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OCT 2 8 1969

NBS TECHNICAL NOTE 510

Use of a Time-Shared Computer System

To Control a Hall Effect Experiment

' •: ' •'.' -" •. '•

U.S. DEPARTMENT OF COMMERCE
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards ' was established by an act of Congress March 3, 1901 . Today,

in addition to serving as the Nation's central measurement laboratory, the Bureau is a principal

focal point in the Federal Government for assuring maximum application of the physical and

engineering sciences to the advancement of technology in industry and commerce. To this end

the Bureau conducts research and provides central national services in four broad program

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standards, (3) technological measurements and standards, and (4) transfer of technology.

The Bureau comprises the Institute for Basic Standards, the Institute for Materials Research, the

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Sciences and Technology, and the Office for Information Programs.

THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United

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uniform physical measurements throughout the Nation's scientific community, industry, and commerce. The Institute consists of an Office of Measurement Services and the following technical

divisions:Applied Mathematics—Electricity—Metrology—Mechanics—Heat—Atomic and Molecular Physics—Radio Physics -—Radio Engineering 2—Time and Frequency -—Astrophysics -—Cryogenics.2 THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research leading to im- proved methods of measurement standards, and data on the properties of well-characterized

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provides technical services designed to aid Government agencies in the selection, acquisition,

and effective use of .automatic data processing equipment; and serves as the principal focus

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and computer languages. The Center consists of the following offices and divisions: Information Processing Standards—Computer Information — Computer Services — Sys- tems Development—Information Processing Technology.

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accessibility of scientific information generated within NBS and other agencies of the Federal government; promotes the development of the National Standard Reference Data System and a system of information analysis centers dealing with the broader aspects of the National Measure- ment System, and provides appropriate services to ensure that the NBS staff has optimum accessibility to the scientific information of the world. The Office consists of the following

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UNITED STATES DEPARTMENT OF COMMERCE

Maurice H. Stans, Secretary

NATIONAL BUREAU OF STANDARDS • Lewis M. Branscomb, Director

NBS TECHNICAL NOTE 510

ISSUED OCTOBER 1969

Nat. Bur. Stand. (U.S.), Tech. Note 510, 46 pages (Oct. 1969)

CODEN: NBTNA

Use of a Time-Shared Computer System

To Control a Hall Effect Experiment

W. Murray Bullis and W. R. Thurber

Electronic Technology Division

Institute for Applied Technology

T. N. Pyke, Jr., F. H. Ulmer, and A. L Koenig

Information Processing Technology Division

Center for Computer Sciences and Technology

National Bureau of Standards

Washington, D.C. 20234

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.

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C, 20402

(Order by SD Cataloa No. CI 3.46:510), Price 50 cents.

CONTENTS

pAQE

LIST OF FIGURES iv

LIST OF TABLES v

1. General Introduction 1

2. Use of a Time-Shared Computer System 2 2.1 The Time-Shared Computer Concept 2 2.2 Response from Time-Shared Computer Systems 2 2.3 Shared Resources 3 2.3.1 Central Processor Units 3 2.3.2 Storage 3 2.3.3 Communications 3 2.4 Configurations Using Time-Shared Systems 4

3. The Hall Effect Experiment 4 3.1 The Effect 4

3 .

2

Apparatus 6 3.3 Adaptability to a Time-Sharing Environment 10

4. The Experiment/Computer Configuration 12 4.1 Over-all Configuration 12 4.2 The Time-Shared Computer System 12

5. The Hardware Interface 16

5.1 Measurements Automation Modules 16

5.2 Configuration of the Modules 16

5.3 Reed Relay Scanner Module 16

5.4 Digital Voltmeter 18

5.5 Digital Scanner Module 18

5.6 Teletypewriter Interface Module 18

5.7 Control Module ' 19 5.8 Special Interface Module 20 5.9 Control Program 20 5.10 Assembly of the Interface System 22

6. Software Design 23

6.1 Over-all Software Design 23

6.2 List of Symbols 23

6.3 Subroutines 26 6.4 Program Details 28

6.4.1 Initialization 28

6.4.2 Temperature Check and Set Up for Data Input 28

6.4.3 Data Input 33 6.4.4 Calculations and Completion of Run 33

7. System Operation 33 7.1 Experimental Procedure 33

7.2 Limitations of the System 38 7.3 Results 39

8. System Evaluation 39 8.1 Influence of the Computer on the Experiment 39 8.2 Influence of the Experiment on the Computer 40

REFERENCES 41

in
LIST OF FIGURES „„„„ PAGE

1. Hall Effect Sign Convention 5

2. Hall Specimen Geometry 7

3. Specimen Holder 8

4. Hall Effect Experiment with Computer Interface 9

5. Electrical Circuit for Measuring Specimen Voltages 11

6. Experiment/Computer Configuration 13

7. Block diagram of NBS computer research facility 14

8. NBS CORD Time-Shared Computer System 15

9. Interface Block Diagram 17

10. Control Module Program 21

11. General Program Flow 24

12. Heater i Current Calculation Subroutine 27

13. Initialization 29

14. Temperature Stability Check 30

15. Initialize Field and Specimen Current 31

16. Data Input 34

17. Data Input 35

18. Calculation of Field, Resistivity, Hall Coefficient, Hall Mobility,

and Average Thermocouple Voltage 36

19. Print Results of Calculations, Save Data, and Check for Completion 37
LIST OF TABLES „.„„ PAGE

1. Maximum Thermocouple Voltages (MTV) for each Temperature Range as a Function

of Coolant 25

2. Heater Current Parameters a and b as a Function of Coolant and Temperature

Range 25

3. Organization of Stored Data 32

4. Field Computation Coefficients 32
ABSTRACT

An experimental time-shared computer system has been used to control and

collect data from a Hall effect experiment. In addition to selection of -the voltage sequence and control of specimen current and magnetic flux directions,

the computer exerts positive control over the specimen temperature in accordance

with a series of thermocouple voltages entered by the operator at the beginning

of the run. The interface circuitry is constructed from four general purpose

modules and a single special purpose unit. Details of the interface system and

the programs used to control the experiment and to perform intermediate calculations are given. It was found that the time-shared system was well-suited to

control an experiment of this type. The demands on the computer were essentially

similar to those required by a human user interacting in a conversational mode.

Substantial savings in time over manual data collection and reduction were realized; a temperature run with 38 to 40 data points between 10 and 320 K could

be completed in less than 4 hours.

Key Words: computer- controlled experiment; data acquisition system; digital

scanner; experiment-computer interface; Hall effect; system control module;

teletypewriter interface; time-shared computer.

VI
1. GENERAL INTRODUCTION

An experimental time-shared computer system has been used to control and collect data

from an attached Hall effect experiment. Operation of the experiment together with its

computer interface in a time-sharing environment has demonstrated that some types of experimental apparatus can be serviced on-line by a general-purpose time-shared system while that

system is servicing a variety of other more typical users. This note, in addition to documenting a particular experiment, points out the characteristics of the experiment that make

it amenable to time-shared operation. From these characteristics a class of experiments

that can be practically assisted by a time-shared system may be identified.

Successful operation of an experiment in a time-shared computer environment requires

that the experimental equipment place a low or moderate load on the computer system. In general, it is essential that the combination of experiment plus interface not require

"critical real-time" service from the computer system. Critical real-time operation may be

described as that in which the system must meet a strict upper limit in response time to service requests; this limit may require response within a few milliseconds or even a few

microseconds

.

Since most time-shared systems have attached teletypewriter or display terminals which

interface individual users to the system and since these users can usually operate acceptably with a response time of a few seconds or more, most systems have been designed for this

type of response, and in fact, the economics of their design depend on the acceptability of

this response time. The actual response time of such multi-access systems is usually a few

seconds, but may range from a few tenths of a second up to several seconds. This response

is usually adequate for human users, and so the system is "real-time" with respect to their

needs

.

Since this type of response can be expected from most general-purpose time-shared

systems, any interface to attached experimental apparatus must usually be designed to meet

this constraint. The class of experiments referred to above is further restricted, then,

to that which can benefit from on-line computer service with a response time of a few seconds or more.

The computational load of an experiment on an interface and associated computing

equipment is variable. Some experiments may require a small computer or a large amount of

special-purpose hardware adjacent to the experiment to satisfy their critical real-time

response requirements. In some cases the small computer may provide all of the necessary

computational capability. In other cases data logging can be done with either specialpurpose hardware or a small computer using a machine-readable medium such as punched paper

tape, punched cards, or magnetic tape. This data can then be used as input to a larger

computer on a batch processing basis for analysis and summary.

In other cases, however, it is advantageous to have a larger computer system directly

on-line with the experiment. This may be necessary because of a need for greater com- putational capability than is available with a small computer or special-purpose hardware.

Such a large on-line computer can provide additional experimental flexibility with the

power of more advanced languages and other software, faster computation, larger memory,

and other features not available on a small computer. Depending on the nature of the

experiment, it may be economically advantageous to be on-line with a larger computer system

which is shared by several users, rather than to use a small computer solely for one or a few experiments

.

The larger, usually remote, computer referred to above may service a number of experiments at one time or it may be only partially dedicated to servicing on-line experiments.

If partially dedicated, its remaining capacity may be used to service general-purpose teletypewriter, display, or background batch processing users. The terms "small" and "large"

computer as used above are relative, although there are identifiable classes of computer

systems according to various measures, such as speed, instruction repertoire, and storage

size. The amount of computing capability needed and the relative scales of computers in

multi- computer configurations must be decided on an individual basis for each system.
The increasing availability of time-shared systems and the more generalized software

which enables such systems to perform functions other than that of an advanced desk calculator, combined with several years experience in using small to medium-sized computers

to control experimental equipment such as nuclear reactors and linear accelerators, has led

to the application of such computer systems to more modest experiments [1].

