_On-Line
Data-Acquisition Systems
in
Nuclear Physics, 1969_
Ad Hoc Panel on On-Line Computers in Nuclear Research
Committee on Nuclear Science
National Research Council
_NATIONAL ACADEMY OF SCIENCES
Washington, D.C. 1970_
This is a report of work under Contract
NSF-C310, T.O. 47 between the National
Science Foundation and the National
Academy of Sciences and under Contract
AT(49-1)3236 between the U.S. Atomic
Energy Commission and the National
Academy of Sciences.
_Available from_
Committee on Nuclear Science
2101 Constitution Avenue
Washington, D.C. 20418
PREFACE
The first digital electronic device employed to collect nuclear data
was the binary electronic counter (scaler) of the 1930's. In the next
decade single and multichannel pulse-height analyzers appeared, still
using vacuum tubes. In the 1950's the development of multichannel
analyzers continued vigorously, with vast improvement of the
analog-to-digital converter sections and with the introduction of
computer-type memories, based first on acoustic delay lines and a
short time later on ferrite cores. The replacement of vacuum tubes by
transistors beginning in the latter half of the 1950's accelerated
the pace of development and application of all types of electronic
laboratory instruments.
The 1960's was the decade of the computer. Before the 1960's almost
no on-line computers were used in nuclear research, but since about
1962 the computer has moved into the nuclear laboratory. It provides
the research worker with an immensely flexible, powerful, and
accurate tool capable of raising the research output of a laboratory
while eliminating the most tedious part of the experimental work.
The phenomenal speed of development of computer hardware, software,
and methodology contributes to the difficulty experienced by
everybody involved in decision-making processes regarding
data-acquisition systems. Since the cost of a computer system is
often a sizable fraction of the total cost of a new laboratory, there
is urgent need for a set of guiding rules or principles for use by a
laboratory director planning a system, a reviewer going over a
proposal for support, or a potential funding agency considering
proposals and reviews. The purpose of this report is to assist in
filling this need. The material presented is current through 1969.
Although we deal with a field that is developing rapidly, we hope
that a substantial portion of the material covered will have
long-lasting value.
The report was prepared by the Ad Hoc Panel on On-Line Computers in
Nuclear Research of the Committee on Nuclear Science, National
Research Council. Appointed in March 1968, the Panel first met in
Washington, D.C., on April 22, 1968.
The original members of the Panel were H. W. Fulbright, H. L.
Gelernter, L. J. Lidofsky, D. Ophir (through late 1968), L. B.
Robinson, and M. W. Sachs. In June 1968, this group prepared an
interim report. L. J. Lidofsky was on sabbatical leave in Europe and
therefore could not participate during the academic year 1968-1969.
Early in 1969 J. F. Mollenauer and J. Hahn joined the Panel.
The Panel has reviewed the present state of the field and has
attempted to anticipate future needs. We have agreed on many
important matters, including especially useful design features for
computers employed in data acquisition, as well as types of
organization of data-acquisition systems suitable for various
purposes, types of software that manufacturers should supply, and
approximate costs of systems, and we present a number of
recommendations in these areas. However, the Panel makes no
recommendation on standards for computer hardware, such as logic
levels and polarities, because of a conviction that these are now
rapidly being established as a result of sound engineering progress
and the pressure of economic competition in the fast-moving computer
business.
Throughout this report we have expressed opinions based on our own
experience and on the best information at our disposal. The nature of
the report seemed to demand some discussion of properties of specific
computers by name. We have tried to be neither misleading nor unjust
in our evaluations.
We wish to thank everyone who has aided us, especially P. W.
McDaniel, C. V. Smith, and G. Rogosa of the U.S. Atomic Energy
Commission and the many scientists in AEC-and NSF-sponsored
laboratories who supplied the basic data on which the economic survey
chapter is based. We are indebted to several members of the staff of
the Department of Physics and Astronomy of the University of
Rochester for assistance in the preparation of the manuscript,
especially Mrs. Brignall and Mrs. Hughes. We also received initial
directions and many helpful suggestions from D. A. Bromley, Chairman
of the Committee on Nuclear Science, F. S. Goulding, Chairman of the
Subcommittee on Instrumentation and Methods, W. S. Rodney and P.
