This is

accomplished by obtaining the address signal from the first decade

of the address counter, instead of the second decade. When the train

of 101 pulses are supplied to the counter to initiate a recording, the

first decade goes through zero ten times, furnishing ten record

pulses. Thus, the first word will go in locations 1, 11, 21, .... 91;

the second will go in locations 2, 12, 22 .... 92; etc.

-30-
2. 6 Record Circuit

In normal operation the record circuit is triggered by a

pulse from the address counter as the counter goes through zero.

When triggered, the circuit functions for the duration of one word,

and is then reset. Before analyzing the logic circuitry it is

desirable to discuss the actual recording circuitry.

2. 6. 1 Record Package

NRZ (non-return-to-zero) recording is used on the magnetic

drum, since it permits the circuitry to operate at one -half the

speed that would be required for recording complete pulses. With

NRZ recording, erasing is not necessary, no bias signal is needed,

and single words can be changed without disturbing adjacent words.

NRZ recording is carried out in the following way: During the

time that a word is being recorded a current is maintained through

the recording head which develops a field strong enough to produce

saturation of the magnetic coating on the drum. Whenever a "one"

occurs, the direction of the current is reversed, resulting in a

crossover in the magnetization on the drum, to saturation in the

opposite direction. Wherever there are "zeros", the direction of

magnetization remains the same. Since the head is always producing

a field strong enough to saturate the coating, in one direction or the

other, any previous information is completely eliminated, and

replaced by the new information. At the end of the recording the

current through the head is turned off, so that it has no further effect.

In playback, each crossover in the direction of magnetization

on the drum results in a rounded pulse from the read head. Since

the crossovers must alternate, t the pulses from the read head

alternate in sign. Each pulse corresponds to a "one"; a bit location

having no pulse is thus a "zero". A string of zeroes would furnish

no indication as to where a bit starts and stops; therefore, it is

necessary that the sync channel supply this information. The sync

channel has a "one" in every bit position.

The circuitry in the record package is very simple, due to this

method of recording (Fig. 16). The same head is used for both

recording and reading, and actually has two identical windings

which can be connected either in series or parallel. These windings

are connected in series -aiding, and the unction is fed from a

transistor which acts as a constant-current source. The free ends

of the windings connect to two switching transistors which in turn

are connected to opposite sides of a flip-flop. Thus, only one

winding at a time can carry current. The direction of the magnetic

-31-
25 Q +Vi

Record

Turn-on

Current

Information to be recorded.

Complementary signals.

All transistors 2N414, except as noted.

Figure 16. Record Package

-32-
!

field produced by one winding is opposite to that produced by

the other. The constant-current transistor is operated by a

switching transistor which causes it to be turned on for the

duration of the word that isjDeing recorded. Operation of this

transistor as a current source is based on the fact that the

variational resistance of the collector is very high when the

base is driven by a voltage -source and a suitable emitter

resistor. is used. An NPN transistor is used in this particular

circuit. This transistor will be off when its base is at -Vcc

volts, which occurs when its accompanying switching transistor

is turned off. Turning on the switching transistor results in a

flow of current through the potentiometer which furnishes the base

drive, causing the base of the NPN transistor to move positively

toward ground. Since this potentiometer is only a few hundred

ohms it is a reasonable voltage source for the constant -current

transistor. The potentiometer is adjusted to establish a collector

current of 30 to 40 ma. which is sufficient to saturate the magnetic

coating of the drum.

The sequence during recording is as follows: The constant - current transistor is turned on via its switching transistor. Then, as

the various bit positions reach the head the recording current is

switched from one winding to the other whenever a "one" is to be

recorded. It can be seen that there may be an odd or even number of

reversals depending upon whether there is an odd or even number of

one's in the 5*-bit word being recorded. However, it is highly desirable

to have the recording circuit start and stop in the same state. This is

necessary in order to use the head for reading and also to prevent

transients from being recorded due to switching the recording current

on and off between words. The desired situation is achieved by the

incorporation of a sixth bit which is used to provide even parity. If

a word has an odd number of bits, the parity bit is made a "one";

otherwise, it remains a zero. Thus, in addition to the very desirable

features mentioned above, the possibility of a parity check is gained,

and this has been included in the read circuits. If a word being read

from the drum contains an odd number of one's, an error is indicated.

It will be noted that there is a diode in the lead connecting one of the

switching transistors with its winding. Also, there is a pull-up

resistor attached from the base of this transistor to +12 V. This

transistor is normally off whe;n the circuit is not being used for

recording, and the opposite transistor is normally on. Thus, one

end of the series -connected head windings is grounded, and the other

end sees a high impedance. This end is connected to the read package

and turns out to have twice the voltage that would be developed across

a single winding. In the present equipment these signals are about

0. 2 volt peak, which is well above noise, and only moderate gain is

-33-
required in the read circuit. The diode connected between this

point and its associated collector prevents shorting of positive

pulses by the "off" transistor. This is necessary for the

following reason. The collector circuit is normally energized

from a negative supply (for a PNP transistor) and is back-biased

for this polarity. However, it becomes a forward-biased diode for

positive signals and will conduct even when turned off. The diode

is therefore connected to prevent current flow for positive signals.

The pull-up resistor at the base of this transistor helps to assure

that it gets turned completely off, so that the transistor will not

load the read signal.

Switching speeds in the record circuit are leisurely, as

permitted by NRZ recording, together with ample bit spacing on

the drum. By using a head with a large inductance, a large read

signal is obtained, but switching speed is reduced. Two times are

of interest here: the time to get the recording current turned on,

and the time to switch from one winding to the other. Switching

requires reversal of the flux in the head, which presents a highly

inductive load. The recording current is turned on and off between

words; the pattern on the sync channel provides a space of two bits

for this purpose. The word one -shot (Eb) is adjusted so that its

trailing edge occurs in the center of this space. Since the address

counter counts on the trailing edge of this pulse, the signal to

initiate a recording will occur at this time. The recording

current remains on for one word and is turned off at the center

of the next interval between words. The time required for the

recording current to go from zero to 30 ma. is about 60 microseconds. The turn-off time is about 80 microseconds. When the

recording current is turned off, a large positive voltage appears

at the collector of the constant current transistor, due to the

inductive load. To prevent damage, this point is clamped to +12

volts through a diode. This slows the turn-off time somewhat, but

it is still well within the time allotted. In order to switch the

current from one winding to the other, the flux in the recording

head must be reduced to zero and built up to a maximum in the

opposite direction. This requires about 110 microseconds, which

is within the 120 -microsecond bit spacing on the drum.

2. 6. 2 Operation of Record Circuit

The signal to begin a recording is derived from the address

counter when it cycles through zero. This is indicated by the

output of the last flip-flop of the counter going positive (pin 21 of

Em). This signal triggers a flip-flop (Ee) which opens several

gates for a single word (Fig. 15). One of these turns on the

-34-
recording current. It will be recalled that the address counter

counts trailing edges of the word one -shot (Eb) and these are

adjusted to fall in the spaces between words. In order to allow

time for the build-up and decay of the recording current the

timing of this one -shot (Eb) is quite important. It is triggered

by the sixth bit from the sync channel and should be set for 180

microseconds, +^20 microseconds. The flip-flop (Ee) will remain

in the "set" state until the trailing edge of the next word pulse.