This report discusses how a particular experiment was attached to a research-oriented

time-shared system. It discusses factors influencing the load placed on the system, including pertinent attributes of the experiment itself and of the interface hardware. In addition, general comments from a user's point of view concerning the advantages and disadvantages of using automated logging and control devices for experimental equipment are included. The successful operation of this experimental configuration shows the feasibility

of operating so-called general-purpose time-sharing systems in a dual mode—servicing ex- perimenter and general terminal user at the same time.

2. USE OF A TIME-SHARED COMPUTER SYSTEM

2.1 The Time-Shared Concept

A large number of computer systems that have been implemented during the past several

years have been labeled "time-sharing" systems [2]. Although these systems vary in capability, most of them have the general property of being able to serve a number of independent on-line users simultaneously. Some systems may limit user independence; however, all

systems allow for apparent simultaneity of operation by multiple users. This is accom- plished by time multiplexing various resources within the system. If use of such resources

can be completed very quickly with respect to the response time expected by the individual

users, then each user is given the illusion that he alone is using the system.

2.2 Response from Time-Shared Computer Systems

Interaction between a user and a time-shared computer system usually consists of a sequence of message pairs. Each pair consists of a message to the computer and a message

from the computer. For example, the user types a string of characters on a teletypewriter

keyboard and a response is received from the remote computer. This order of transmission

may be reversed; a query from the computer may be followed by a reply from the user.

This strict pairing need not be maintained and implied groupings of transmissions in

either direction may be used for specific interactions. Whether the interaction be user- directed or machine-directed, there is a lag within the computer system between the time a message is transmitted to it and the time a response is formulated. The "response time"

may be defined as the period beginning with completion of transmission of the last character

of a message to the computer and ending with receipt of the first character of the responding transmission to the user.

Response times of a few seconds are permissible for a human user who is interacting in a conversational mode. . For some applications a response time of several seconds or even minutes may be adequate. Since the internal operating speed of most computers is a function

of a basic cycle time which is of the order of microseconds, the response time for a simple

response to a single user might be a fraction of a second. If, however, there are a number

of users requiring service, and if some or all of these users require substantial computation or other time-consuming system activity, then typical response times will be considerably longer.

Experiments are usually connected to a computer system through intermediary interface

hardware. A simple interface may consist of a buffer and a small amount of logic. ' The

effective response time of the "computer system plus interface" is that which is of concern

to the experimenter. Since a complex interface can make up for a slow computer response

time in some cases , interface design depends on both the requirements of the experiment and

computer capability. If only a few simple tasks require extremely fast response time and

the rest can survive with longer response time but need substantial computation, then a very simple interface can provide the fast response time necessary for the critical tasks

as well as provide interconnection to a computer system.
2.3 Shared Resources

A prime objective in operating a computer system in a time-shared mode is to share a common set of resources among several users. Economic gain for each user is possible providing that the users adhere to several limitations with respect to the available resources.

These restrictions may be met quite easily for some applications, yet be hardships on others,

By examining briefly those resources that are shared in a time-shared system the basis limitations on users become clear.

2.3.1 Central Processor Units

A time-shared system contains one or more central processor units (CPU's). These are

the basic modules for controlling the system and for performing arithmetic and logical

operations. The capability of each CPU is fixed by its basic logic speed, number of internal registers, instruction repertoire, and basic organization. Some of this capability must

be reserved for executive or bookkeeping functions within the system; that is, the dynamic

allocation of system. The remainder of the CPU time is available for sharing among the

various users.

The total demand for CPU usage by all users over any specified time period of a few

seconds or longer must not be more chan the time available on the CPU(s). Users requesting

large amounts of CPU time are usually given low priority, so that system response time for

such users is substantially longer than for highly interactive users. Part of the basic justification for time-sharing is that users need substantial com- puting capability for very short periods, but that these needs arise only occasionally. For

example, a user may require 50 ms of CPU time and then not again request service for several

seconds until the computer's response has been typed out and the user's next message formulated and typed into the system. It is possible that when any one user requests CPU time no other user may have an active request pending. It is likely, however, that the user's program must wait in a "queue" for its slice of CPU time. Successful operation of time-sharing

systems depends on successful estimates of the probability of usage of each of the resources

by the various users. Elementary queuing theory can be applied to estimate loads in various

parts of time-sharing systems based on predicted user behavior.

2.3.2 Storage

Another resource shared in a time-shared system is storage. Main storage, usually

core memory, is very expensive relative to other parts of the system. Usually not enough

is available to allow a program associated with each user to remain in core all of the time.

It is necessary to swap programs or parts of programs in and out of main core before and

after they use their share of CPU time. Backup memory, drum or disc, is used to store

active user programs when they are not in core memory. It is obvious that the less core a given user requires for an application, the less burden he is placing on the expensive core memory resource within the system.

Backup memory—drum, disc, or magnetic tape—is also shared, A large amount of such

memory is usually .available , but since there must.be some limit even to this amount of

backup storage, restrictions are usually placed on the number of files each user may store.

2.3.3 Communications

The communication link between users and the computer system is another important re- source. The design of the interface between computer system and remote terminals must en- sure that no data be lost when communicating at normal data rates. It is necessary to provide a limited amount of hardware for each communication line, since queues cannot be held

in a simple remote terminal which has no buffers . Because the servicing of communications

lines requires either CPU or the action of a special-purpose controller, each user is re- stricted in frequency and size of messages. A basic restriction may be to limit him to the

capacity of a single telephone line.

Because of the expense of communications lines and equipment and because individual

lines are quite likely not shared as are resources within the time-shared system itself, it

is economically advantageous for individual users to use the minimal communication facilities necessary for their applications.
2.4 Configurations Using Time-Shared Systems

A variety of configurations are possible for interconnecting experimental apparatus to

a time-shared computer system. Three examples of such configurations are described below,

and one can incorporate any combination of these in a system designed to service a specific

set of applications.

(a) An experiment may be connected via a specially designed hardware interface to a time-shared system that services this and a few other specially interfaced experiments. An example of this configuration is the use of a small computer to monitor several experiments

connected to a nuclear reactor, providing service on-line to a number of experimenters simultaneously.

(b) An experiment can be connected via a special-purpose hardware interface to a large

general-purpose time-shared computer system. The hardware interface can be designed such

that the burden on the time-shared system is of the same order as that placed on the system

by more usual teletypewriter or display terminals. The experiment described in this report

is an example of this configuration.

(c) An experiment can be connected to a small computer which itself is connected as a remote terminal to a larger time-shared computer. An experiment which requires servicing

of a number of critical real-time needs, yet requires substantial computation available on- line but with no such critical response time, is a candidate for this type of configuration.

3. THE HALL EFFECT EXPERIMENT

3.1 The Effect

The Hall effect is widely employed in the characterization of metals, semiconductors,

and insulators. Its importance arises because of the direct relationship between Hall

coefficient and free charge carrier density in a solid. If electric currents are carried

dominantly by one charge carrier, the Hall coefficient, Rjj, is given by

R - JL

H nq (3.1)

where n is the free charge carrier density, and q is the charge of the carrier. The proportionality factor, r, which is of the order of unity, depends on details of the energy

band structure of the solid, the scattering mechanisms, temperature, magnetic flux density,

and current and flux directions.

Use of the Hall effect in the study of semiconductor crystals [3, 4-] has been particularly significant since the charge carrier density in these materials depends on the

density and kinds of impurities in the crystal. Additional information concerning carrier

transport can be obtained if the Hall coefficient is combined with the resistivity. In the one carrier case the Hall mobility, u„, is the ratio:

l RHl

y H

= -Lf- , (3.2)

where p is the resistivity measured at zero magnetic flux density, and R

R

is measured in

the low flux density limit.

The Hall effect results from the combined action of electric and magnetic fields on the free carriers in a solid. In the most usual case, mutually perpendicular electric and

magnetic fields are impressed on a solid, and the charge carriers are deflected in the

third mutually perpendicular direction. If the current distribution is constrained to re- main unchanged when the magnetic flux is applied, as for example in a long, bar-shaped

specimen, a transverse potential difference, called the Hall voltage, is developed to oppose

the deflection of the charge carriers as a result of the build-up of deflected carriers at the edges of the specimen. The Hall coefficient is the ratio of the transverse electric

field due to the Hall voltage to the product of the current density and magnetic flux

density:

Rw

= E /J B , H y x z

where J is the current density along the length 'of the bar (the x direction), B^ is ^ the

magnetic flux density (in the z direction) and E is the Hall field (in the y direction).

The sign convention for the Hall effect is shown in Fig. 1. As expected from Eq. (3.1)

the sign for p-type semiconductors, in which the carriers are positively charged "holes,"
1 T) x H

a)

<D tH

Xi 4h P

rH C H

•rH (fl «

T3

a) a) •h x: H 4-> a

a «

CO c

o

XJ -a H P

0) o

•rH 0) Mn fc •H O 13) •H 1 ^ N P

O <D

a) x: H p

(D c

C -H

(0 >i

-C P

P -H

• rH W 3 C

0) T3 C X . O 3 a

•H H o P Mh •rH C P

(1) O o

> -H 0) C 4-> & O 0) •rH O C T3 M 1 C (0 >, m e

•rH 0) W X) J3 C P

+j rtf O C

Q) C •rH Mh O

4h -H M

0) P rH O (0 rH d) 0) H rH Cu

tfl tH ft I X) rt H

0)b3

bO •H

Ph
is positive while for n-type semiconductors, in which the carriers are negatively charged

"electrons," it is negative.

Many different specimen shapes can be used for Hall measurements [5]. Specimens used

in this experiment were ultrasonically machined bars of germanium or indium antimonide. The

specimens had two side arms on each side and enlarged end contacts as shown in Fig. 2. The geometry of these specimens conforms to the ASTM standard [5], In this configuration,

one can measure two Hall voltages (between arms 3 and 4 or between arms 5 and 6 and two

resistivity voltages (between arms 3 and 5 or between arms 4 and 6). In order to minimize

the contribution from misalignment and thermomagnetic effects, it is necessary to measure the

Hall voltages for both directions of current and magnetic flux [6] and take a suitable

average. It is also customary to measure the resistivity voltage with no magnetic field for

both directions of current.