Donovan of the National Science Foundation, and Charles K. Reed,
Executive Secretary of the Committee on Nuclear Science.
H. W. Fulbright, _Chairman_
H. L. Gelernter J. F. Mollenauer
J. Hahn L. B. Robinson
L. J. Lidofsky M. W. Sachs
CONTENTS
1. THE TASKS AND THE COMPUTER 1
A. Introduction 1
B. The Tasks 2
C. The Computers 3
D. Matching Computers to Tasks 5
E. On Characteristic Features of Computers and
Related Equipment 6
2. DATA-ACQUISITION SYSTEMS 16
A. Introduction 16
B. A Small Time-Shared Data-Acquisition
System Based on a PDP-7 Computer 19
C. A Small System Based on a PDP-8 Computer 23
D. A Medium-Sized On-Line Computer System 28
E. A Large System Based on a Single Computer
(The Yale-IBM Nuclear-Data-Acquisition
System) 32
F. Multiple-Computer Systems 39
G. A Process-Control System: The Brookhaven
Multiple Spectrometer Control System 48
H. Relationship to a Remote Computing Center 54
3. A REVIEW AND ANALYSIS OF EXPENDITURES 58
A. The Nature of the Data 58
B. Breakdown of Data for Analysis 59
C. Types of Computers 61
D. Some Total Costs 61
E. Breakdown of Costs by Systems 64
F. Rotating Memory Devices 65
G. Systems On-Line with Computing Centers 65
H. Anticipated Future Expenditures 65
I. Investment in Accelerators, Computer
Systems, and Laboratory Budgets 66
J. Process-Control Application 67
4. SUMMARY AND RECOMMENDATIONS ON SYSTEM PLANNING 68
A. The Need for On-Line Computer Systems 68
B. Where Should Large-Scale Calculations
Be Done? 69
C. Exercising Economic Judgment in Planning 70
D. On the Utility of Small and Medium-Sized
Computers 71
E. Growth Considerations 71
F. Short Summary of Conclusions Regarding
System Planning 73
Appendix A: TABLES OF PROPERTIES OF SMALL AND
MEDIUM-SIZED COMPUTERS 79
Appendix B: BACKGROUND INFORMATION FOR CHAPTER 3,
A REVIEW AND ANALYSIS OF EXPENDITURES 86
Chapter 1
THE TASKS AND THE COMPUTERS
A. INTRODUCTION
On-line data-acquisition computer systems are made in a wide range of
types and sizes. In all cases at least one electronic computer is
involved--a stored-program machine--because wired-program devices
such as pulse-height analyzers are not considered to be computers.
The rest of the system typically consists of input/output (I/O)
devices such as analog-to-digital converters (ADC's), printers,
cathode-ray oscilloscopes, plotters, and control devices, which may
include, in addition to the console typewriter, switch boxes to
simplify the control of special types of operations and perhaps a set
of logic circuits associated with the input system, used to provide
preliminary selection of incoming data. In a small but increasing
number of cases a computer is seen dedicated entirely to a
"process-control" application such as the automatic adjustment of the
shim coils of a variable-energy cyclotron or the control of data
acquisition in a nuclear-scattering experiment, adjustments such as
changing the angle of observation being made essentially under direct
automatic control of the computer. The smallest on-line systems use
the smallest commercially available computers; the largest use
computers bigger than those which until recently served most
computing centers. Large systems sometimes include one or more
satellite computers. The cost of individual systems ranges from
$25,000 to $1,000,000, approximately. The total cost of computer
systems in low-energy nuclear laboratories is estimated by now to
have reached about $20,000,000. (There has been a larger expenditure
in the high-energy nuclear field, where computer systems have been
employed extensively for some years longer and where experiments are
so expensive that the economic advantages of computer use were
quickly recognized.)
B. THE TASKS
We first list the main uses to which on-line computer systems have
been put. We start with the simple operations, which we call Class 1.
_Class 1 operations_:
a. Accepting digital data from external devices and storing it in
computer memory.
b. Preliminary processing of incoming data, on-line, before storage.