These pulses are constantly fed into the flip-flop via its internal gate

(pin 1). However, a "reset" pulse is developed only when the flipflop has been in the "set" state for more than a few microseconds.

The flip-flop (Ee) thus remains on for a duration of one word, starting

and ending in the intervals between words. In addition to turning on the

recording current this flip-flop opens a gate which allows pulses

corresponding to the one's being recorded to enter the counting input

of the 'switching flip-flop (Ee). This flip-flop causes the recording

current to flow alternately in the two separate windings of the recording

head, as explained in the previous section. A third gate goes to the

read circuit. During manual operation this provides a signal for

reading single words. This will be explained in the section on the

read circuitry. Another flip-flop (Ea) is used to establish whether

the single -word circuitry is being used for reading or recording.

It is automatically triggered into the record state whenever a word is

to be recorded during automatic operation. It remains in the record

state until the record function is ended, as indicated by the closing of

the one -revolution gate. During manual operation this flip-flop is

triggered by the "Write" button. When this flip-flop is in the record

state it allows the single -word flip-flop (Ee) to turn on the recording

current and allow one's into the switching flip-flop. When it is in the

read state it gates off thes e signals and instead allows the single

word signal to be sent to the read.circuit. Note that the switching

flip-flop (Ee) is always reset at the end of a word by the leading edge

of the word pulse. This flip-flop "counts" back and forth in accordance

with the one's being recorded, which occupy the first five bit positions

of a word. The word pulse occurs at the sixth or parity bit position.

If an even number of one's has been recorded, the flip-flop will end up

in the reset state and the parity bit will be a zero. If there were an

odd number of one's, the word pulse will reset the flip-flop, making

the sixth bit a one. Triggering of the record circuit is done by means

of an "end of scan" pulse which is applied through gates (Ed) to the readrecord flip-flop (Ea) and to the input flip-flop (Eh) of the one -revolution

gate.

-35-
2. 7 Read Circuitry and Comparator

During the analysis cycle the information stored on the

magnetic drum is scanned in order to obtain answers to the

various questions of interest. These are restated below, in

terms of the equipment operation:

1) Which altitude number occurs most frequently on

the drum?

2) What is the lowest altitude number that occurs 5

or more times?

3) What is the highest altitude number that occurs 5

or more times?

4) How many altitude numbers occur up to the critical

altitude?

Answers to these questions are developed in the following

way: The altitude number counter is advanced to the first altitude

number and this is compared with each of the 100 words stored on

the drum•* A coincidence pulse is generated whenever the word

on the drum is the same as the number. The output of the comparator

will thus be a train of coincidence pulses, the number of pulses

equaling the number of times that the particular altitude number

occurs on the drum. One revolution (0. 1 second) is required for this

comparison, after which the altitude counter is advanced to the next

number. This is again compared with the contents of the drum and

another train of coincidence pulses is developed. The process is

repeated until all 23 altitude numbers have been generated, after

which the analysis cycle is terminated. The comparator circuit

has thus furnished 23 trains of coincidence pulses, from which the

answers to the various questions are to be obtained.

In order to determine which altitude number occurred most

frequently, the trains of coincidence pulses are sent to the peakvalue detector. This device furnishes an output whenever an

incoming train contains more pulses than the longest previous

train. These output pulses cause the altitude number associated

with the incoming train to be shifted into a buffer register. Thus,

at the end of the analysis cycle the buffer will contain the altitude

number which occurred most frequently on the drum.. This number

is then shifted into the display register from which it is decoded

to the numeric readout, as the predominant cloud height.

-36-
The trains of coincidence pulses also go to the max-min

circuit and the critical height counter. The max-min circuit

contains a preset counter which gives an output whenever it

receives a pulse train containing more than the preset number

of pulses. These output signals transfer the altitude number

associated with the incoming pulse train into two buffers: the max

buffer and the min buffer. However, after the first output signal

the input to the min buffer is gated off, so that only the lowest

altitude with five or more counts appears in this buffer. Altitude

numbers continue to be transferred into the max buffer as long

as their pulse trains exceed the preset count. Thus, when the

analysis is completed, the max buffer contains the highest

altitude which occurred five or more times. The critical height

counter starts counting coincidence pulses at the beginning of

the analysis and continues until a preset altitude number,

corresponding to the critical height, is reached. Its input is then

cut off. Therefore, its final count is equal to the number of cloud

observations up to the critical height. Details of these circuits

will be discussed in their respective sections.

2.7. 1 Comparator Circuit

The comparator circuit makes a bit -by -bit comparison of the

altitude numbers and the words on the magnetic drum. Since the

words come serially from the drum it is logical to make the

comparison in this fashion, and much less equipment is required

than for a five -bit parallel comparator. It is required that the

altitude numbers be serialized, also, and this is accomplished

quite simply, by using the five commutating pulses from the sync

channel. The altitude numbers are sampled, one bit at a time, by

these commutating pulses and a flip-flop (Dc) is caused to set up

according to whether the bit is a one or a zero (Fig. 17). A second

flip-flop (Dc) is set in accordance with the bits from the drum. The

outputs from these flip-flops are connected to a gating structure

consisting of three OR -inverters (Dd) which are connected to give an

output if the flip-flops have opposite states. This function is

represented by the Boolean expression (AB or AB). (This would be

the sum digit for a half-adder, also called the Exclusive -OR function.)

If the two flip-flops do not agree, the output of the comparator triggers

a flip-flop (Df) which remains set until the end of the word. At the

end of the word (start of the sixth bit) a sampling pulse is sent to a

gate (Df) controlled by this flip-flop. If the flip-flop has been triggered,

the gate will be closed, and the sampling pulse will not get through.

If, however, all five bits of the altitude number and the word from

the drum agreed, then the flip-flop would remain in its untriggered

state and the sampling pulse would pass through the gate to become

-37-
a coincidence pulse. The sampling pulse for this circuit is the

word one-shot pulse; the trailing edge of this pulse resets the

flip-flop between words, readying the circuit for the next

comparison. The comparator circuit functions continuously,

even during recording (during which errors might arise due to

transients) but the output gate (Dj) is inhibited except while reading.

A note of explanation is necessary with regard to the timing

of information read from the drum. The read circuit reads only

one's from the drum, due to the use of NRZ recording. Hence,

if a one does not appear in a given bit position, the bit is assumed

to be a zero. However, if a one does appear in a bit position it

will occur about 40 microseconds after the start of the bit, with

respect to the sync channel. This is due to the appreciable time

required for the record circuit to write a one on the drum. For

this reason, time must be allowed for the word flip-flop to set up,

before it is compared with the altitude -number flip-flop. This is

achieved by means of a strobe pulse which activates the comparison.

