DATA-ACQUISITION SYSTEMS
A. INTRODUCTION
1. History
The movement toward computer systems began in earnest about 1962.
Much of the early work depended on the use of magnetic tape for
storage of data, either raw or partially digested, the analysis of
data being carried out later, off-line. More recently, computers have
been used increasingly for on-line processing. The early work is well
known and will not be described here. Some of the more recent systems
are basically very close descendants of one or another of the early
systems. Many varieties are now in service. Most incorporate small or
medium-sized computers, however, extensive new experience has been
gained during the past two or three years of operation of a few large
time-shared systems, in particular those in the tandem Van de Graaff
accelerator laboratories at Yale and at Rochester, perhaps the first
large systems in operation which were planned systematically for
nuclear research. Both operate with multiprogramming monitor control,
background calculations being possible, on a low-priority basis,
simultaneously with data acquisition.
2. Possible Systems
Simple rules for the design of various types of data-acquisition
systems cannot be stated, but some examples of possible systems can
be given. (See Figure 1.)
a. A simple system for pulse-height analysis work can be assembled
from a small computer, a 5-in. Tektronix CRO, an ADC unit, and a
teletype with paper-tape attachment for a cost of about $30,000,
providing that a competent engineer is available, not counting
programming and engineering costs. A Calcomp plotter could be added
for about $6000. To maintain and operate the system at least a
half-time technician-programmer would be required.
[Illustration: FIGURE 1 Basic data-acquisition system.]
b. A general-purpose system for use in an accelerator laboratory can
be assembled from a medium-sized computer, two typewriters, four
13-bit (0-8192) ADC's, six 18-bit (0-262,144) counters, a 10 in. x 10
in. CRO display unit with light pen, two tape drives (for IBM tape),
a Calcomp plotter, and a fast paper-tape reader for about $175,000
plus the cost of engineering service and programming. At least one
full-time technician-programmer would be needed for maintenance and
programming.
c. A large shared-time system of the smallest configuration which
makes much sense consists of a large computer with a 16k memory, two
typewriters, a fast punched-tape reader, four DEC tapes (or the
equivalent), one IBM-compatible magnetic tape, one CRO with light
pen, one incremental plotter, input devices for experimental data
(ADC's, counters, etc.), plus an interfacing system to link the
external input-output devices to the computer. The interfacing system
may include a fixed-wired "front end," such as that used at Yale, a
small computer, such as that used at Rochester, or both. The hardware
would probably cost over $400,000 exclusive of engineering, and to
this must be added a large expense for programming, even if the
manufacturer supplies a satisfactory shared-time monitor plus all the
usual software. Three men would be needed to assemble, maintain, and
operate the system: an engineer, technician-programmer, and a
full-time programmer, or some equivalent combination, assuming use of
the system in a large laboratory with an active and continually
developing research program. Thus the cost of this "stripped-down"
system must be expected to reach $500,000 before it is in full
operation, and the cost of keeping it going, including salaries,
overhead, and replacement parts will likely exceed $50,000 per year,
although this could perhaps be trimmed somewhat once the system is
running. Furthermore, to run efficiently, the system would need
additional components: another 16k (at least) of core memory, another
IBM-compatible magnetic-tape drive, and a line printer. A rotating
memory device would also be helpful. These would raise the cost by
well over $100,000. It is apparent that large time-shared systems are
so expensive that they can ordinarily be justified only in the
largest, most lively research establishments.
3. Small Computers as Satellites
In medium and large systems the use of small computers for coupling
input and output devices to the main computer offers a number of
attractive advantages, especially now that mass production and
competition have brought the prices down so low that a large amount
of hardware nearly ideal for the purpose is available at a bargain.
Some advantages: (1) The small machine can control data acquisition,
accumulating blocks of data while the large machine is doing
background calculations, interrupting those calculations only
occasionally to transfer raw or partially processed data. (2) The
small machine can continuously control the monitor CRO. (3) It can
control output devices such as a plotter, line printer, rotating
memory, or tape drive. (4) It can carry out many logic operations on
the incoming data. Experience has shown that such operations are
numerous, and from the economic point of view they should not be
allowed to tie up the larger machine, which, at the same time, can
better be engaged in complicated calculations. In some cases the use
of two small satellite computers can easily be justified. The chief
disadvantage: Programming can be complicated. However, if the larger
machine already has a time-shared monitor which recognizes the small
machine as a typical input-output device (as is the case with the
PDP-6 plus PDP-8 system at Rochester) the programming problem is not
bad.
In the following five sections descriptions of a number of
data-acquisition systems of various types and sizes will be given in
order to illustrate concretely some practical system configurations.
In each case a breakdown of costs and a discussion of the lessons
learned in connection with planning, construction, and operation will
be included. The systems are of the following types: two small, one
medium, one large, two multiple-CPU, and one process control.
B. A SMALL TIME-SHARED DATA-ACQUISITION SYSTEM BASED ON A PDP-7
COMPUTER
1. Introduction
In 1966, two identical computer systems based on PDP-7 computers were
set up in two different locations at the Lawrence Radiation
Laboratory (LRL), to be used by several groups of experimenters (see
Figure 2). Assembly of the hardware for the first system was
completed 6 months after delivery of the computer. Assembly of the
second system required only 3 months. Two years after operation
commenced, the first satisfactory time-sharing monitor was completed
and put into service. The basic use of these systems is pulse-height
analysis. In principle many other types of operation are possible.
[Illustration: FIGURE 2 PDP-7 data-acquisition system at Lawrence
Radiation Laboratory.]