This usually involves only operations of logic and simple arithmetic.
c. Controlling the presentation of data via cathode-ray oscilloscope
or typewriter, often for the purpose of monitoring the progress of an
experiment.
d. Controlling the recording of digital data on magnetic tape, paper
tape, or other storage medium.
e. Controlling an incremental plotter.
f. Controlling the output of large quantities of data via a line
printer.
g. Transmission of quantities of data between two computers or
between a computer and a pulse-height analyzer or other device having
a magnetic core memory.
* * * * *
Several operations of intermediate complexity we will label Class 2.
_Class 2 operations_:
a. Processing of data already accumulated and stored either in memory
or on tape or other medium (off-line processing). This data reduction
is often more complicated and lengthy than the preliminary on-line
processing referred to in (Class 1b).
b. Calculation of information required by the experimenter during the
experiment, for example, kinematics tables and particle energies
corresponding to field strengths in analyzer magnets.
c. Process-control operations, in which the computer directs or
regulates a sequence of events in an experiment. Under program
control the computer monitors the course of the experiment and
supplies signals that cause automatic changes in experimental
conditions, such as starting and stopping times of event counting,
angles of observation of scattered particles, and accelerator
energies. Such applications are designed to relieve the experimenter
of unnecessary labor and to reduce the probability of error in
routine operations.
* * * * *
Our final class involves even more complex calculations.
_Class 3 operations_:
a. Complicated treatment of reduced data, including least squares and
curve fitting.
b. Large-scale calculations such as those required for the evaluation
of theoretical nuclear scattering and reaction cross sections, e.g.,
DWBA calculations, which may each require running times of the order
of minutes, even at a modern computing center.
* * * * *
Apparently Class 3 operations do not always have to be done during
the course of the experiment; in fact, they can in most cases be
carried out later, leisurely, at the local computing center.
Nonetheless, calculations of the first type, and to a lesser extent
the second, are currently being done at laboratories having large,
powerful computers in their on-line data-acquisition systems.
C. THE COMPUTERS
1. Introduction
Because computers have proved useful in so many fields, many
varieties are now on the market, quite a few of them having
properties highly suitable for nuclear-data acquisition. The
properties particularly useful are, first, the ease with which a
great variety of external input and output devices can be attached
(interfaced to the computer); second, provisions for rapid, efficient
response to interrupt signals from external devices; and third,
usually a means of transferring data from external devices directly
into blocks of memory without use of the central processor, the
transfer possibly requiring only a single memory cycle per word.
(This is referred to as direct memory access through a direct data
channel.)
Several types of small computers have appeared on the market during
the past year, some having 8-bit words, but they are too small for
general data-acquisition use, although valuables for special
applications. For present purposes, the smallest useful machines have
a minimum memory size of 4096 (4k) 12-bit words, which can usually be
enlarged to 32k words by the addition of memory modules, while the
larger machines have minimum memories of at least 8k, with provision
for expansion to several hundred k. Regardless of their size, the
machines of the present generation all have memory cycle times around
1 or 2 µsec.
2. Rough Classification of Computers
Before proceeding with the discussion it is convenient to find a
simple scheme for classifying computers. The scheme adopted here is
to divide them into three loosely defined classes--small, medium, and
large--essentially on the basis of the properties of the basic
central processors:
Small
Word length 12 to 18 bits
Useful memory size 4k
Number of bits in instruction 3 or 4
Floating-point hardware orally offered
Approximate cost range $8500 to $40,000
Medium
Word length 16 to 24 bits
Useful memory size 8 to 16k
Number of bits in instruction 4 to 6
Floating-point hardware option sometimes offered
Approximate cost range $30,000 to $120,000
Large
Word length 32 to 48 bits
Useful memory size at least 16k
Number of bits in instruction 7 or more
Floating-point hardware
Approximate cost range $150,000 or more
Computers do not fall neatly into these three classifications,
especially since manufacturers offer many optional features;
therefore, some argument about the assignment of a particular machine
to one or the other class is possible. This is especially true with
respect to the small and medium types. The properties of a large
number of small and medium-sized computers are given in Appendix A.
Information on larger machines can be found in the Adams Associates
_Computer Characteristics Quarterly_.
D. MATCHING COMPUTERS TO TASKS
Having classified both the computers and the jobs that they may be
called on to do, we now ask this question: How suitable is each of
the three types of computers for each of the three classes of jobs,
given that in every case the acquisition system consists of a single
computer coupled to all necessary input and output equipment?