A one -shot (Da) is triggered at the start of each bit, and is set for

80 'microseconds, in order to allow plenty of time for a one to occur,

if it is present. This one -shot triggers the strobe one-shot (Db)

which furnishes a 20 -microsecond strobe pulse to the two AND-gates

of the comparison circuit (Dd). Thus, the comparison circuit is

strobed near the end of each bit to test whether a coincidence has

occurred. The word flip-flop (Dc) is reset to its zero state at the

beginning of each bit, and will remain in this state unless a one

occurs. Resetting at this time cannot result in a conflict between

the reset pulse and a one from the drum, since the one's are always

at least 20 microseconds late.

Mention should be made of the effect of the parity bit on the

comparison circuit. The altitude number flip-flop (Dc) takes on

states corresponding to the five bits of the altitude number, while

the word flip-flop (Dc) takes on the state of the parity bit, in

addition to the five information bits. The comparison circuit operates

for all six bits, which means that a lack of coincidence will be indicated

whenever the parity bit is not the same as the fifth bit of the altitude

number. This will not prevent a coincidence pulse from being generated,

however, since the output is sampled before the occurrence of the strobe

pulse for the parity bit.

2. 7. 2 Parity Check Circuit

The five -bit altitude numbers may have an odd or an even number

of one's. However, as pointed out in the section on recording, it is

desirable that all words recorded on the drum have an even number of

-38-
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-39-
one's. (A word containing all zeroes would be acceptable; in

fact, the drum can be erased by recording all zeroes.) For

this purpose, space has been allowed on the drum for a sixth

bit, which is the parity bit. This is made a one or a zero as

needed in order to make an even number of one's in each word.

This permits an even parity check to be applied during the reading

process as a check against errors in recording and reading. The

check consists in counting the number of one's in each word, and

noting whether there was an odd or an even number. This is done

by sending the one's from the drum into the counting input of a flipflop (Dh). A Parity Reset pushbutton initially sets this flip-flop to

the correct state. This flip-flop controls a gate which is sampled

at the end of each word. The gate is normally kept closed by the

counting flip-flop; however, if an odd number of one's has occurred

during a word, the flip-flop will end up in the opposite state, and

the sampling pulse will pass through, triggering the parity flip-flop

(Dh). The parity flip-flop operates a relay which can be used to

furnish an error signal. In the present equipment the relay causes

a red background to appear behind the numeric readout. In many

cases a parity error might have no effect on the actual output, but

it serves as a warning that the answer may be questionable, and

if it occurred repeatedly it would indicate a failure. An indicator

light is also operated by the parity flip-flop, and is normally on. This provides a fail-safe indication; if the light were connected so

as to be normally off, a failure in the lamp circuit would mask a

parity indication. Similarly, the parity relay is normally held

closed, so that a failure of the relay circuit causes the parity

indication to appear. A parity error may be due to a variety of

causes, including power line transients, etc. , and need not mean

that all succeeding results are in question. Hence, the parity

circuit is reset at the beginning of each analysis cycle. This reset

pulse is applied to both the parity flip-flop (Dh, pin 13) and the

counting flip-flop (Dh, pin 5, via a gate on package Dm).

It will be noticed that there is a one -shot (Cm) which "squares

up" the one's coming from the read circuit. Due to a peculiarity

of the recording scheme it is possible to occasionally produce a

double -peaked "one", instead of a single, rounded pulse.

Nevertheless it is a perfectly valid, well-defined "one". The read

package will, in turn, produce a double pulse. This would not

affect the comparison circuitry (or the shift register described below)

but would cause the counting flip-flop to count twice on a single

"one". Thus, a parity error would be indicated. This is prevented

by having the read circuit trigger a one -shot (Cm) which is set for

40 microseconds. This is long enough to discriminate against

multiple pulses of the type described. However, the "T" input cannot

-40-
be used; hence, the revolution pulse is sent to the reset side

(Cm, pin 11) in case it should ever hang up.

2.7. 3 Read Package

The Read Package receives digital signals from the magnetic

drum and produces negative output pulses of standard amplitude.

The signal developed across the Brush BK1110 head used on the

magnetic drum is about 0. 3 volt, peak. Thus, it is not necessary

to provide significant voltage gain, but merely to provide a reasonably high input impedance. When used with NRZ recording,

successive input pulses alternate in polarity, which requires that

the signals be full-wave rectified.

A block diagram of the package appears in Fig. 18, together

with a schematic diagram and typical waveforms. The emitter - follower stage presents a high input impedance and provides low

output impedance for driving the phase -inverting transformer.

This transformer has a center-tapped secondary which drives the

bases of a pair of transistors having a common collector load.

Both transistors are normally cut off, since their bases and

emitters are at ground potential. Since they are driven in opposite

phase, when one conducts, the other is cut off even further. Thus,

only one transistor at a time draws current through the load

resistor. The transistors conduct on negative signals; hence, the

signals at the load are positive pulses for either polarity of input

pulse. The emitters of these two transistors go to opposite ends of

a balance potentiometer which has the sliding contact grounded.

This potentiometer is adjusted to obtain equal -amplitude pulses at

the load resistor.

The full -wave rectified pulses are differentiated to provide a

signal which has a sharply-defined positive -to -negative zerocrossing. Since the input pulses to the differentiating network are

positive, the first portion of the differentiated signal is positive.

Thus, the following amplifier stage, which is normally cut off,

remains off. When the peak of the pulse occurs, the derivative falls

sharply to zero, and as the pulse decreases, the derivative goes

sharply negative, turning on the following stage. This results in a* pulse whose leading edge is closely timed to the peak of the input

pulse. This signal is d-c coupled to an NPN transistor, which also

is normally off and which starts to conduct when its base goes positive.

Together, these two amplifier stages furnish sufficient gain to cause

saturation at the collector of the NPN output transistor, resulting in

12 -volt negative pulses whose leading edges coincide closely with the

peaks (both positive and negative) of the input signals to the Read

-41-
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T. P. - Test Point

All Transistors Type 2N414

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Push-pull

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Rectifier

Differentiator

Direct -coupled

Complementary

Amplifier -- Output

Figure 18. Read Package

-42-
Package. 3y passing the output pulses through an OR-inverter

stage, the rise time can be speeded up to less than one microsecond, resulting in positive pulses which will readily operate

flip-flops and other standard packages in this series.

2.7.4 Storage Re gister

At, the end of the analysis cycle the altitude number occurring

most frequently is stored in a buffer register. Since this buffer

is also us ed during the input cycle the answer must be transferred

into a storage register (Fig. 17), from which it can be decoded into

the numeric readout. Since this register drives the output display

it is also convenient to use it during manual operation. Accordingly,

it has been made into a shift register, and single words can be shifted

into it, from the drum. It is six bits long, so that it can include the

parity bit. The Address and Record Circuit has a flip-flop (Ee)

which is triggered by the address counter and which stays on for a

duration of one word. When the equipment is operated manually on

"Single Word", and the "Read" button is pushed, the signal from this

Hip-flop is gated via an OR-inverter (Ec) to a pair of gates which

furnish signals to the shift register. One of these (Bl) feeds one's

from the read circuit into the first stage of the register (Bm). The

other gate (Bl) provides shift pulses.' Simultaneously, it shifts

zeroes into the first stage of the register. Thus, the first shift

pulse move8 the word one space to the right (to the left on the front

panel) and sets the first stage to zero. If the first bit of the word

being read is a one, the one will arrive a short time later, setting

the stage to a one. Then the next shift pulse occurs, moving the

word one more place to the right, and setting the first stage to zero.