At the minimum, the complete sequence of measurements includes the following: B(t),

I(+,+), V34(+,+), V56(+,+), I(+,-), V34(+,-), V56(+,-), B(-), K-,+), V34(-,+), V56(-,+),

I(-,-), V34(-,-), V56(-,-), B(0), 1(0, + ), V35(0,+ ), V46(0,+ ), 1(0,-), V35(0,-), and

V46(0,-). In these expressions, the first symbol in the parenthesis represents the condition

of the magnetic field; the second represents the condition of the current.*

Measurements of Hall coefficient at a single point are frequently used in routine

determinations of carrier density in order to deduce the impurity density in the crystal.

However, much more information can be obtained if the Hall coefficient is determined as a function of some variable such as temperature or magnetic flux density. The independent

variable in these experiments was the specimen temperature; the magnetic flux density was

fixed.

3 . 2 Apparatus

The contact areas of the Hall bar were tinned with indium-tin solder. Small diameter

wires were attached to the soldered regions for later use in connecting the specimen to the

electrical terminals in the holder. Fig. 3 shows the various parts of the specimen holder.

The Hall bar was soldered to a small copper block which in turn was secured to the holder.

The bar was electrically insulated from the copper holder by a 0.5 mm thick sapphire plate

which provided good thermal contact. A copper cover completed the enclosure for the specimen. An outer cylinder, with a 500-fi heater of manganin wire wrapped around it, surrounded

the enclosure. The specimen holder was connected by a long piece of thin-wall, stainlesssteel tubing to a header through which passed the electrical leads and thermocouples. This

assembly was lowered from above into the double glass Dewar system shown in Fig. H. The

header mated with the stainless-steel fittings above the inner Dewar flask to complete an enclosure which could be evacuated. The tail of the Dewar system was centered in the gap

of an electromagnet with 6-in. diameter pole faces. The holder was positioned so that the

magnetic flux was perpendicular to the two flat faces of the specimen.

Usually measurements were made with a flux density of 0.6 T, but it was variable up to

a maximum of 0.67 T with a 2. 5- in. gap between the pole faces. A commercial Hall generator

was mounted on one of the pole faces, and its Hall voltage, calibrated against a nuclear

magnetic resonance fluxmeter, was used to determine the value of the magnetic flux density.

The current through the magnet coils must be turned on and off and reversed during the

data collection. These changes were initiated by relay closures supplied by the command

decoder discussed in Section 5.8. These closures complete the circuit for the 110-V relay

coils which turned the magnet current on and off. The magnetic field was reversed by first

turning the current off, then reversing the contacts for the magnet current using a two-coil,

latching-type relay, and finally turning the current on. A time-delay relay prevented the

current from coming on during the 5 s required for the reversing procedure.

A combination of heating and cooling was used to raise the specimen holder to any desired temperature above that of the coolant. The liquid level was below the holder and its

normal boiling provided a steady efflux of cold gas which cooled the holder. The computeroperated temperature control circuit supplied sufficient power to the heater to increase the

5

'f It will be convenient later to replace these symbols with subscripts as listed in Table 3.

6
&p

<uso

Q)bOCe

•HO

0)ftW

DC

bO •H
Figure 3. Specimen holder. Hall specimen (A) was soldered to copper block (B) and

secured to holder with sapphire plate (C) providing electrical insulation.

Lead wires from each arm of the specimen were attached to respective pins

(D) for electrical connection to measurement circuit. Cover (at bottom)

was placed over specimen, and cylindrical jacket (at top) with heater

windings completed the assembly. Outside thermocouple (E) was attached to

metallized ceramic plate (F) on jacket.
Figure 4. Hall effect experiment with attached computer interface. Double Dewar

flask (A) was supported between poles of 6-inch electromagnet (B).

Connections between the specimen holder assembly and the external circuitry were made at the header (C). Equipment rack contained digital

voltmeter (D), reed relay scanner (E), special interface module (F),

digital scanner (G), teletypewriter interface (H), and control module

(J). Operator communicated with experiment through teletypewriter (K).
temperature of the holder quickly until the desired temperature was reached. Then the

heater current was reduced to that needed to balance the cooling effect of the cold gas

flowing past the holder. The control circuit compensated for slight variations in the cooling rate which occured during a run as the liquid level fell in the Dewar flask, and from

run to run as the flask "softened." A flask goes "soft" because helium gas diffuses through

glass and reduces the vacuum between the walls of the flask. This results in increased heat

transfer from the outside to the inside wall and a corresponding increase in the boiling

rate of the coolant

.

The normal evaporation of the liquid provided some cooling which corrected for slight

overshoot in the heating. For more cooling it was possible to reduce the pressure over the

liquid but it was found that a steady cooling rate was difficult to maintain by pumping, so

it was seldom done. However, if it is desired to lower the temperature of the sample

quickly, pumping is helpful as the cooling rate of the normally boiling liquid is fairly

slow. Also, blowing warm gas down the transfer line into the cyrogenic liquid in order to increase its boiling rate was found to enhance greatly the cooling rate. Passing current

through a small resistor in the bottom of the inner Dewar flask would serve the same purpose.

The mass of the specimen holder was sufficient for good temperature stability, but

small enough so it could change from one temperature to the next fairly rapidly. One

thermocouple was placed in the main part of the holder near the specimen and another was placed on the outside cylinder just above the heater windings. Because the holder was composed of several parts with the heater on the outside, the inside thermocouple was slightly behind the outside one in its response to the applied heat. The power was con- trolled to keep the outside thermocouple at the desired temperature and allow the inside one

to come into equilibrium with it.

Gold (with 2.1 atomic percent cobalt )-chromel thermocouples were used to determine the

temperature. The sensitivity is 11 uV/K at 10 K and increases to 50 yV/K at 100 K [7], A thermocouple from the same wire was calibrated by the NBS Temperature Section. Frequent

calibration checks at 4.2 and 77 K were made on the thermocouples in the system as it is

known that the gold-cobalt is not a stable thermocouple element [8], Gold-iron compositions

are more stable and more sensitive than gold-cobalt below 20 K, but unfortunately the output

of the gold-iron wire changes in a magnetic field [9], A germanium or carbon resistance

thermometer is a more appropriate temperature sensor below 10 K.

The electrical circuit used to measure the Hall and resistivity voltages is shown in

Fig. 5. The current source was a regulated power supply. The current through the specimen

was determined from a measurement of the voltage across a standard resistor which was variable in decade steps from 1 to 10 10 Q. Selection was made by means of a manual rotary

switch.

3.3 Adaptability to a Time-Sharing Environment

The relationships between Hall coefficient or resistivity and temperature are complex

functions which have many inflection points. The greatest amount of information can be deduced from experimental data if the points are taken sufficiently close together that the curves can be accurately determined. Because of the large number of measurements which must

be made at each temperature, the number of data points which can be taken during a run is

severely limited if the data are collected by hand. It is in this application that computer

control of the experiment is most valuable because both the time between points and the time

required to obtain the data can be significantly reduced.

The time involved in the operation of the Hall experiment are long compared with the

cycle time of the computer. For example, temperature need be sampled only every few seconds even during the time when the computer is controlling the changing of temperature from one stop point to the next. In changing magnetic field, delays of 10 s or more are required to permit the field to stabilize. A set of data is collected only every 5 to 10 min so detailed

computations are required only at infrequent intervals. Hence, this experiment is an ideal

candidate for use on a time-shared computer system.

Although the interest in the present experiment was restricted to Hall effect and

resistivity as a function of temperature, extension to determine other galvanomagnetic and

thermomagnetic effects would have been a simple and straightforward matter since all involve

10
CURRENT

REVERSING

RELAY

CURRENT

SOURCE

T.

i

R,

VOLTAGE

SCANNER

CHANNEL

6 6

VSR

4

6 6

V35

5

6 6

V46

6

6 6

V34

7

6 6

V56

8

Figure 5. Electrical circuit for measuring specimen voltages.

11
the measurement of voltages, either directly on the specimen as in the Hall effect or as the

output of thermocouples as in the Seebeck or Nernst effects [3],

4. THE EXPERIMENT/COMPUTER CONFIGURATION

4.1 Over-all Configuration

The Hall-effect experiment together with its associated hardware interface was con- nected to the NBS CORD (Computer with On-line Remote Devices) Time-Sharing System using a teletypewriter-rate communication line. The experiment was designed to interface to the

time-sharing system in a manner identical to more conventional remote terminals. Communication, CPU, and memory requirements do not exceed that normally encountered with usual

teletypewriter operation.

Figure 6 shows the over-all experiment/computer system configuration. The interface,

switch-box, teletypewriter and experiment shown in Fig. 4 are located on another floor and

approximately 30 meters from the computer system. Interconnection is made with a two-wire

shielded cable which allows full duplex communication between the experiment and the communications controller of the time-sharing system.

The switch box provides for either local or computer-controlled operation. When in

local mode , everything typed on the teletypewriter is transmitted to the interface and con- trol characters from the computer may be simulated. Output from the interface is printed on the teletypewriter. In computer-controlled mode:

a. Everything typed on the teletypewriter is transmitted to the experiment interface

and to the computer.

b

.

All computer output is both transmitted to the experiment and printed on the teletypewriter.

c. All experiment output is both transmitted to the computer and printed on the teletypewriter.

4.2 The Time-Shared Computer System

The NBS CORD Time-sharing System is designed to serve a limited number of remote devices

such as teletypes, display consoles, a flying spot scanner, and special experiments. The

system is built oh the MOBIDIC B dual computer system, which has been modified to meet NBS

time-sharing requirements . The two computers within the system have specific functions , the

executive for system control and the user for executing programs associated with remote terminal users. Core memory of the user computer is partitioned among the users active at any

given time and also includes a small supervisory program.

Figure 7 shows the connection of the experiment to the computer system via the communications controller. The controller, also shown at the right in Fig. 8 is a manually switched

device that allows interconnection of combinations of remote terminals with the executive

processor. This processor is responsible for communication with all remote devices, control

of the user processor, and access to backup memory.