1. Large Computers
We start with the large computer system. All classes of jobs can be
handled by this powerful system. However, we should question the
wisdom of assembling a system based on a large machine unless a
substantial amount of numerical calculating is anticipated, because
the essential advantage of the large computer--the advantage that
costs so much--is its capacity for rapidly executing highly accurate
floating-point arithmetical operations.
2. Small Computers
The small computer system can handle the jobs of data acceptance,
data manipulation, and output characteristic of the simple Class 1
operations, but they are suitable for very few jobs involving
floating-point arithmetic. In fact, we must usually be skeptical
about the use of small machines for any of the Class 2 operations
except those of the process-control type, which in many cases would
involve little if any arithmetic. (Process-control applications have
been rather few to date, but a rapid increase can be expected in this
field, especially because of the convenience and low cost of small
modern computers.) It is apparent that these machines have been
designed as economical instruments specifically intended to handle
Class 1 jobs. The smallest word length of a machine in this group, 12
bits, is sufficient for storing in one word the output of a
4096-channel ADC unit, but it is not quite so convenient for handling
the output of a typical scaler, which would likely require the use of
two words. The capability of even a small computer system to convert
experimental information into digital form, to transfer it into
memory, to manipulate it, and to present it for inspection in a
digested, convenient form, all at a high rate and essentially
without error, is of immense value to an experimenter who has to cope
with the abundant outflow of data from a modern nuclear experiment.
3. Medium-Sized Computers
The capabilities of medium-sized computers are less clear. These
machines are superior to the small ones mainly in two respects: they
have a more flexible command structure (i.e., they have a larger set
of wired-in operations), and, usually, they have a longer word
length. These features make them easier to program and give them a
limited, but important, capability to execute floating-point
operations sufficiently quickly and accurately for many purposes,
even though these operations must in most cases be programmed, in the
absence of floating-point hardware. We can reasonably conclude that
the medium-sized machines will serve for any use listed in Classes 1
and 2. Certain simpler calculations of Class 3a are also expected to
prove feasible, but few, if any, of those of Class 3b.
E. ON CHARACTERISTIC FEATURES OF COMPUTERS AND RELATED EQUIPMENT
The value of any feature depends on its need in the application
involved; therefore detailed, absolute statements regarding each
characteristic usually cannot be made. However, the Panel has
discussed various features at some length, and we present here some
general comments on the pros and cons of these features. Among the
items discussed are some, such as word length and cycle time, that
represent basic, inherent properties of the computer; while a great
many others, such as priority interrupts, are customarily offered as
options.
1. Word Length
The shorter the word length the cheaper the hardware, generally
speaking, but the less the accuracy in calculations unless multiple
precision is used. For example, although the 12-bit words of the
PDP-8 match the accuracy of data from most ADC's, they are too small
not to match the output data from most counters; furthermore,
indirect addressing is often required because a single word is too
short to include both the operation code and the absolute address of
a memory location. Apart from addressing considerations, a 12-bit
word is too small for many uses, e.g., in general-purpose
pulse-height analyzer applications where 16 bits or, better, 18 bits
should be considered a minimum. Fortran programs for numerical
calculations are in general best run on machines having at least
32-bit words, although 24-bit words are usually acceptable here when
double precision can be used.
2. Number of Memory Words
In general the more words that a system can retain the better; but
the greater the memory, the greater the expense. The cost must be
weighed against the need. For simple handling of data, a 4k memory
may be adequate, but in a large shared-time general-purpose machine a
16k or greater memory is essential. In the latter case, the resident
shared-time monitor will probably occupy at least 6k of the memory,
so with a 16k memory only 10k would be left accessible to users, and
experience has shown that this much can be taken up completely by one
user compiling a Fortran IV program. A 4k memory is adequate for many
process-control applications, but it is too small for many other
applications such as general-purpose pulse-height analyzer use, where
an 8k memory is highly desirable. Adding a supplemental rotating
memory device (disk or drum), at a cost per word about 1 percent that
of core storage, is often preferable to adding core memory. See 6
below.
3. Cycle Time
For most purposes the typical memory cycle time of 1 to 2 µsec is
quite adequate. Some of the modern computers have cycle times under 1
µsec.