The first stage either remains at zero or is set to a one in accordance

with the second bit being read. In all, there will be six shift pulses

to a word, after which the new word is in the register, together with

its parity bit. It then appears decoded in the numeric display.

2-7. 5 Max-Min Circuit

The max-min circuit detects the highest and lowest altitudes

at which the number of cloud observations are equal to or greater

than a preset value.

The max-min circuit consists of two 5-bit storage registers

with provisions for decoding to numeric displays, and a counter

which can be preset for numbers from 1 to 16 (Fig. 19). The

counter contains four flip-flops, and either side of each flip-flop

can be connected to a four -input OR -inverter (Gm). The counter

is driven by trains of coincidence pulses from the comparator circuit.

-43-
Whenever the number of pulses in a train equals the value for

which the counter is preset, the OR -inverter provides an output,

gating off the counter input (via flip-flop Gn and OR ^inverter Gm).

This output signal drives additional OR-inverters (Gl) which

cause the concurrent altitude number to be shifted into the registers.

The gates feeding the lower register are driven by a flip-flop (Gn)

which is triggered by the first output signal, thereby transferring

only one altitude number into this register, which displays the

minimum altitude. After each pulse train the counter and its input

flip-flop (Gn) are reset by a pulse from the analysis control circuit.

The counter thus continues to furnish output pulses each time a

pulse train exceeds the preset value, and altitude numbers continue

to be transferred into the other register, which displays the maximum

altitude

.

2. 7. 6 Circuit for Counts Up to Critical Altitude

This circuit contains a counter which counts the number of

cloud observations that occur up to a preset height. This would

ordinarily be preset to the critical altitude for a given airport. Since

the drum contains 100 altitude numbers, and all of these could fall

below the critical altitude, the counter must be capable of registering

100 counts. This is accomplished by cascading two decade counters,

the first being a units counter and the second being a tens counter

(Fig. 20). They are wired to count in 1-2-2-4 fashion.

The input to the counter is the series of coincidence pulse trains

from the comparator circuit. These pulses are passed through an

OR -inverter (Hf) to the input of the counter. This OR -inverter is

controlled by a flip-flop (He) which can be used to gate off the pulses

at the appropriate time. This flip-flop (He) receives a trigger pulse

from a gate circuit which is operated by the altitude number counter.

When the critical altitude is reached, this gate circuit triggers the

gating flip-flop (He), causing the input pulses to be gated off. Thus,

at the end of the analysis cycle the counter will have registered the

number of cloud observations up to (but not including) the critical

altitude. (Observations up through the critical altitude can be included

simply by wiring the gating circuit to recognize the next highest

altitude number.)

The counter is reset to zero at the beginning of each analysis

cycle by means of reset pulses applied to two OR-inverters (Hj).

The reset pulses also reset the gating flip-flop (He). Pressing

either the "Read" or "Write" button in manual operation will cause

this reset pulse to occur, thus zeroing the counter. However, this

circuit receives input pulses only during analysis.

-44-
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2. 7. 7 Analysis Control

The analysis cycle requires some 25 revolutions of the

magnetic drum, consuming about 2. 3 seconds. During this

time the altitude number counter is advanced by one count after

each revolution. In automatic operation the equipment goes

through an input cycle and then produces an end-of -scan pulse.

This pulse causes one word to be recorded on the drum, after

which the analysis cycle is initiated. This signal is derived

from the closing of the one -revolution gate and triggers the

analysis flip-flop (Df). (See Fig. 18.) This flip-flop opens a

gate (Dj) which allows revolution pulses to go to the input of

the altitude number counter. This is also used as the "transfer

up" pulse in the Peak-Value Detector, and as the reset pulse

for the preset counter in the max-min circuit. This flip-flop

also furnishes a negative signal to the input of an OR -inverter

(Dg), the output of which goes positive, opening the output gate

(Dj) for the comparator circuit, allowing coincidence pulses to

be produced. The analysis ends at the moment the altitude

number counter reaches 24, which causes the counter to reset to zero,

simultaneously resetting the analysis flip-flop. When the analysis

starts, the altitude number counter is in the zero state, since it

is set to all zeroes by the end-of-scan pulse. Normally, this would

not constitute a valid word, since the altitude numbers start with

10000, and the word 00000 is not used. Hence, it should not be

included in the analysis. It is eliminated by the arrangement of the

timing of the analysis cycle. The one -revolution gate closes at

the leading edge of the revolution pulse. This furnishes the trigger

signal for the analysis flip-flop, causing the gate (Dg) to open in

time to allow the same revolution pulse to advance the counter

immediately to the state 10000. Thus, word zero is not included

in the analysis. However, should it prove desirable to include

word zero, it is simply necessary to bypass one OR -inverter in

the one -revolution gate circuit tEi 7-12). This will cause the

analysis trigger signal to occur on the trailing edge of the revolution

pulse. The analysis will therefore start with word zero.

The analysis control circuitry is used extensively in the various

manually -ope rated test functions of the computer. The purpose and

use of these functions are described in a separate section; however, it

is convenient to discuss the circuit operation at this point. The three

manual modes of operation, which are selected by a front-panel switch,

are:

1. Read or write single words

2. Read a single word for one revolution

3. Analysis cycle.

-47-
There is a push-button marked "Write" and one marked

"Read". The "Write" push-button is only used with the single -word

function, since it is only possible to record one word at a time.

The "Read" push-button is used with all three manual functions.

Pressing either the "Read" or "Write" button triggers a one-shot

(Eg) which furnishes a "start" pulse for the selected function. This

pulse activates the one -revolution gate circuit by triggering flipflop Eh.. Thus, the one -revolution gate circuit is activated for all

of the manual functions. These will now be described individually.

1. Reading Single Words

The "Read" button triggers the one -revolution gate, as described above. This causes the address counter to receive 101

counts, thus cycling through zero, as described in a previous section.

When the counter goes through zero, it triggers a flip-flop (Ee) in

the record circuit. Notice that, since the address counter is counting

the trailing edges of word pulse-s, this trigger will occur at the trailing

edge of a word pulse. At the same time, this flip-flop is constantly

receiving word pulses at its opposite input, from the word one -shot

(Eb). These pulses attempt to reset the flip-flop at the trailing edge,

but they are only effective after the application of a "set" pulse.

Thus, the flip-flop (Ee) will provide an output signal of one word

length. This signal is sent to the read circuit via an OR -inverter

(Ec) and is used to gate one word into the display storage register.

Pressing the "Read" button again will result in the next successive

word being read.

2. Writing Single Words

Pressing the "Write" button triggers the one -shot (Eg) as

described above, but in addition to triggering the one -revolution gate,

the "start" pulse is passed through an extra set of contacts on the

"Write" switch. This pulse triggers the read-record flip-flop (Ea)

into the record state. Now when the selected address occurs, the

record package is activated for one word, and the altitude number

residing in the altitude buffer is recorded. At the end of the selected

word the one -word flip-flop (Ee) is reset, gating off the recording

process. At the end of the revolution the read-record flip-flop is

returned to the "read" state.