Magnetic tape has been the primary backup memory; however, disc has now been added

along with a completely revised system for file storage. Storage for the Hall-effect experiment has been on magnetic tape, both for programs and data.

A general set of system commands is available to teletypewriter users for program

loading and execution as well as for file manipulation. Several language processors are constructed as subsystems within the general system framework. These include an assembler,

CAS (CORD Assembler), a compiler (similar to BASIC), and a list processing system, CL6 (a

conversational system based on Bell Laboratories Low Level Linked List Language). All subsystems interact with terminal users.

Programming for the Hall-effect experiment was done in CAS Assembly Language and program execution is controlled by the use of system commands at the teletypewriter associated

with the experiment.

12
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Figure 8. NBS CORD time-shared computer system

15
5. THE HARDWARE INTERFACE

5.1 Measurements Automation Modules

The hardware interface between the Hall effect experiment and the time-shared computer

system has been constructed utilizing a set of general modules for data logging and computer

interfacing. These modules are engineered for flexible interconnection and operate under

the asynchronous control of a supervisory module equipped with a patch board for input of

the control program. Each module is constructed with pluggable printed circuit cards and

contains its own power supply, indicator lights, manual controls, and standard connectors

for both control and data signals. This permits the convenient assembly of systems tailored

to the particular requirements of individual applications such as the Hall effect experiment.

5 .

2

Configuration of the Modules

The interface consists of four general purpose modules, one special purpose module, and

a commercial digital voltmeter. Figure 9 indicates how these modules are interconnected and

shows their relative position in the system.

All parameters and variables that need to be measured are available from the Hall

effect experiment as d-c voltages. This makes it possible to take all measurements with a single instrument, an auto-ranging digital voltmeter. The voltmeter is connected to the

experiment through a reed-relay scanner module. The output from the digital voltmeter is

converted to a bit-serial signal so that the data can be transmitted to the computer in

teletypewriter-compatible form. This conversion is accomplished in two steps. The digital scanner takes the parallel data word from the voltmeter and converts it to a serial string

of characters ; then the teletypewriter interface module converts each character to a serial

string of bits.

The hardware needed to decode control commands from the computer is contained in an interface module designed for the Hall effect experiment. Single-character codes sent by

the computer can start one of two preset scanning and digitizing sequences. Once a scan

code is received, the control module assumes its supervisory role; checking status signals

and issuing commands to the rest of the modules until all the voltage readings have been

transmitted to the computer.

Control of the magnetic field in the experiment (on, off, forward, reverse) is also

accomplished by single-character codes from the computer. In this case the control module

is not involved since the interface module serves directly as the command decoder and relay

driver.

The special interface module also decodes computer commands for control of specimen temperature. Two methods of temperature control have been used. The first employed a singlecharacter code to indicate a heater current pulse; the pulse duration and magnitude of cur- rent were preset manually. Experience with the system showed that this method was inadequate

so another method was developed in which current is supplied continuously to the heater but

the computer adjusts the magnitude of this current. This method requires two code characters

The first character indicates that the next character is to be processed as a heater current

value. The second character is then switched to a digital-to-analog converter which in turn

drives a programmable power supply that supplies current to the heater.

5.3 Reed Relay Scanner Module

The reed relay scanner module is a sequential multiplexing device which can scan up to twenty separate analog data channels and connect them to a common output line. The multiplexing is done through double-pole, single-throw, normally-open reed relays. The twenty

channels are arranged such that they can be separated into two independent groups which are designated SCAN A and SCAN B, both of which are under the direction of the control program.

The length of either cycle is continuously adjustable by means of selector switches, but the

combined length of both is limited to the 20 input capacity of the scanner. Scan cycle A always begins at input 1, and ends at the input selected by switch A. Scan cycle B always

begins at input adjacent to the end of Cycle A and ends at the input selected by switch B. The scanner will go to an error state and halt if switch B is set to an input which is within scan cycle A.

16
EXPERIMENT

r~

>Y A A A A

SPECIAL INTERFACE MODULE

Command Decoder"! Relay Drivers D"A Converter

:lv y y y y v

REED RELAY

SCANNER

MODULE

DIGITAL VOLTMETER

DIGITAL SCANNER

MODULE

TELETYPEWRITER

INTERFACE

MODULE

CONTROL

MODULE

TELETYPEWRITER

COMPUTER

Figure 9. INTERFACE BLOCK DIAGRAM

17
Each of the scan cycles is controlled by two commands: RESET SCAN, which readies the scanner to sample the first input of the cycle, and ADVANCE SCAN, which connects the digital

voltmeter to the next input signal, initiates a "convert" command to the voltmeter and

readies the scanner to sample the next input. If the next sequential input is outside the

preset limits, the scanner returns to the first input of the cycle. The ADVANCE SCAN- com- mand for cycles A and B can be the same signal, in which case the currently employed scan

cycle advances

.

5.4 Digital Voltmeter

The digital voltmeter is a commercially available unit in which measurements are made

by a dual-slope integration technique which minimizes the effects of long term drift in

component values in the measuring circuits. Manufacturer's rated accuracy is ±0.01 percent

of reading ±2 digits. Voltage ranges of ±0.1, 1.0, 10, 100 and 1000 V full scale are selected automatically, manually, or by remote control. Measurement time is 250 ms on all

ranges or 50 ms on the four high ranges, depending on the setting of a rear panel switch.

These measurement times do not include the time required for range changing in the automatic

mode which, in the worst case, is 2.2 s.

The digitizing is done with four full decades plus a fifth digit to allow a maximum

reading of 16 000 on all ranges. Parallel readout in 8-4-2-1 BCD code is proveded at rear

panel connector. Included in the readout is an indication of polarity and decimal point

location.

The conversion cycle can be triggered internally at a rate governed by a front panel

control or externally by a command pulse. When the conversion cycle is in progress, a con- trol signal indicates that the unit is busy; when the cycle is complete, a "print" pulse is

generated.

5.5 Digital Scanner Module

The digital scanner module is a multiplexing device for connecting parallel digital

data sources to serial-by character output devices. It can switch sequentially up to sixteen

inputs of eight lines, each to a common set of eight output lines. The switching is done by

solid-state digital circuitry built from an NBS family of circuit cards [10].

The scan cycle is always sequential but its beginning and end points can be selected by

front panel switches to any of the channel numbers, through 15. If the channel number for

the start of the cycle •is higher than the end number, the scanner will advance to channel 15,

back to 0, then on to the end point. The scanner will continuously select one channel if

"Start" and "End" are set to the same number. The scanner is controlled by two commands:

SCAN, which sets the scanner to the "Start" channel, and ADVANCE, which steps the scanner to the next channel in the sequence.

5.6 Teletypewriter Interface Module

The system described in this report uses an eight-bit code for each character. The

maximum transmission rate between the experiment and the computer is ten characters per

second. The signal that is transmitted and received is a serial stream of eleven bits for

each character: eight data bits preceded by a start bit and followed by two stop bits.

Characters may be sent to the teletypewriter asynchronously at any rate up to the maximum of

10 per second but the timing of the bits within the character is fixed and must be carefully

controlled. However, machines with different transmission rates can be accommodated by the

interface module.

The signals described above differ considerably from those of most other digital instruments and devices used in system instrumentation. The latter typically generate data

with four bits per character and the bits are made available simultaneously on parallel

lines. The conversion of the parallel lines data to serial form is the primary function of

the teletypewriter interface module.

Code conversion is another requirement for the interface. Generally the parallel data

is binary-coded decimal (BCD), while the teletypewriter accepts only the USA Standard Code

for Information Interchange (USASCII). This module can convert any four-line code (sixteen

characters maximum) to a subset of the USASCII code.

18
The module may be connected to the serial data line in much the same way as the teletypewriter is connected, i.e., half duplex (send and receive units in series) or full duplex

(separate lines for send and receive). A current of 20 mA for the signal line is supplied

by the module.

The module has an eight-bit (one character) buffer register which is essential to both

input and output operations. A LOAD command pulse causes parallel-line data which has

passed through the code conversion network to be loaded into the register. The loading

operation is followed automatically by a serializing operation which puts the code on the

serial line. Incoming signals appearing on the serial data line are loaded into the buffer

register by a shifting operation. The contents of the register are retained until a new

character appears on the line or until another output command is executed. For test purposes, the register may be loaded manually by front panel pushbuttons. A TRANSMIT command

initiates the serializing operation without loading new data into the register.

When using a teletypewriter as a data recording device it is usually necessary to insert certain format control characters into the data in order to obtain a readable copy on the printer. For this reason, provisions are made for forcing codes into the data register

without going through the normal data inputs. Three codes may be transmitted this way and

the code transmission is initiated by command pulses. One of the command inputs will give

two characters in succession, CARRIAGE RETURN and LINE FEED. Another command will produce

LINE FEED only and a third will produce a SPACE. Other codes may be used in place of those

given above; coding is done with a patch panel on the rear of the module.

Paper tape readers are included as a part of many teletypewriters, and in some systems

it is necessary to be able to start and stop the reader by remote control. The interface

module provides for this control in two ways: either one character will be read when a READ

command pulse is received or a START READ command may be used to cause the reader to run un- til a, STOP READ command is received.

The interface module generates a variety of busy signals to inform the system controller

of its status. Termination of these signals is used to initiate each new action by the

system controller. MECHANICAL BUSY is a negative level that is present for 0.1 s, starting

when a character begins to be serialized out or in; this could be the result of a LOAD,

TRANSMIT, READ, or special format command or the start of a signal coming in on the serial

line. REGISTER BUSY will begin at the same time as MECHANICAL BUSY but will end as soon

as the buffer register is quiescent which will be at least 18 ms prior to the end of the

inception of a LOAD Command or one of the special format character commands until the

loading of the register has been accomplished. READ BUSY is a negative level present

from the inception of a READ or START READ command until the required reading has been accomplished.

5.7 Control Module

The control module exercises supervisory control over the other interface modules

.

Depending on the status of the system, as indicated by signals on the condition lines (busy

signals, time signals, switch positions), the control module generates "do" commands to the

other modules in the system.