4. Direct Data Channels
These allow sequential depositing of digital data from external
devices directly into blocks of computer memory without intervention
of the central processor (direct memory access, DMA). Such input may
require only one computer cycle per word, that being the next cycle
after the one during which the interrupt signal arrives. This is the
fastest means of getting data into memory, but it requires more
external hardware and more complex interfacing than input through an
accumulator of the central processor. Most data-acquisition machines
provide both possibilities. Direct data channels can be valuable for
interfacing to magnetic disks, drums, and tapes.
5. Priority Interrupts (Nested)
These can be very useful. They may cost as little as $125 each,
depending on the machine, and can be used to reduce greatly the
overhead running time losses of the computer. In complicated
data-taking applications many interrupt lines are desirable; 8 to 16
priority levels are generally adequate. The usual Fortran compiler
cannot compile programs that respond properly to interrupts, although
a relocatable object code generated by the compiler can always be
assembled with a machine-language subroutine designed to handle
interrupts. Enlargement of Fortran compilers for data-acquisition use
to include statements designed to handle interrupts is desirable.
(See, for example, the discussion of the Yale-IBM system, Chapter 2,
Section E.)
6. Mass Storage
Magnetic media--drums, disks, and standard magnetic tapes--are
employed here. DEC tapes are useful and reliable, but they have only
a small capacity. The use of such microtapes is also limited by their
incompatibility with typical computer-center equipment. Reliable,
inexpensive incremental magnetic tape units are now available which
can be operated asynchronously at about 300 Hz, too slow for many
purposes. Some of them can also be run much faster in a synchronous
mode. Drums and disks are highly desirable because they provide
program-controlled rapid access to great volumes of data. Typically,
access times are of the order of 17 µsec. In the past few years, good
and inexpensive disks have been developed which are now on the
market. Some suppliers are IBM, CDC, Datadisk, Burroughs, DEC, and
SDS. Disk storage is cheaper per word than core storage by two orders
of magnitude; therefore, it is preferable for applications where data
can be organized serially and where access and transfer time
requirements can be relaxed somewhat. For example, a small DEC disk
system for the PDP-8 holds up to 128k 12-bit words and has an average
access time of 17 µsec and a transfer rate of 16,000 12-bit words per
sec. It costs $6000 for the first 32k of capacity, plus $3000 for
each additional 32k, including interfacing through the direct data
channel. Larger and faster versions are available. Disks (or drums)
should be important in future systems. Magnetic tapes of the
IBM-compatible type are valuable, especially for communication with
machines at computing centers, but tape drives and interfacing are
usually expensive. It often costs $25,000 or more to get a single
tape drive in service, although the next few are usually less
expensive. The cheapest tape drives available cost about $5000. The
cost of interfacing depends greatly on the particular computer. It
may be as little as $5000, but it is often in the neighborhood of
$15,000 or $20,000.
7. Program Input Method
Because they provide immediate access, the most satisfactory program
storage media are magnetic disks and drums, followed by the IBM tape.
The most satisfactory cheap device for input of programs is the
high-speed, punched-tape reader, but the advantages of using small
"cartridge-type" magnetic tapes have recently been emphasized.
Recently, card readers have appeared which are much cheaper than the
older IBM models. They can read 200-300 cards per minute. They cost
about $2000 plus interfacing. Examples: Soroban, General Devices,
Uptime.
A simple means of restoring the basic loader program (other than
toggling!) is desirable. Many computers have this feature, e.g., the
IBM 360 series; the SDS Sigma 2, Sigma 5, and 910 PDP-9.
8. Memory Protection
Hardware memory protection is necessary in multiprogram systems. It
is very helpful in any machine with a batch-processing resident
monitor and in other special situations.
9. Parity Check
This feature is useful for purposes as detecting memory failures, but
it is usually not worth its cost in computer speed and in capital
investment in the case of a small system.