3. Reading for One Revolution

In this mode a pulse for the duration of one revolution is sent

to the read circuit. This pulse is obtained from one of the flip-flops

(Eh) in the one -revolution gate. The trigger signal from the address

-48-
counter is interrupted by a switch, so that the one -word gate

is not actuated in this mode.

4. Analysis

This function takes about 24 revolutions of the drum.

It is necessary that the analysis start at the beginning of a

revolution. This is achieved by using the pulse from the onerevolution gate to trigger the analysis flip-flop (Df) in the read

circuit. Pressing the "Read" button causes the one -revolution

gate to produce an output signal for one revolution; the trailing

edge of this pulse initiates the analysis cycle, which then

continues for 23 revolutions.

2. 8 Altitude Interval Generator and Control Circuit

The rotating searchlight of the Ceilometer is driven by a

synchronous motor at a speed of exactly 5 revolutions per

minute. A switch actuated by the rotating shaft produces a

pulse once each revolution. This pulse is used to synchronize

a counter which counts pulses at the power line frequency. Thus,

the counter is in step with the rotating searchlight, and its count is

related to the angular position of the searchlight. The counter

consists of three 1-2-2-4 decade units, giving it a capacity of

1000 counts (Fig. 21). It is pulsed at a 120 -cycle rate, receiving

2 pulses for each cycle of the line frequency. The active portion

of each input scan is three seconds in duration, representing a

count of 360. Each decade of the counter drives a decimal-decoding

package, which has ten separate ouptuts for the numbers 1-10.

These in turn drive a number of 3-input diode AND-gates which put

out pulses corresponding to the various altitude intervals. There

are 23 of these gates, and thejr outputs are all combined in a

single OR-gate which supplies pulses to the altitude number counter.

Therefore, the altitude number counter is advanced each time that

the start of a new interval is indicated by the altitude interval

generator.

2. 8. 1 Detailed Operation of Altitude Interval Circuit

The altitude intervals are shown in the accompanying table,

Figure 11. The required resolution is shown by the highest

interval, 8500 to 9500 feet, which corresponds to about 1/4° of

angular rotation. The searchlight rotates 90° in three seconds,

so there are 3 x 60 = 180 cycles at 60 cps for one scan. This is

2 cycles per degree of rotation. By counting both positive and

negative half-cycles of the line voltage (using a full-wave rectifier)

-49-
it is possible to get 4 counts per degree, each count equaling l/4

of rotation. Thus, the interval from 8500 to 9500 corresponds to

one count at the 120 pps rate. In the table, the angles corresponding

to the various altitude intervals have been rounded to the nearest

1/4 degree, and in most cases the approximation was quite close.

The various counts corresponding to the 23 intervals are shown in

the table

.

The source for the 120 pps signal is the same as that used to

trigger the sampling process for the analog -digital converter. This

consists of a flip-flop (Am) triggered by pulses from a full -wave

rectifier, with the pulses occurring at the zero-crossings of the 60-

cycle line voltage. The flip-flop (Am) triggers a string of three oneshots (Fd, Hc,Fe) which furnish signals for the analog -digital converter..

The output from the second of these one-shots (He) is used to drive

the altitude interval counter, via an OR -inverter (Ao). (This OR - inverter is used to inhibit counts which might occur during the

occurrence of a synchronizing pulse from the ceilometer. This is

discussed in the following section. ) Attached to each decade of the

altitude interval counter is a decimal decoding package, and these

packages drive the 23 three -input diode AND-gates whose outputs

correspond to the various altitude intervals. The diode gates are

on special package with six gates per package. The outputs of these

gates are combined in a 2 3 -input diode OR -gate, constituting an

additional package. The output of this package is a series of negative

pulses which are passed through an OR -inverter (Di) in order to

increase the amplitude and shorten the rise time. One problem arises

in counting these pulses, in that the last two correspond to consecutive

states of the counter. Thus, the output of the OR -gate is a single pulse

two counts in length, whereas all the other pulses are distinct and are

one count in length (8. 3 milliseconds). This is resolved by the use of a

strobe pulse, applied to the input of an OR-inverter (Ca), together with

these signals. The strobe pulses are obtained from one of the one -shots

(Fe) which is operating at the 120 pps rate..

2. 8. 2 Control Circuit

The control circuit determines whether the computer is in the

input cycle, or the analysis cycle. This circuit furnishes the proper

signals to the various portions of the computer, and also handles the

synchronizing pulse from the- rotating searchlight. The control circuit

receives three -second pulses from the altitude interval generator

which are developed in the following way:

An extra AND-gate is included in the altitude interval generator

and is set for a count of 359. When this count is reached, a one-shot

-50-
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(Ap) is triggered (Fig. 22). This one -shot is set for about 20

microseconds, and the trailing edge of its output pulse is used to

set the interval counter to all one's. (Were the counter stages

set to zero, instead of one, false carries might occur, with

uncertain results.) The next input pulse then resets it to all

zeroes, causing the counting sequence to be repeated. Each threesecond input scan is followed by three seconds during which the

Cellometer is inactive; therefore, only alternate sets of altitude

pulses are desired. The inactive time is used by the computer to

analyze the stored data. The "359" pulse is used, therefore, to

trigger the computer back and forth between the "input" and

"analysis" cycles.

The "359" pulse triggers a flip-flop (Am) which determines

the mode of operation of the computer. Several functions are

affected by the control circuit. The control circuit operates in

the following way:

Assume that the computer is in the input cycle. Pin 21 of

flip-flop Am is positive, holding OR-inverter Ca open and allowing

counts from the altitude interval generator to be sent to the altitude

number counter. Also, two OR -inverters in the Analog -digital

converter are held open. One of these (Fb) allows "transfer up"

pulses to be sent to the peak-value detector. The other (Fb) allows

the pulse trains from the converter to be sent to the peak-value

detector. The converter runs continuously, but its output signals

are used only during the input cycle. When the "359" pulse occurs,

it triggers flip-flop Am into the "analyze" state, causing pin 21 to go

negative, thereby gating off signals from the converter. As pin 16

becomes positive it opens an OR -inverter (Bh) allowing the "359"

pulse to pass through. . The output of this OR-inverter is a negative

signal referred to as the "end-of-input" pulse. It triggers the address

and record circuit, causing the new altitude number to be recorded,

after which analysis takes place. The "359" pulse would normally

persist for 1/120 of a second, since this is the rate at which pulses

are sent to the altitude interval counter. However, the "359" pulse

triggers a one -shot (Ap), as explained previously,, which sets the

counter to all one's. As soon as this happens, the count of 359 no

longer exists, and the "359" pulse ends. Thus, the width of the "endof-input" pulse is equal to the pulse from one -shot Ap, which is about

20 microseconds. Note that this gate only passes every other "359"

pulse. All of the "359" pulses are present at the input to this gate;

however, the next pulse will trigger flip-flop Am at its leading edge,

causing Am to close the gate in time to prevent passage of the pulse.