The actions of the control module are governed by a program which can be stored wither

on a plugboard or on matrix cards which can be inserted in the control module card cage.

The plugboard is employed when frequent changes in the control program are expected, such as during initial programming. The fact that the plugboard is easily changed can also become

a handicap since it means that the program can be inadvertently changed. Therefore, as soon

as the program is debugged, it is transferred to matrix cards.

The control module performs an instruction every 50 us. The instruction is:

Energize indicated "output lines" (the "do" command);

Test , simultaneously, the selected conditions; and

Go to Next Step as determined by the state of the selected conditions.

The program specifies two "next steps"; the control goes to one when the selected conditions are met , and to the other when the

selected conditions are not met.

19
The control module has a capacity for sixteen instructions, eight commands, and fourteen condition lines

.

Also available in the control module are various control elements , including three J-K

flip-flops, two adjustable one-shots, and an adjustable-RADIX, four-bit counter.

5.8 Special Interface Module

The modules described above are all general purpose modules, that were designed without

a particular system in mind but with sufficient flexibility to allow their use in many data

gathering systems. Nearly every system has its own unique requirements. These requirements

for the Hall effect experiment are handled by the special interface module.

One major function contained within this module is impedance matching and voltage level

shifting required for the digital voltmeter. Another function is that of command decoding

of characters received on the teletypewriter line from the computer. Decoding is done by a diode matrix whose inputs are connected to the outputs of the buffer register in the teletypewriter interface module. The outputs of the decoder are used to drive relays, signal

the system control module, or initiate other control sequences in the interface module. The

following is a summary of the USASCII codes which are decoded, their function and the method

used to accomplish the function:

" (Quotation Mark) Magnetic Field On/Off. The decoder output line drives a relay

that energizes switches which control the magnetic field.

! (Exclamation Point) Magnetic Field Direction. This function is accomplished in the same manner as Magnetic Field On/Off.

# (Number Sign) Pulse Heater. The decoder output line drives a DPDT relay via a monostable multivibrator with an adjustable run time. Current may be supplied to

the heater through the relay contacts.

( (Left Parenthesis) Heat Code Coming. This decoder output arms control circuits so that the next character received over the teletypewriter line will be stored and

gated to a digital-to-analog converter. The resulting analog voltage is used to

control a programmable power supply which supplies current to the heater.

$ (Dollar Sign) Read All Data. This decoder output is connected to the system con- trol module to cause the controller to initiate a SCAN A sequence. This causes all

data channels to be sampled.

% (Percent Sign) Read Temperature Only. This causes the controller to initiate a SCAN B sequence' which reads only the two temperature channels.

All control characters sent by the computer to the interface are printing characters

so that they may be observed on the teletypewriter print-out. Teletypewriter output con- tains a complete record of all transmissions to and from the experiment as well as the

results of calculations of each stop point. Formats for all data transfers have been se- lected for ease of computer input and output and for convenience formatting on the teletypewriter printout. The selected special control characters are unobtrusive, but available if

needed.

There is one DPDT relay in the special interface module which is controlled by command

from the system control module. The function of the relay is to reverse the direction of

current through the test specimen. The relay contacts are used to control toggle and latch

relays that control specimen current.

5.9 Control Program

The right half of Fig. 10 shows the details of the oontrol module and its program, and

the remaining modules with their control connections are shown on the left. Control program

steps are indicated by rectangles. Each rectangle is divided into three parts: the righthand part indicates the number of the step, the upper left-hand part indicates the "do"

commands which are energized, and the lower left-hand part indicates the selected conditions

to be tested. Transitions between steps are indicated by lines which emanate from bottom

corners of each step rectangle, indicating the two transitions which are possible from the

step.

In the quiescent state of the system, the controller is in a "wait" loop consisting of

step and step 10. Step checks for the presence of a "Read All Data" code and step 10

checks for the presence of a "Read Temperature Only" code.

20
FROM

4

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SCANNER

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DIGITAL VOLTMETER

SCAN BUSY

1

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»»T0 STEP 2

Figure 10. CONTROL MODULE PROGRAM

21
Assume that a "Read All Data" code is received. The program goes from step to step 1 where a command is issued to the reed relay scanner to put it in the SCAN A mode. Step 1 is

followed by step 2 which sends an ADVANCE command to the scanner, causing it to sample the

first channel in the SCAN A sequence. The scanner responds with a busy signal which will

remain until the digital voltmeter has measured the voltage on this first input. Another

condition line to step 2 checks the status of the teletypewriter. The presence of a busy

signal on either condition line causes a temporary halt condition because the program goes

from step 2 back to step 2. When the voltmeter has finished its task, and if the teletypewriter is not busy, the program advances to step 3, which starts the cycle of the digital scanner module. Again, there is a temporary halt while this scanner is busy. The

digital scanner switches the data from the digital voltmeter to the teletypewriter interface module one character at a time. The control signals which cause the digital scanner to

advance and the teletypewriter interface to load and transmit are not generated by> the con- trol module in this case. Instead, there is interaction of busy signals between the scanner

and the teletypewriter interface which causes the process to proceed at its own pace without

superivsory control.

When the digital scanner has completed transmission of the last digit of a measurement

to the teletypewriter interface, the program proceeds to step 4 where it checks to see if

the last channel of the SCAN A sequence has been reached; if not, step 4 is followed by step

6 which is a similar step for SCAN B. Since SCAN B has not been set (we are still doing

SCAN A), the program will branch back to step 2 which will advance the reed relay scanner to

the next channel. This loop, steps 2, 3, 4 and 6, will be repeated until the last channel

of SCAN A has been measured, then the program will go from step 4 to step 5, where it waits

for the teletypewriter to finish printing, then on to step 7. Step 7 sends a CARRIAGE RE- TURN/LINE FEED command to the teletypewriter interface module and waits for that command to

be accepted before proceeding to step 8. Step 8 checks a control flip-flop, the significance of which will be explained in the next paragraph. For the case under discussion,

step 8 will be followed by step 9 where the current through the Hall specimen is reversed.

From step 9 the program returns to the quiescent "wait" loop, step and step 10.

Now consider the case where a "Read Temperature Only" code is detected. This time the

program leaves the "wait" loop by going from step 10 to step 11. Step 11 sets the reed re- lay scanner to the SCAN B mode and .sets a control flip-flop so that the controller will

"remember" that this is .a SCAN B operation. From step 11 the" program goes to step 2 and

through the rest of the program in the same manner as it did for SCAN A. It will branch out

of the 2-3-4-6 loop from step 6 into step 5 when SCAN B is finished. The one major difference in this sequence is the branch that is made in step 8. This is the step that tests the

flip-flop set in step 11 and causes the specimen current direction not to be changed after

this type of scan. Therefore, the program branches back to the quiescent "wait" loop without going to step 9.

5.10 Assembly of the Interface System

Most of the engineering effort for the Hall effect experiment went into planning the

system configuration and the design of circuits for the special interface module for the

Hall effect experiment. Before interconnecting the modules, they were tested on the bench as individual units. Since each of the general purpose modules has its own power supply and

manual controls, the testing can be quite thorough, yet complex test jigs are not needed.

Only simple tests could be performed on the special interface module this way because of its

unique relationship to the experiment. The next step was to mount the modules in a rack and

interconnect them. Some of the interface system programming was done at this point because

selection of cable connections and rear panel patching is part of the programming process.

The control module was initially programmed by inserting pins in a matrix plugboard; whenthe system had been completely installed and tested, the program was wired on printed

circuit cards which were inserted into the module, replacing the matrix plugboard.

The assembled system underwent more tests before it was connected to the experiment.

During these, the teletypewriter was used to generate the command characters which normally

would come from the computer. The same testing method was used again after the system was connected to the experiment. The connection to the computer was the final step in the process; from this point on, further testing depended on the computer system, including the

special software developed for the Hall effect experiment.

22
6. SOFTWARE DESIGN

6.1 Over-all Software Design

The software directly supporting the Hall effect experiment consists of a user program

written in CAS Assembly Language that runs in the user processor of the time-shared system

during the time slices allocated to the experiment. The program uses system commands for input and output and for conversion from character input to floating point internal form. It also uses the floating point software package that is an integral part of the user processor

supervisor.

Figure 11 shows the general program flow for a run of the Hall effect experiment. The

parameters for the run include the specimen geometry and a set of stop points for thermocouple voltage representing temperature values. The specimen is brought to the temperature

corresponding to the first stop point and a full set of data is taken from the experiment.

During the data input sequence the heater current, which controls specimen temperature, is

adjusted to minimize temperature drift.

The magnetic field, resistivity, Hall coefficient, and Hall mobility are then computed

for this stop point. These results are printed on the local teletypewriter, and, if the

operator has specified storage of raw data all of the input data are stored on magnetic tape.

If there are additional stop points, the control program is set for the next one and the

temperature control cycle begins again. When the last stop point has been reached, the program halts after printing an appropriate message on the teletvDewriter.