10. Ease and Cost of Interfacing
This is a big subject, partly because the organization of computers
for input and output of data varies with the manufacturer. Some
computers such as the Hewlett-Packard and the DEC models are
especially easy to interface, whereas the automatic channels of the
SDS Sigma computers and the ordinary IBM machines (e.g., the 360
series) are very difficult. The IBM machines require an expensive
control unit. It is said that before a competent engineer could order
plug boards for Sigma interfacing he would have to study the system
for a month or two. However, once interfaced, these machines permit
rapid input of data. Interfacing a $5000 Calcomp plotter to the
automatic channel of an IBM or Sigma series machine may cost much
more than the cost of the plotter.
11. Typewriters
Many small computers use teletype machines as console typewriters.
The ASR-33 teletype has not performed well, but it has recently been
improved. The ASR-35 and KSR-35 have excellent records, and the newer
ASR-37 and KSR-37 (15 characters/sec) are very good. The IBM
Selectric has had a mixed reliability record which is, however,
improving. In every case, expert routine maintenance is required.
12. Index Registers
These are a valuable asset to efficient programming. At least one,
and preferably more, is desirable, especially in the medium and large
computers.
13. Line Printers
These are of great use for obtaining a permanent ("hard copy")
record, especially when large volumes of output are produced;
however, they are expensive, usually costing $20,000 or more
(including interfacing). In order to avoid tying up a large central
processor during typewriter output of masses of data, a line printer
is not only very useful, it is essential for efficient operation (and
to spare the typewriter). A line printer can be _immensely helpful_
and can save much time in the process of developing and debugging
programs. The cost, however, will often preclude its addition to a
modest system. If the system has an IBM-compatible tape drive, the
computer output can be written on tape and later carried to a
computing center for printing. Several industrial concerns are known
to be working on new types of printers, some being dry-copy,
nonpercussive types. One type which has already been marketed, the
Inktronic printer, operates by spraying ink at the paper from small
tubes. The characters are well formed. It operates at about 120
characters per second and costs $5600. Conveniently, it requires
standard Teletype interfacing, and it can be ordered with an optional
keyboard. Although it has exhibited a few new-product ailments in its
first 8 months or so of use, it shows promise of becoming a very
useful device. Another printer operating on a similar principle has
just appeared--the A.B. Dick Company's Videojet printer, priced at
about $4900.
14. Plotters
The overwhelming favorite is still the incremental machine called the
Calcomp plotter. It costs about $5000 and is easily interfaced to
many computers. It is very accurate (about 0.01 in. in each
direction) and provides valuable output to the experimenter. It can
be programmed to plot experimental points and theoretical curves
together on white paper in India ink, relieving draftsmen of
considerable work and doing a more precise job. Other incremental
plotters are now on the market, e.g., the Houston Instruments
version. Varian has developed an electrostatic plotter to sell for
about $15,000.
15. Cathode-Ray Tube Displays
At least four types are in use. The standard scheme involves the
displaying of bright spots under control of the computer, which has
generated appropriate words to cause _x_ and _y_ deflections of the
spot when those words have been transformed by ADC's in the CRT unit.
The pattern is rewritten continuously. A light pen held against a
particular part of the display pattern can be used to signal the
computer. This scheme works well but may produce a flickering image
if the computer is interrupted frequently to handle higher priority
jobs or if the display is so complicated that the rewriting period
exceeds 1/30 sec. The expensive hardware option called a character
generator is considered not worthwhile unless large amounts of text
are to be displayed. On a 10 in. x 10 in. raster a matrix of dots
1024 x 1024 is sensible.
A second scheme involves a disk or drum on which the computer writes
the words to generate the pattern. Separate reading heads send the
words to the CRT unit. Thus the display, automatically rewritten over
and over, is updated from time to time by the computer. The
light-spot cursor and joy-stick method replace the light pen in this
case. (In passing, it is worth remarking that a light pen is only as
effective as the computer program allows it to be, that the effort of
programming for light-pen control is usually not trivial, and that a
substantial amount of core storage may be required. A means of
display control perhaps not so popular as it should be is
sense-switch control.)
A third scheme makes use of a modern storage CRT. The computer sends
the pattern to the CRT only once, and the display can persist until
erased. This method is flicker-free and inexpensive, but the pattern
is not so distinct and sometimes not so bright as in the above
schemes. However, it is cheap. Furthermore, the storage tube can be
used alternately as an ordinary CRT with quite satisfactory
resolution. A storage version is thus possible which reverts to the
standard scheme, for high-resolution inspection, when a button is
pushed. The storage-tube scheme is probably the best buy for use in a
typical small system. The Tektronix Company has recently announced a
storage-tube device, Type 4501, which is said to generate a
continuous video signal suitable for driving large-screen television
monitors.