Every "359" pulse triggers the one-shot (Ap) which leads to the

resetting of the altitude interval generator. The pulse from Ap serves

-52-
as a reset pulse for several portions of the computer as it cycles

back and forth between the "input" and "analysis" cycles. These

"reset" pulses are sent to the peak -value detector, where both

the adding and subtracting registers are set to zero. They also

reset the altitude number counter, the max-min counter, and'

the critical altitude counter.

2. 8. 3 Synchronizing Pulse from Ceilometer

Synchronizing of the altitude interval generator with the pulse

from the rotating searchlight is somewhat intricate, because of

the associated control circuitry. This "rotation pulse" only occurs

once per rotation of the searchlight shaft, whereas there are two

active scans during this time, since there are two searchlights

mounted back-to -back on the shaft. It is necessary, then, that

the equipment cycle back and forth between input and analysis whether

the synchronizing pulse occurs or not. This enables the equipment

to remain in operation even if a sync pulse should fail to occur.

The sync pulse, where it occurs, represents the start of an active

scan and the computer should be set to the start of the input cycle.

This means that the altitude interval counter should be reset and

flip-flop Am should be set to its "input" state. There are four

possible conditions to consider:

1) absence of sync pulse,

2) sync pulse at correct time (normal condition),

3) sync pulse occurring early,

4) sync pulse occurring late.

If no sync pulse occurs, the "359" pulse will continue to trigger

the equipment back and forth between the input and analysis cycles.

One pulse will signify the start, of an input cycle; the next, the start

of an analysis cycle. When a sync pulse occurs, it closes a relay

of the type normally used in the Ceilometer indicator units. This

is a Western Electric 276-B with mercury-wetted contacts, and does

not exhbit contact bounce. When the relay closes, it grounds the

input of a one -shot (An) which is normally held negative through a

high resistance (Fig. 22)* This positive -going impulse triggers the

one-shot, developing a synchronizing pulse. The leading edge of this

pulse triggers flip-flop Am to its "input" state, by means of an

external gate. It also starts the one -shot (Ap) which prepares the

altitude interval generator to be reset, also furnishing a reset pulse for

the computer. The sync pulse one -shot (An) performs two "inhibit"

-53-
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functions, which are necessary to prevent the occurrence of

conflicting pulses from the control circuit. The 120 -cycle

pulses to the altitude interval generator are cut off, via ORinverter Al, in order to allow one -shot Ap to complete its

cycle and set the counters to all one's without any conflicting

counts. Also, the "359" pulse is inhibited, since it should be

replaced by the sync pulse when it is available. The duration

of An is at least 20 microseconds greater than Ap, in order to

allow the altitude interval counters to settle down before additional

counts 'are let in. (An « 50 microseconds.

)

Operation of the control circuit in terms of the arrival of

the sync pulse can now be discussed. If the pulse arrives early,

the altitude interval generator will not have reached 359, and

the computer will still be in the analysis cycle. The sync pulse

will trigger the sync one-shot (An). The leading edge of this pulse

will set flip-flop Am to its input state, and will also trigger oneshot Ap, resetting the computer and terminating the analysis.

(Analysis takes about 2. 4 seconds, so the sync pulse could be 0. 6

seconds early without chopping into the analysis.) The altitude

interval counter will not be set to all one's until Ap goes off,

requiring about 20 microseconds. During this time there is a

small possibility that the count of 359 will occur. This would

result in the production of a "359" pulse which would switch

flip-flop Am away from its input state. For this reason the

"359" pulse is inhibited during this time. The sync pulse will

always trigger one -shot Ap, unless it happens to be already

triggered, in which case the equipment may be considered to

be in sync. Since this 20 -microsecond interval is very small

with respect to .the time scale of the ceilometer, it is unlikely

that the sync pulse will occur during this time; hence, the sync

pulse will usually trigger this circuit. However, the equipment

will generally be within one pulse at the 120 -cycle rate, since this

represents an interval of 8. 3 milliseconds. Since the control

circuit is using the 120 -cycle pulses as a clock, it cannot, in fact,

be set closer than 8. 3 milliseconds, which therefore, constitutes

being in sync. If the sync pulse is early, the altitude interval

counters are reset to all one's before reaching 359, and hence

359 will not occur, since the counters will start again from zero.

However, if the sync pulse is late, the count of 359 will have occurred

and the control circuit will have automatically switched to the input

cycle. The sync pulse simply serves to restart the altitude number

generator and to reset the computer to zero.

The flip-flop, Am, which controls whether the computer is in

the "input" or "analysis" state, is connected to two indicator lights.

-55-
One of these lights, located above the "Write" pushbutton, is

on during the input cycle. The light above the "Read" pushbutton

is on during the analysis cycle.

2. 9 Output Decoding and Display

Several quantities are displayed by the computer as answers

to the various factors which it analyzes. These are:

1) The altitude at which the most frequent cloud activity

occurred.

2) The minimum altitude at which five or more counts

were observed.

3) The maximum altitude at which five or more counts

were observed.

4) The number of cloud occurrences up to the critical

altitude

.

The first three quantities are altitude numbers and are

represented in the machine as five -bit binary words. The last

quantity appears as a binary-coded decimal number (1-2-2-4

code) in two decade registers. There are two requirements for

displays. First, there must be illuminated decimal numbers at

the computer (Fig. 23), and second, there must be contacts available for activating a remote read-out device, which should be

electrically separate from the local display. The four quantities

can each be represented by two decimal numbers, the three altitude

numbers being heights in hundreds of feet from 00(00) to 99(00)

and the critical counts being represented directly from 00 to 99.

For both the local and remote devices it is desirable that each

decimal number be represented as a contact closure on one of ten

lines. From the standpoint of the number of contacts required,

and the decoding involved, the most economical solution was

through the use of stepping switches. There are 24 possible

values for the altitude numbers, permitting the use of a 26-point

switch for each altitude number. The two decade numbers for the

critical-counts counter were handled by an 11 -point switch for

each decade.

2. 9* 1 Altitude Number Stepping Switches

There are 3 of these 26-point switches, each for a separate

altitude number. They have 10 banks of contacts which are wired in

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the following way: The first five banks are wired to represent

the binary numbers from to 23. Two leads are brought from

each bank to opposite sides of a corresponding flip-flop in one

of the buffer registers (Figure 24). These lines are filtered to

remove switching transients produced by the stepping switches,

which might otherwise alter the numbers in thebuffers. The

wiper arms for the five banks are connected to an AND circuit.

If the setting of the switch does not correspond to the state of its

associated buffer, the AND circuit Will have one or more negative

inputs and will energize the drive circuit for the switch. Thus,

the switch will step along until it is in agreement with the buffer.

Of the remaining five banks on the switch, two banks are used

for driving the local display at the computer and two are available

for the remote requirements. One bank is unused. Each bank

represents a decimal numeral. Hence, two banks together

comprise a ten's bank and a unit's bank. It is these banks which

actually decode the altitude numbers into heights. Thus, the word

OHIO, which is the binary representation of 14, corresponds to

20(00) feet. At this position of the switch, the unit's contact would

connect to the digit of the unit's display, and the ten's contact

would connect to the 2 -digit of the ten's display. The digit of the

unit's bank would be energized in several other cases, namely

00, 10, 20, , 90; for all of these positions the unit's contacts

would be wired together. The wiring for the contact assembly is

shown in Figure 24.