6.2 List of Symbols

a heater current calculation parameter

b c

'^ heater unit calculation parameter

coolant index (0, LN2; 1, LHe)

CONTROL numerical value of one of the characters [!, ", $, %, (] used to control

CHAR. experiment

d']_ distance between specimen arms 4- and 6 d2 distance between specimen arms 3 and 5 Dp ptn set of stored data (see Table 3) F magnetic tape file name

HCC heater control character

i index

Ipj heater current (mA) IHP Hall generator current

IHPi IHP of ith stored data set (same for VHP, VSR, V46,...)

k index on stop points (1 f k < n, n number of stop points)

L magnetic field off flag

MTVC ^ maximum thermocouple voltage as a function of coolant (c) and termperature

range (i)

OUT save flag (0 or 1) p index on data sets

R„ Hall coefficient calculated from voltages measured between specimen arms

5 and 6

Rfl' Hall coefficient calculated from voltages measured between specimen arms

3 and 4 %iav average Hall coefficient

RN record number of file F Re standard resistor (used for measuring specimen current)

t

s specimen thickness

VHP Hall generator voltage

Vj inside thermocouple voltage

Vj av average inside thermocouple voltage

Vj^ k stop point voltage

Vq outside thermocouple voltage

VSR voltage across standard resistor (specimen current)

V35 resistivity voltage (see Fig. 5)

V46 resistivity voltage (see Fig. 5)

V34- Hall voltage (see Fig. 5)

V56 Hall voltage (see Fig. 5)

Wg specimen width

23
(

STflRT

)

i

INPUT RUN PARAMETERS

SET FOR FIRST STOP POINT

BRING SPECIMEN TO CORRECT TEMPERATURE

INPUT DATA FROM EXPERIMENT

Compute

-MAGNETIC FIELD -RESISTIVITY

-HALL COEFFICIENT

-HALL MOBILITY

PRINT RESULTS OF COMPUTATION

FOR THIS STOP POINT

STORE RAW DATA

FOR THIS STOP POINT

ON MAGNETIC TAPE

SET FOR

NEXT STOP POINT

Figure II. GENERAL PROGRAM FLOW

24
X 1 -10.45 -12.90

1 - 9.44 -12.45

2 - 6.68 - 11.40

3 - 3.71 - 10.00

4 — —

_ f0,LN2 coolant

c "ll , LHe coolant

i = temperature range index

(all voltages in mV)

Table I. MAXIMUM THERMOCOUPLE VOLTAGES (MTV) FOR EACH TEMPERATURE

RANGE AS A FUNCTION OF COOLANT

a:

K 1 0.56050 0.1060

1 0.2 600 0.1060

2 0.2172 0.1160

3 0.1954 0.1260

4 0.1954 0.1360

X 1 0.04770 0.0059

1 0.01710 0.0059

2 0.01000 0.0059

3 0.00674 0.0059

4 0.00526 0.0059

ft

LN2 coolant

LHe coolont

i = temperature range index

Table 2. HEATER CURRENT PARAMETERS a and b AS A FUNCTION OF COOLANT

AND TEMPERATURE RANGE

25
Vif-j Hall mobility

p A resistivity calculated from voltages measured between specimen arms 4 and 6i pg resistivity calculated from voltages measured between specimen arms 3 and 5

p average resistivity

6,3 Subroutines

Full understanding of the detailed program description in the next section requires a description of two basic subroutines. They are:

(a) WAIT (n). This routine uses the system clock to delay n seconds

before continuing the program.

(b) CALCUR (m,n). Given m, the present thermocouple voltage, and n, the desired

thermocouple voltage, the heater current necessary to approach or maintain

the desired thermocouple voltage is calculated. This value is converted

into the appropriate character code and is transmitted to the experiment.

Details of the heatejr current calculation subroutine are shown in Fig. 12. The

primary entrance, CALCUR Cm,n), requires as input the present and desired thermocouple

voltages. An alternate input, CALCUR' (n), requires only the desired thermocouple voltage.

The routine then reads the temperature data directly from the experiment and uses this to

set m.

There are two thermocouples, either of which may be used for the value of m in calling

CALCUR. These are labeled Vj and Vq , for inside and outside thermocouples, The inner one

is located near the specimen and the outer one is external to the specimen holder, but near

the heater. In general, equilibrium at a stop point is reached when the inner thermocouple

is within 15 yV of the desired value and the difference between the inner and outer thermocouple values is less than 40 yV.

For the CALCUR 1 program entrance the desired temperature data is read by transmitting

the [%] control character to the experiment and then executing a character string input

command. The values of V-r and Vq are transmitted back to the computer and this transmission

is terminated with a carriage return character. The carriage return causes termination of

the input command executed by the program. V-j- is then coverted into internal floating

point form and stored in m.

The contents of m is next compared to the maximum thermocouple voltage for each of four

temperature ranges. Table 1 shows these maxima, where i, the index on the ranges, varies

from to 4-. There are separate sets of ranges, one for liquid helium and one for liquid

nitrogen coolant. All thermocouple voltages algebraically greater than -3.71 or -10.00,

depending on range, are in range 4. This table was derived by examination of the plot

of thermocouple voltage as a function of temperature. A set of straight lines that approximate the curve in five different regions were identified graphically. The table values are the maximum thermocouple voltages for each temperature range as a function of coolant.

The computation of heater current, 1^, is based on present and desired thermocouple

voltages. This computation uses a set of empirically determined coefficients which depend

on voltage range and coolant (see Table 2). The first term, ac ^, introduces a constant

current for each temperature range. The second term, n • b

c ^, subtracts a correction for

the specific thermocouple voltage desired within that range.' At equilibrium, the current

specified by these two terms should balance the effect of cooling by effluent gas and hold

the temperature constant. The contribution of the third term is proportional to the difference between present and desired temperature. This term is especially significant when

moving from one stop point to the next since it adds a varying amount to the heater current,

thereby bringing the specimen holder to the desired temperature quickly, but with little

overshoot. The term is also important to compensate for drift during the data input sequence.

During system design several approaches for temperature control were suggested. The

initial one was simply to output a control character which initiates a fixed length pulse

to the heater. Since the effective heat added to the specimen is a function of the integral

of heater current over a period of time, varying the pulse frequency would provide limited

temperature control. It was found that the constantly varying heater current disturbed

measurements being made on the Hall effect specimen. It was also found that drift during

data input was too great and, therefore . a more sophisticated controller was required.

26
f CALCUR'(n) J

fCALCUR(m,n) ^

READ TEMPERATURE DATA Vr

INTO m

:3C

I =

» i = i+ I

*H = °c,i— n bc ,i ~ (m -n) x IO :

Ih = 160 Ih =100

HCCs

[ 0.00625 J

HCC = HCC+ I

TRANSMIT HCC

TO EXPERIMENT

C RETURN

J

Figure 12. HEATER CURRENT CALCULATION SUBROUTINE

27
Hardware was then implemented to allow 256 increments of heater current as specified

by a single 8-bit ASCII character from the computer system. This control mechanism was first used with the first two terms of the equation for It, shown in the flowchart; however,

the time required to reach each stop point was much longer than was considered acceptable.

Drift during the data input sequence was also unacceptable. The third term in the equation

was added and its coefficient was adjusted empirically so that the time required to move between stop points is acceptable, yet the overshoot introduced by the thermal inertia of

the specimen and its environment can easily be corrected and the temperature brought to

equilibrium for the data input sequence. Additional sophistication, such as the addition

of a second order derivative term, was not found necessary for successful operation of the

experiment

.

If Iy is negative, then HCC, the heater control character, is set equal to 0. If c = (coolant is liquid nitrogen), then a maximum heater current of 160 mA is allowed. If the

coolant is helium, the maximum is 100 mA. The heater current is then translated to an HCC

value of from to 256. HCC is the numerical code of the character to be transmitted to the

experiment. HCC is then compared with the list of control characters. If HCC is a control

character, it is incremented by one unit and then checked again. This check is necessary

since the hardware interface at the experiment does not turn off its control character

detection circuit while it is receiving the heater control character.

Finally, HCC is transmitted to the experiment in the following manner. First, the

character [(] is transmitted to the interface, this indicates that the next character

transmitted is the heater control character. HCC is then transmitted and the interface sets

the new heater current and holds it constant until the next HCC value is received.

6.4- Program Details

6.4.1 Initialization

. As shown in Fig. 13, the thermocouple stop points V-,, V^,..., V are first entered by

the operator of the experiment using the local teletypewriter. Conversational parameter

entry continues with input of the value of the standard resistor, R , used to compute

specimen current, followed by input of specimen geometry: specimen width and thickness and

distances between contacts . Entry of the coolant type , LHe or LN 2 is followed by a query

for setting the data save mode. If the raw data is to be saved, the tape file record number

is set to 1 and the tape file name is entered by the operator and stored in F. The index

stop point index, k, is set equal to 1.

6.4.2 Temperature Check and Set Up for Data Input

Temperature data is read from the experiment in a manner identical to that described

for CALCUR' . Figure 14 shows that the absolute magnitude of the inside thermocouple voltage

is compared to the desired stop point voltage. If this difference is less than 15 uV the

magnitude of the difference between inside and outside thermocouple voltages is then checked.

If this difference is less than 40 uV, adequate stability has been achieved at the new stop

point and the program continues. If either test fails, CALCUR is called using the outer

thermocouple voltage; then a delay of five seconds is introduced to wait for this heater

current change to take effect and the temperature is again checked.

Figure 15 details the remainder of initialization and the beginning of the data input

sequence. Flag L, used to initiate signalling the operator if the magnetic field is manually

turned off, is first set to 0. CALCUR is called, this time using the inside thermocouple

voltage Vj . The field is then turned on using the ["] control character. A delay of 12 s is triggered by a [$] control character and results in input of Vj , inside thermocouple voltage, VHP and IHP, for Hall generator voltage and current (used for measuring magnetic field),

and VSR, voltage across the standard resistor used to measure specimen current. Four voltages across terminals on the specimen are also measured. The absolute magnitude of the Hall

generator voltage is next checked to determine if the field is actually on. If the magnitude of VHP is not greater than 5 mV, Flag L is set to 1 and the loop continues with another

call to CALCUR. Every time this loop is re-entered when Flag L equals 1 a message is printed

on the teletypewriter and a 5 s delay is introduced to allow the operator to turn on the

magnetic field manually.