A fourth scheme involves the generation of a video (analog) signal
corresponding to the display, written on a disk or drum by the
computer. Reading heads then send the video information to a CRT
having a TV raster synchronized with the rotation of the medium. This
is a good scheme where many displays are needed, but it is too
expensive for many applications, costing upwards of $20,000 for the
first unit. (For example, the Data Disc System 6500 Display costs
about $23,000.)
One display feature considered desirable by many nuclear physicists
is rotation of isometric data plots. This can be accomplished in one
of two ways: recomputing every displayed dot or using an appropriate
analog device (potentiometer). Because the latter is so cheap,
clearly its use is more desirable than the recomputation of the
rotated view. Also, using a light pen on a recomputed display is
especially difficult because the inverse computation has to be
performed in order to maintain proper correlation with the original
data. However, it should be noted that the TV raster technique is
limited in this respect: rotating potentiometers cannot be used, and
the image must be recomputed. The technology of displays is
developing rapidly.
16. The Role of External Devices
In many cases, especially where typical standard operations are
involved, it is preferable to use external devices to handle
preliminary selection and sorting of events, rather than to ask the
computer to do the entire job. For example, particle identification
by use of signals from two counters involves one or two
multiplications and additions, which can be carried out almost
instantly by a fairly simple external analog device, whereas a small
computer would likely require at least 500 µsec for the job, assuming
calculation, and perhaps 40 µsec, assuming table look-up.
17. Time Sharing
Computers as small as a PDP-8 have been successfully time-shared by
several users in special applications. The justification given is
that all the peripheral hardware can be shared also, so that the
added constraints and programming difficulties are balanced by
savings in hardware costs. Computers have also been shared for
simultaneous on-line data-taking in low-data-rate experiments. In
working out the economics of time-sharing, the added hardware (such
as CRT's and remote consoles and memory protection) needed to allow
simultaneous access by more than one user, as well as the extra
memory space needed by the time-sharing monitor, should be
considered. The greatest costs, however, lie in the added constraints
placed on each of the users and in the greatly increased cost of
programming. In many cases the use of two or more identical computers
is preferable. However, in large, expensive systems time-sharing can
be very useful.
18. Software That Should Be Supplied by Manufacturer
Complete documentation should be provided, including listings,
step-by-step user instructions, and some fully worked out examples.
a. Hardware diagnostic routines: To test memory addressing,
instruction set and to test correct operation of every
peripheral and special hardware feature.
b. Systems to edit, assemble, and debug programs in symbolic
machine language: These should efficiently use any special
I/O device such as magnetic tape, disk, or line printer.
c. Efficient subroutines should be provided for operation of
any special peripheral device purchased from the computer
manufacturer. Symbolic language source tapes or card decks,
listings with comments, and examples of use should be
included.
d. Conversational Fortran-type programs provided by some
manufactures are useful for supplemental calculations.
NOTE: The following points apply particularly to the medium and large
machines and become increasingly important as the computer becomes
larger and more complex.
e. Fortran compiler and operating system, with convenient
method to insert machine language instructions and
subroutines. Good compile and run-time diagnostics are
essential.
f. Mathematical subroutines should be provided in binary and
source language.
g. Complete specifications and documentation for the
programming system should be supplied, so that programs
prepared by users can be made compatible. It may be objected
that this will cost too much, but not to do so will be very
costly and frustrating to many users.
19. Note on the Cost of Programming
Experience at Brookhaven and Berkeley has shown that a programmer can
produce between 10 and 20 debugged and documented lines of program
per day, depending on such factors as experience, when he is working
on reasonably straightforward programming. When working on a
complicated monitor system he would be considerably less productive.
System programming is obviously very expensive, therefore the average
person exploring the computer market would be well advised to
consider the software support along with the hardware offered in each
case. Manufacturers vary greatly in this respect. A major
contributing factor to the persistent popularity of the PDP-8 is that
the software support is so extensive. _In general, the newer a
computer, the less software is likely to be available._