2. 9. 2 Critical Counts Stepping Switches

The two decimal numbers from the critical -counts counter are

decoded by the 11 -point switches in much the same way. However,

only a single decimal number is obtained for each switch, rather

than a pair of numbers, as above. Furthermore, the decade counters

have only four flip-flops instead of five as for the altitude numbers.

These stepping switches have 6 banks. Four banks are wired to

correspond to the states of the 1-2-2-4 counters, one bank is used

for a local decimal digit, and one bank is used for the remote output.

The wiring for the contact assemblies of these switches is shown in

Figure 25.

2. 9. 3 Stepping -Switch Drive Circuit

The drive circuit for the stepping switches is complicated by the

fact that the actual stepping of the wiper arms occurs as the coil is

de -energized. Thus, if they are being operated in a self -interrupting

mode, the drive contacts will already have closed, and be starting to re -energize the switch by the time the contacts move to the new position.

Then if the new position is found to be the desired one, it will be

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necessary to interrupt the drive circuit quickly, before the

coil is completely energized. This can be solved through the

use of fast relays; however, the drive relay for. this application

was selected to be compatible with the transistor packages used

throughout the computer and as such it was not a high-speed type.

In order to assure that adequate time would be available for the

AND circuit and drive relay to function, the drive relay and

stepping 'switch were connected together as a sort of oscillator.

The stepping switch drive circuit is illustrated in Figure 26.

This circuit works as follows:

Assume that one or more of the inputs to the AND circuit

is negative, indicating that the stepping switch setting does not

correspond to the number stored in its respective buffer. The

AND circuit will send a negative signal to an indicator driver

which in turn energizes the drive relay. The drive relay closes,

energizing the stepping switch. Note that the current for the

drive relay must pass through two contacts on the stepping switch.

Current can flow through the first contact and through a resistor

to the relay coil, and it can flow through the second contact directly

to the coil. The resistor has a value such that it will pass enough

current to hold the relay closed, but it will not pass enough current

to close the relay if it is open. This resistor, therefore, provides

a latching action. The stepping switch contacts are adjusted to

operate in sequence. As soon as the stepping switch starts to move,

the direct path to the relay coil is opened, leaving only the path

through the latching resistor. As the stepping switch reaches the end

of its cocking stroke the next contact opens, breaking the latching path.

This causes the drive relay to drop out, deenergizing the stepping

switch. Now the stepping switch armature begins to drop back,

stepping its wiper contacts to the next point. As soon as it starts

back the contact in the latching circuit closes, but this does not pass

enough current to close the drive relay. Not until the switch drops

back far enough for the wipers to reach the next point does the second

contact close, completing the direct path to the drive relay. By this

time the AND circuit will have had a chance to function, if the new

position is the desired one, and the drive for the relay will be cut

off, before another step is initiated. If the new position is still not

correct, the AND circuit will continue to furnish a signal to the drive

relay, and the stepping action will continue. The stepping rate is

about 15 to 20 steps per second with this circuit.

Contact protection for jthe drive relay contacts, which must

interrupt the stepping switch current, is provided by an R-C network

of 100 ohms in series with a 0. 1 uid capacitor across the contact. (Fg.27)

The stepping switches require 110 volts, d-c, and the coil resistance

-61-
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is about 400 ohms. The Current is about 0. 3 amperes per coil

and since there are five stepping switches the power supply

must be able to furnish a peak current of about 1. 5 amperes."

The power supply, which is mounted on the stepping switch

chassis , consists of a half-wave rectifier using two silicon

diodes in series. No transformer is used, so one side of the

d-c output is connected to the a -c line. The rectifiers used

are lN1124's with a current rating of 5 amperes (with proper

heat sink) and a peak inverse voltage of 200 volts. This is not

sufficient to allow a single rectifier to be used, since the peak

value of the 110 volts line is about 160 .volts, and this is the value

which the filter capacitor will attain. Thus, during the nonconducting half -cycles, the rectifier has across it the 160 volt d-c

output voltage plus the 160-volt peak voltage of the line, or 320

volts. For this reason, two rectifiers are used in series, with

10K ohms resistors across them to assure equal voltage division

in the back direction. The power supply is protected by a 2-

ampere fuse.

One additional feature was included with the stepping switches

to prevent excessive display changes. Certain of the buffers

change several times during the analysis cycle, and it is undesirable that these momentary changes appear in the display.

For this reason, a relay has been added to the power supply which

turns it off during the analysis cycle. Since a double -pole relay

was available for this purpose, one contact is placed in the a-c

line and one in the d-c output. Thus, the power supply is not

energized continuously, reducing the dissipation on the power

supply components. This relay is closed for all manual modes of

operation.

3. MANUALLY OPERATED TEST FUNCTIONS

In normal operation the Ceilometer computer operates in

the automatic mode, either one -minute, or 10 -minute, as

determined by the appropriate switch setting. However, there

are several manual settings of this switch, to provide for testing

of the computer (Figure 28)* The switch settings are as follows:

AUTOMATIC MANUAL

One -Minute Single Words

Ten-Minute One Revolution

Analyze

Just above this switch are two push-buttons labelled "Write"

and "Read". The "Write" push-button is used only for recording

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single words. The "Read" push-button is used for reading single

words, reading for one revolution, or analyzing, (NOTE: These

buttons should not be pressed faster than about 2 or 3 times per

second. This limit is imposed by the access time of the drum,

3. 1 Single Word Operation

This function permits any of the 5-bit altitude numbers (including

00000) to be recorded or read in any desired location on the drum.

The procedure is as follows:

1) The mode switch is set to "Single Word".

2) The Altitude Number Counter, Address Counter, and PeakValues Detector are zeroed by means of their respective reset

buttons

.

3) The desired word is set up in the Altitude Number Counter

(top row of lights) by means of the five small buttons beneath it. These

buttons cause the lights to turn on, indicating one's. (NOTE: Internal

feedback does not allow the 8 and 16 lights to be on simultaneously.

)

4) The selected word is transferred to the Buffer by means of

the "Transfer" button. (NOTE: If the Control light on the PeakValue Detector is on, the word will not transfer into the Buffer.

Hence, the Peak-Value Detector should be reset.)

5) The desired address is selected by pushing the small buttons

next to the address lights. (NOTE: These are 1-2-2-4 counters,

connected in such a way that if either of the upper two lights are on,

the second light will be on also.

)

6) The desired word and address have now been selected.

Pressing the "Write" button causes the word to be recorded. The

Address Counter lights will be seen to blink momentarily, indicating

that 101 counts are being gated to the counter. The address will be

seen to advance by one. Thus, if the "Write" button is pressed again

it will record the same word in the succeeding address, and so forth.

The word may be recorded as many times as desired.

7) To read a single word it is only necessary to select the address

and press the "Read" button. Again, the address advances by one each

time a word is read. The word will be displayed in the Storage Register,

in binary form, and decoded into the Output Display in decimal numbers.

-66-
8) The drum may be erased by recording all zeroes in every

address. This is done by resetting the buffer to zero and recording

100 times.