28
f START J

INPUT THERMOCOUPLE

STOP POINTS V, ,V2

, Vn I

INPUT STANDARD RESISTOR

VALUE Rs

INPUT SAMPLE GEOMETRY

wS.rS. d 'l > d 2

INPUT COOLANT TYPE

C =

C=l

INPUT RAW DATA SAVE MODE

OUT =

OUT « I RN * I I

INPUT FILE NAME

I

k= I §

Figure 13. INITIALIZATION

29
READ TEMPERATURE DATA Vj-.Vq

WAIT (5)

I

«rALCUR(V Vk)^#-

Figure 14. TEMPERATURE STABILITY CHECK

30
PRINT bells + PLEASE

TURN ON MAGNETIC FIELD

/ WAIT(5) \

READ DATA Vi, VHP, IHP,

VSR, V34,V56,V35,V46

L= I

REVERSE FIELD

N4- WAIT(IO)

0—'-

Figure 15. INITIALIZE FIELD AND SPECIMEN CURRENT

31
EACH DATA SET, Dp, CONSISTS OFA SET OF DATA VALUES:

Vj ,Vhp,IhP,VSR,V35,V46,V34,V56

p FIELD CURRENT

1 + +

2 + -

3 - +

4 - -

5 +

6 ^

Table 3. ORGANIZATION OF STORED DATA

i e f

1 111930 294

2 111930 294

3 1 1 1 800 285

4 111800 285

Table 4. FIELD COMPUTATION COEFFICIENTS

32
If the> magnitude of VHP exceeds 5 mV, the direction of the field is checked. When VHP

divided by IHP is less than 0, the field is negative with respect to an arbitrary convention

and must be reversed. Reversal is achieved with the [!] control character and is followed

by a delay of 10 s to allow the field to stabilize. At this point, if V34 is less than 0,

the specimen current is not positive with respect to an arbitrary convention and the program returns to point II. Note that every use of [$], the read data control character, reverses the specimen current; therefore, the next time the test for current direction is

reached V34 will be greater than or equal to 0, indicating a positive current. Once the

field is on and in a positive direction and. the current is in a positive direction, the

data input sequence begins.

6.4.3 Data Input

The first data set, for positive field and positive current, has been read during the

initialization cycle. Figure 16 begins with storage of this data set with label D-, . Table 3 outlines storage of the six data sets taken during a run. These sets are indexed

by the variable p and consist of all combinations of positive, negative, and zero fields

with positive and negative specimen current.

CALCUR' which inputs, computes, and transmits the drift-compensating heater current is

then called. After a delay of 5 s for this change to take effect, a new data set is taken

and stored as set D~.

After field reversal, a delay is necessary to wait for the field to stabilize. During

this delay, two calls on CALCUR' are made to minimize temperature drift and then data set

D~ is read and stored. Note that data set D„ is for positive specimen current. Following

input of Do, however, specimen current is negative and, following another CALCUR' and a delay of five seconds, data is read and stored as data set Dm.

The program continues in Fig. 17 with reversal of field in preparation for the next

run followed by a delay which includes a call to CALCUR' . The field is then turned off and

the following delay also includes a call to CALCUR' . Data is read and stored as data set

Dg and a final call to CALCUR' and a delay of five seconds precedes the input and storage

of the last data set, Dg.

6.4.4 Calculations and Completion- of Run

The magnetic flux density is computed from the Hall generator voltage and current for

each of the first four data sets. Figure 18 shows this computation and Table 4 lists the

coefficients used in field computation.

Two resistivities p A and p B are computed as shown in the flowchart, the Hall coefficients Rfl and RH

' are computed by the formulas shown in the flowchard, and these are aver- aged to compute Ru . The four magnetic field values are averaged and the Hall mobility

and average thermocouple voltage during data input are computed.

Figure 19 shows the printing on the teletypewriter of the various computed values. If

the output flag OUT has been set to 1, all six data sets are stored in record PN of file F,

the record number is increased, and a "data save" message printed. Otherwise a "data not

saved" message is printed.

If there are additional stop points , the program continues at point I . When the last

stop point, n is reached, an HCC of is transmitted to turn the heater current off, and

"run completed" is printed on the teletypewriter. The program then halts.

7. SYSTEM OPERATION

7.1 Experimental Procedure

The first step in preparing for each run was to transfer liquid helium into the precooled Dewar flask. For a morning run the cryostat was precooled with • liquid nitrogen overnight. During the helium transfer it was helpful to read the voltages of the thermocouples

attached to the specimen holder to see how the transfer was progressing. This could be done

on the local mode by typing the [%] control character on the teletypewriter. Usually the

transfer was stopped when the helium level reached the specimen holder as the first data

33
STORE DATA SET D I

(CALCUR(Vk )) I

WAIT(5)

READ DATA

±

STORE DATA SET D2

REVERSE FIELD

^CALCUR(Vk n

WAIT (5)

READ DATA

STORE DATA SET D3

I

XALCUR(Vk r

WAIT(5)

WAIT (5)

CALCUR(Vk )]

READ DATA

STORE DATA SET D4

WAIT (2)

Figure 16. DATA INPUT

34
©

REVERSE FIELD

WAIT (5)

I

'CALCUR(Vk)j X

WAIT (5)

TURN OFF FIELD

WAIT(5)

I

fCALCUR(V„r

©

WAIT (2)

READ DATA

STORE DATA SET D5 I

TALCUR(Vk3>

WAIT (5)

I

READ DATA

STORE DATA SET dTH

Figure 17. DATA INPUT

35
i - 1

-*

\f

"s

a

(VHPi)iej) . . Di - + T

i

IHPi

<^ i = 4 v N i= i +1 J

/ V46 6 V46« v ( Rs ) (WS ) (ts ) Pa~[ VSR5 "vSR6 / 2(d,')

/^ . / v355 _ v356 \ ( Rs) (ws) (ts) B " \ VSR5

~ VSR6 ' ~ 2(d2 )

/>OV=£(/>0+/>b)

I

RH = (2.5xl07 )(R s

t s )

RH=(2.5xl07)(R s

t s )

r V56, V56,

(B,)(VSR|) (B2 )(VSR2 )

V34, V34,

_(B,)(VSR,) (B 2 )(VSR2 ) (B4KVSR4) (B3HVSR3)

V564 V563 "J (B4MVSR4) (B 3 ){VSR 3 )J

]

Rhov- i (Rh + Rh)

V

B=^ 2 Bi

i = l I

**- -p

IRaayi OW

Vlov s 6 SVli

Figure 18. CALCULATION OF FIELD, RESISTIVITY, HALL COEFFICIENT, HALL MOBILITY, AND

AVERAGE THERMOCOUPLE VOLTAGE

36
PRINT Aiv.^/VRho^Rh.Rh',

0- k =k + I

STORE RAW DATA D,, D 2 ,....D6

IN FILE F, RECORD RN

RN = RN + I

PRINT DATA SAVED

TURN OFF HEATER *

PRINT "RUN COMPLETED"

i f C HA LT )

PRINT DATA NOT SAVED

Figure 19. PRINT RESULTS OF CALCULATIONS, SAVE DATA, AND CHECK FOR COMPLETION

37
point desired was about 15 K. Sometimes helium was transfered above the holder so a calibration check at 4.2 K could be made on the thermocouples.

Next, the specimen current was turned on and adjusted to the desired value. The magni

tude was that necessary for an electric field of 0.01 to 1 V/cm in the specimen. Readjustment was often necessary when the temperature for the first data point was reached. A standard resistor was chosen which gave a voltage reading of at least three significant

figures for VSR. The standard resistor could usually be kept at some fixed value, but if

the current was very small, a larger resistor than normal was needed. If the resistor was changed during the run, it was inconvenient to stop the data taking to type in the new value

since all of the information called for must be reentered, so the output affected was just

scaled by the appropriate power of ten. At low temperatures it is important to keep the

voltage across the specimen small in order to avoid hot-carrier effects . If the specimen

resistance changed significantly during the run, the current was adjusted manually to maintain sensitivity and avoid heating.

As a check, specimen data were obtained in a local mode of operation by entering the

appropriate control characters on the teletypewriter. Of course there was no computation,

but from the values of the voltages which were printed on the teletypewriter it was possible

to determine that the experiment was operating satisfactorily.

Time-sharing operation began by switching to the computer mode, logging in, typing in

the thermocouple voltages corresponding to the temperatures desired, and answering the

questions posed by the computer concerning the standard resistor, specimen dimensions, and

coolant type. Normally about 20 data points were taken with liquid helium as the coolant.

This usually required 1-1/2 hours . Sometimes it was desirable to repeat a point because of

a poor choice of specimen current, temperature flucuations, or other reasons. This could

be done before the intermediate output was printed by striking the. abort key on the teletypewriter and then typing the appropriate command. To repeat the point at a later time

involved reentering that thermocouple voltage along with any others remaining to be done and

answering the questions again. Since the sample dimensions do not change, it would have

been more convenient to answer these questions before typing in the voltages and then have

an alternate entry in the program after these questions.

Several times measurements were made above 200 K with good temperature control using

helium as the coolant. However the usual procedure was to boil away the liquid helium

rapidly with warm helium gas and then fill the inner Dewar flask with liquid nitrogen. The

next set of thermocouple voltages was entered with at least one point overlapping the helium

set. It took about 15 min to switch coolants and resume data collection. About 2 hours were needed to obtain 18 data points from 80 K to near room temperature. For example, it

took 5 min to change temperature from 125 K to 135 K and 2 min to obtain the data and print

the intermediate output.

It was desirable to have an operator in the room with the equipment so that any difficulties could be quickly remedied. Sometimes an error occured which stopped the computer

control of the experiment. A message was automatically typed, following which the operator

must respond with a command in order to restart the operation. In other situations com- puter personnel had to be notified of terminal troubles , as the run was usually not monitored at the computer. Also there were occasions when the specimen holder would not reach the

stop point temperature. It was possible for the operator to increase the current through

the heater by entering certain control characters on the teletypewriter. In instances where

the temperature stabilized above the stop point, the heater power supply could be turned off

to cool the specimen so that another approach could be made to the point. The operator had

2 to 3 hours during the run that he could use for other activities , such as plotting data

from previous runs

.

7.2 Limitations of the System

The 1 Gft input impedance of the digital voltmeter limited the measurements to source resistances of less than 10 Mfi for loading error of less than 1 per cent. Specimen contact

resistance must be included when determining the loading effect. Since the calculations

involve ratios of voltage, it is possible to reduce the error by selecting a standard re- sistor which is about the same as the source resistance. By using an electrometer in front

of the digital voltmeter, measurements can be made on specimens whose resistance exceeds the

input impedance of the digital voltmeter [11].

38
Acceptable data could be obtained at temperatures above 20 K. Below that three factors

limited the accuracy of the results. First, the thermocouple voltage could only be read to four significant figures by the voltmeter. Second, the sensitivity of the thermocouples is

less thao at higher temperatures. Last, below 20 K the specimen holder responded very

quickly to a change in the heater power, and temperature flucuations in phase with the con- trol corrections resulted unless the power was almost exactly right for that temperature.

Finer steps in the power and a shorter time between adjustments would have helped, but in

view of the other limitations it was hardly worthwhile to refine the control.

7 . 3 Results

More than 20 measurement runs were made on germanium and 10 on indium antimonide specimens during a period of 4 months. It was possible to measure a specimen each day, since

less than 2 hours were needed to put another one in the holder.

The print out of the calculated values immediately after the measurement sequence was a very useful feature. One could see at a glance if the results were reasonable and, if not,

take corrective action in time for the next point. Since the quantities were available for

plotting immediately, trends could be followed as the run progressed.

Reproducibility of the data obtained by computer-controlled operation was checked at

77 K by repeating the point many times. In each instance the values were the same to four

significant figures. The computer results were in good agreement with those obtained manually. A partial run was made on the specimen for which manual data was available and the

resistivities agreed within 1 per cent and the Hall coefficients within 2 per cent. The thermocouple was read only to the nearest 10 yV with a possible 2 digit or 20 yV error. This gives a possible uncertainty of ±1 K at 20 K decreasing to ±0.5 K above 100 K. However, whenever the digital voltmeter reading was checked with a more sensitive potentiometer, the two values always agreed within ±10 yV. So the uncertinty in temperature due to the voltmeter reading is no more than ±0.2 K above 100 K.

Temperature stability during the taking of a data set was usually such that the spread

of the six thermocouple readings did not exceed 20 yV and was frequently less. The voltmeter was not sensitive enough for a detailed evaluation of the computer temperature con- trol, but above 20 K the control was satisfactory. Another indication of good temperature

equilibrium in the specimen holder was the absence of significant offset in the results froii. the two coolants in the overlapping temperature range. Typically differences of less than

2 per cent were found. The temperature control might have been even better had more sensitive thermocouple readings been available.

8. SYSTEM EVALUATION

8.1 Influence of the Computer on the Experiment

It is interesting to compare this data acquisition system with other methods of obtaining data from the Hall effect experiment. Previous experience with manual measurements

showed that at least 3 days were required to take 4-0 points by the d-c method using a potentiometer. This can be shortened to 2 days if the Dauphinee method [12] is used. This

technique utilizes a chopper which reverses the specimen current and thereby relieves the

operator of this task. Manual measurements are tedious and mentally fatiguing, so usually

only a minimum number of points are taken. The conclusions drawn from limited data may not

be as certain as those from a larger number of points

.

About 2 days are needed to calculate 40 points either with a slide rule or mechanical

calculator. This time can be reduced to less than one day if a programmable, electronic

calculator is available. Thus from 24 to 40 man hours are needed to take data and make calculations that can be completed by the computer system in 4 hours.

Several other automatic data collection systems have been used to make Hall effect measurements [11, 13-15]. These systems, though different in many ways, have the common feature

that the temperature is not controlled. Rather, measurements are made as the temperature of

the specimen slowly drifts upward or downward. In the present case the temperature drift

would be excessive because of the relatively long time (15 s) required for the magnetic

39
field to become stable after being turned on or reversed. This effect is particularly seri- ous in temperature regions where the Hall or resistivity voltage changes rapidly with temperature.

It should be emphasized that the method of controlling the temperature in the present

experiment is not necessarily the best which could be used. Another method, which should

be an improvement , especially at low temperatures , involves the use of a standard temperature controller with the set point supplied by the computer. In this case, an electrical

signal, sent by the computer, is translated into a temperature set point by a digital-to-set

point converter.

In addition to the speed, a second advantage of the computer-controlled system is the

computation and print out of the data immediately after each measurement sequence. This

feature permits the operator to plot the data during the run and to rerun any portion of the

range in which unusual or interesting features are observed. Although rerun of a set of

data points was rather inconvenient in the present system, the problems could be solved

simply by rearrangement of the input sequence.

Another feature of the system, an option to store the calculated quantities on magnetic

tape and then call for a summary print out at the end of the run, was never implemented.

This feature would have been very convenient for plotting the results after a run. Arrangements could have been made to plot the data by machine after storing the values on magnetic

tape or after punching paper tape during the summary print out on the teletypewriter.

Major disadvantages of the system were the cost and availability of terminal time.

Operating cost is estimated at about half that required to obtain the same data by hand.

Other types of data acquisition systems, for which the initial investment is about the same

as the computer-controlled system, can be operated for about one quarter the cost of manual

operation.

Because on-line computer control is not a necessity for the Hall effect experiment,

another data acquisition system was recently obtained and installed. Its output is in the

form of punched cards which are processed at the NBS computer facility. The various function changes, such as reversal of specimen current and magnetic field, are accomplished by

voltage level changes which occur during the data collection cycle. This system uses a standard temperature controller with an operator to adjust the set point or a stepping

switch which connects each set-point voltage in succession to the controller. A clock timer

is set for a period long enough to insure that thermal equilibrium has occurred before the

data cycle is initiated. For this reason it may require 6 hours to obtain the same data

the computer system obtained in 4. The operating cost per run, however, is about half

that for on-line operation. In addition, runs can be made at any time which is an advantage

when there are restrictions on the availability of a computer terminal. It is very inefficient from the user's point of view if the terminal must be released at a certain time

before all of the desired data has been obtained.

All data in the off-line system are processed after the run is completed. Therefore, a run may have to be repeated occasionally because of errors or unusual features which could

have been detected during the run if calculated values had been available immediately. It

is most helpful to have an immediate print out when very little is known about the specimen

being measured.

8.2 Influence of the Experiment on rne Computer

The design objective in attaching the Hall-effect experiment to the time-sharing com- puter was to limit the load on the computer system to that normally encountered with a teletypewriter terminal used for conversational operation. This objective was met; in fact,

the load on the computer system is less than that of a typical time-sharing user.

The program associated with the Hall effect experiment is small enough to be retained

in the core memory of the particular time-sharing system used, and use of disc or magnetic

tape is necessary only for data storage. Although transmission of data to the computer

system is at the maximum teletypewriter rate of 10 characters per second, such transmission

is made for a small percentage of the time and there are relatively long periods when the

program idles waiting for a change in specimen temperature.

40
1. J.PPPP

2. T.

3. E.

4. A.

Even with such a light load on the computer system, the connection of the experiment

to the computer resulted in substantial performance gain, as described above. It is quite

possible to attach this or a similar experiment, with respect to computer assistance requirements, to any time-sharing system that meets certain minimum characteristics. One important

such characteristic is the ability to input and output arbitrary characters and character

strings, either in assembly or a higher level language. Another requirement is that the

worst case response time of the system meet the demands of the experiment.

The use of configurations similar to this introduces the possibility of using generalpurpose time-sharing systems to support large numbers of on-line experiments. It is interesting to speculate about the possibility of using existing commercial and private timesharing systems not only for conversational man-system interaction, but in support of

special-purpose experimental equipment.

REFERENCES

W. Frazer, "Instruments and Computers," Science and Technology, July 1968.

. 41-49; D. F. DeTar, "Computers in the Lab," Scientific Research, November 25, 1968,

. 26-29.

N. Pyke, Jr., "Time-Shared Computer Systems," Advances in Computers 7, 1-45, (1967),

H. Putley, Hall Effect and Related Phenomena, Butterworths , London, T960.

C. Beer, Galvanomagnetic Effects in Semiconductors, Supplement 4, Solid State

Physics, F. Seitz and D. Turnbull, Eds. Academic Press, New York, 1963.

5. "Standard Method for Measuring Hall Mobility in Extrinsic Semiconductor Single

Crystals." (ASTM Designation F76-68) , 1968 Book of ASTM Standards, Part 8 American

Society for Testing and Materials, Philadelphia, 1968.

6. 0. Lindberg, "Hall Effect," Proc. IRE 40 , 1414-1417 (1952).

7. L. L. Sparks and R. L. Powell, "Cryogenic Thermocouple Thermometry," Measurements and

Data 1, No. 2, 82-90 (1967).

8. R. L. Powell, M. D. Bunch, and R. J. Corruccini, "Low Temperature Thermocouples—1. Gold-Cobalt or Constantan versus Copper or 'Normal' Silver," Cryogenics 1, 139-151

(1961). -

9. R. Berman, J. C. F. Brock, and D. L. Huntley, "Properties of Gold + 0.03 per cent

(at.) Irs>n Thermoelements between 1 and 300 K and Behavior in a Magnetic Field,"

Cryogenics 4_, 233-239 (1964).

10. P. G. Stein, "Transistorized Building Blocks for Data Instrumentation," NBS Technical . Note 268, May 26, 1966.

11. R. D. Thomas, "Automated Data Collection System Applied to Hall Effect and Resistivity

Measurements," NASA TMX-1464 (1967) Available from the Clearinghouse for Federal

Scientific and Technical Information.

12. T. M. Dauphine and E. Mooser, "Apparatus for Measuring Resistivity and Hall Coefficient of Semiconductors," Rev. Sci. Instrum. _26, 660-664 (1955).

13. J. J. Duga, J. J. Fruindlich and R. T. Bate, "Automatic Data Recording System for

Semiconductor Research," Rev. Sci. Instrum. 33_, 365-369 (1962).

14. E. Loebner, T. J. Diesel, and C. M. Schade, "ADAC - An Automatic System for Measuring

Hall Effect in Semiconductors," Hewlett-Packard Journal 18, No. 3, 9-16 (1966).

15. R. H. A. Carter, D. J. Howarth, and E. H. Putley, "A Digital Recording System for

Measuring the Electrical Properties of Semiconductors," J. Sci. Instrum. 35, 115-116 . (1958); E. H. Putley, ibid 33, 164 (1956).

<rU. S. GOVERNMENT PRINTING OFFICE : 1969—392-21(6/63

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