The max-min and critical count circuitry do not obtain signals

in this mode of operation, but they are reset to zero when the "Write"

or "Read" buttons are pressed.

3.2 One -Revolution

This -function permits the read circuitry to be operated for a single

revolution of the drum. During this operation the read circuitry will

compare each of the 100 words on the drum with the contents of the

Altitude Number Counter, and the resulting coincidence pulses will be

fed to the Peak-Value Detector. The Peak-Value Detector will then

indicate the number of times that this word occurred. Also, if the word

occurs at least once, the Control Light will come on, and the word will

be transferred into the Buffer. If the word does not occur, it will not

appear in the Buffer, and the Peak-Value lights will remain off.

The procedure is as follows:

1) The mode switch is set to "One -Rev. "..

2) The Altitude Number Counter and Peak-Value Detector are

zeroed by means of their respective reset buttons. The Address Counter

plays no part in this operation and need not be reset.

3) The des ired word is placed in the Altitude Number Counter by

means of the five small buttons. Word "zero" (00000) may be counted,

if desired. It should occur 100 times if the drum has been erased.

4) The "Read" button is pressed. Jf the word occurred on the drum

it will now appear in the Buffer. In this case the Control Liglit will be

on, and the number of times that the word occurred can be read from the

Peak-Value lights.

The "Write" button should not be used with this function.

The max-min and critical count circuitry are not activated during

this function.

3. 3 Analyze

This function permits the computer to be triggered manually to

perform the analysis cycle. The analysis is identical with that performed

-67-
duziing the "analysis" cycle under automatic operation. Answers

will be developed for all of the pertinent factory, as listed below:

a) What altitude number occurs most frequently, and how

many times?

b) What are the lowest and highest altitude numbers which

occur five or more times?

c) How many times did altitude numbers appear which were

less than the critical altitude number?

In this mode of operation the read circuitry counts the number

of times that each of the altitude numbers from 1 to 23 occurs on

the drum. Trains of coincidence pulses are- generated and sent to

the Peak-Value Detector, which causes a new altitude number to be

transferred into the Buffer any time it occurs more frequently than

any previous number. The 23 numbers require 23 revolutions of

the drum, or 2. 3 seconds. During this time the Altitude Number

Counter can be seen to count in binary fashion as the altitude number

increases, indicating that the analysis is in progress.

The procedure is as follows:

1) The mode switch is set to "Analyze".

2) The Altitude Number Generator and the Peak-Value Detector

are zeroed by means of their respective reset buttons. The address

counter plays no part in this operation and need not be reset.

3) The "Read" button is pressed, to initiate the analysis. The

analysis requires 2. 3 seconds, during which time the Altitude Number

lights will be blinking. At the end of this time the displays will contain

answers to all of the questions listed above. The analysis may be

repeated any number of times, and the answers should always be the

same, as long as the data is unchanged. Once an analysis cycle has

been initiated, the "Read" button should not be pressed again until it

is completed.

The "Write" button should not be used with this function.

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USCOMM—DC—

DEPARTMENT OF COMMERCE

Frederick H. Mueller, Secretary

LTIONAL BUREAU OF STANDARDS

A. V. Afltin, Director

THE NATIONAL BUREAU OF STANDARDS

The scope of activities of the National Bureau of Standards at its major laboratories in Washington, D.C., and

Boulder., Colo., is suggested in the following listing of the divisions and sections engaged in technical work. In

general, each section carries out specialized research, development, and engineering in the Aeld indicated by its

title. A brief description of the activities, and of the resultant publications, appears on the inside of the front cover. WASHINGTON, D.C. ELECTRICITY. Resistance and Reactance. Electrochemistry. Electrical Instruments. Magnetic Measurements.

Dielectrics. METROLOGY. Photometry and Colorimetry. Refractometry. Photographic Research. Length. Engineering

Met : ology. Mass and Scale. Volumetry and Densimetry.

HEAT. Temperature Physics. Heat Measurements. Cryogenic Physics. Rheology. Molecular Kinetics. Free

Radicals Research. Equation of State. Statistical Physics. Molecular Spectroscopy.

RADIATION PHYSICS. X-Ray. Radioactivity. Radiation Theory. High Energy Radiation. Radiological

Equipment. Nucleonic Instrumentation. Neutron Physics.

i HKMISTRY. Surface Chemistry. Organic Chemistry. Analytical Chemistry. Inorganic Chemistry. Electrodeposition. Molecular Structure and Properties of Gases. Physical Chemistry. Thermochemistry. Spectrochemistry. Pure Substances.

MECHANICS. Sound. Pressure and Vacuum. Fluid Mechanics. Engineering Mechanics. Combustion Controls. ORGANIC AND FIBROUS MATERIALS. Rubber. Textiles. Paper. Leather. Testing and Specifications.

Polymer Structure. Plastics. Dental Research.

METALLURGY. Thermal Metallurgy. Chemical Metallurgy. Mechanical Metallurgy. Corrosion. Metal Physics.

MINERAL' PRODUCTS. Engineering Ceramics. Glass. Refractories. Enameled Metals. Constitution and

Micrestructure. BUILDING RESEARCH. Structural Engineering. Fire Research. Mechanical Systems. Organic Building Mai ials. Codes and Safety Standards. Heat Transfer. Inorganic Building Materials.

APPLIED MATHEMATICS. Numerical Analysis. Computation. Statistical Engineering. Mathematical

Physics.

DATA PROCESSING SYSTEMS. Components and Techniques. Digital Circuitry. Digital Systems. Analog

Systems. Applications Engineering.

ATOMIC PHYSICS. Spectroscopy. Radiometry. Mass Spectrometry. Solid State Physics. Electron Physics. A : iinic Physics.

INSTRUMENTATION. Engineering Electronics. Election Devices. Electronic Instrumentation. Mechanical

1 struments. Basic Instrumentation,

of Weights and Measures.

Cryogenic Processes. Properties of Materials. Gas

BOULDER, COLO.

CRYOGENIC ENGINEERING. Cryogenic Equipment.

Liquefaction.

IONOSPHERE RESEARCH AND PROPAGATION. Low Frequency and Very Low Frequency Research. Ionosp] ere Research. Prediction Services. Sun-Earth Relationships. Field Engineering. Radio Warning Services. RADIO PROPAGATION ENGINEERING. Data Reduction Instrumentation. Radio Noise. Tropospheric Measurements. Tropospheric Analysis. Propagation-Terrain Effects. Radio-Meteorology. Lower Atmosphere Physics.

RADIO STANDARDS. High frequency Electrical Standards. Radio Broadcast Service. Radio and Microwave

M rials. Atomic Frequency and Time Standards. Electronic Calibration Center. Millimeter-Wave Research. M owave Circuit Standards.

DIO SYSTEMS. High Frequency and Very High Frequency Research. Modulation Research. Antenna Rech. Navigation Systems. Space Telecommunications.

J PPER ATMOSPHERE AND SPACE PHYSICS. Upper Atmosphere and Plasma Physics. Ionosphere and

rhere Scatter. Airglow and Aurora. Ionospheric Radio Astronomy.
NBS