[Illustration: FIGURE 3 A switch panel used for data taking and
control of CRT display in conjunction with the PDP-7 computer. The
switch-setting codes can be read into the PDP-7 accumulator under
program control and are used to select branch points in the program.
As many as eight of these units can be connected to the system. The
lights are used to indicate program status.]
2. Operational Features
Data-reduction jobs currently possible in the shared-time operating
mode include spectrum stripping, normalization, smoothing, storage
and retrieval of data from magnetic tapes, graph plotting, printout,
energy calibration, background fitting, peak integration, and
transfer of data from a remote analyzer. Remote control of the
computer from up to eight experimental locations is possible using
inexpensive switch panels (Figure 3). Remote slave CRT display is
also provided. Multiparameter pulse-height acquisition and analysis
can be done on a time-shared basis but often requires all the
computer's time and memory.
3. Hardware
The hardware configuration is shown in Figure 2. The PDP-7 computer
was supplied by the Digital Equipment Corporation with an 8k memory
(18 bits) extended arithmetic hardware, microtape (Dectape), paper
tape, teletype, and cathode-ray tube (CRT). The other items were
built or interfaced at LRL.
Automatic memory increment and memory-protection hardware, together
with suitable programming, allow a user to carry out simple
data-reduction jobs with a live CRT display while two other users are
independently acquiring separate, 2048-channel, pulse-height spectra
in part of the computer memory, with computer-controlled-gain
stabilization. ADC dead time per pulse is less that 40 µsec. Up To
6144 words of the memory can be used for data (one PHA channel per
word), while machine language programs fill the remaining 2048 words
of memory.
4. Lessons Learned from Operating Experience
The system works well for pulse-height analysis, but for new
applications, e.g., nuclear magnetic resonance magnet control, it
needs additional hardware and programs.
Two groups of experimenters, doing chiefly pulse-height analysis
experiments are very satisfied with the system. Another group, with a
wider range of interests, has been dissatisfied because of the time
lag to implement new experiments.
One programmer is now engaged full time preparing more programs.
Lack of free computer time has become a limitation for both users and
programmers.
Provision for programming at the outset was inadequate. One full-time
systems programmer should have been assigned to these systems for 18
months.
Experimenters need Fortran or similar language capability. A disk, or
more core memory, would make this practical.
The memory size is totally inadequate for multiple users because of
the large amount of data space needed for the high-resolution spectra
now obtainable with Ge(Li) detectors. An external 8k memory is being
acquired for data acquisition in each system so that more of the
computer memory can be used for computing.
Memory CRT's are needed to provide independent displays for each
user. A separate teletype for each user would be invaluable.
A disk memory is needed for rapid overlay of programs and for sorting
of multiparameter data.
A "czar" should have been appointed for day-to-day assignment of
facilities, consultations with users, and routine maintenance and
upkeep of the hardware and programs. The "czar" could be a good
electronics technician interested both in programming and in physics.
5. Costs
System costs for the PDP-7 with time-sharing are given in Table 1.
Fabrication time is included as a dollar cost. Engineering and
programming times shown are one half those charged against two
identical systems.
Additional special-purpose experimental equipment commonly used with
the system includes gain stabilizers, analog pulse derandomizers,
amplifiers, pulse pileup rejection, low-noise preamplifiers, and
Ge(Li) detectors.
TABLE 1 System Costs--PDP-7 with Time-Sharing[A]
---------------------------------------------------------
Items Costs Man-Months
---------------------------------------------------------
CPU (8k, EAE) PDP-7, 1966 $ 52,000
Dual microtape 10,350
Calcomp plotter 4,825
Calcomp interface 200 1/2
CRT controller 2,300
Large screen CRT 3,500
3 small CRT's (5 in.) 1,995 1/2
Mag tape (556 BPI, 30 IPS) 8,690
(with erase head)
Mag tape interface 200 4
Memory protection 2,000 3
Direct memory multiplexer 2,000 2
2 ADC's (4096-channel, 35 µsec 5,000 -
per count)
ADC multiplex interface (automatic 2,000 3
memory increment)
8-parameter input to ADC 1,500 1
(analog multiplexer)
4 remote memory switch panels 1,200 2
Cabling to experiments 1,500 1
8k external, 18-bit, 2-µsec memory 12,000 3
---------- ------
20
Programming 18
---------- ------
$113,260 38
---------------------------------------------------------
[A] The PDP-7 is no longer made. Its modern equivalent is the PDP-15.
C. A SMALL SYSTEM BASED ON A PDP-8 COMPUTER
1. History and Hardware
This second example of a small computer system is also taken from
experience at LRL. It was planned in February 1967 and first put into
operation in the summer of 1967. Data were first taken with the aid
of the system in the spring of 1968, and the system programming was
completed in May 1969. The system is used extensively in experiments
with the Bevatron.
[Illustration: FIGURE 4 PDP-8 data-acquisition system at Lawrence
Radiation Laboratory.]
The computer-system hardware consists of the items shown in Figure
4. The PDP-8 has 4k of memory. The disk is a Data Disc unit with
a removable disk (128 tracks and a movable head) on the same shaft
as a smaller disk and three fixed data heads. Two of the fixed-head
tracks are devoted to the display: they drive a hardware-translator
continuously. A single display track is used when the number of
points does not exceed 2000. For larger displays two tracks are used
alternately.
The display is controlled from the switch panel. On-line operating
functions may be controlled both from the switch panel and from the
teletype.
2. Programming
Three classes of programming have been completed:
_System Programs_: Symbolic Text Editor, Assembler, a general-purpose
library system--all disk oriented.
_Data-Taking Programs_: These cause the ADC to be read, control
elementary sorting, update histograms resident on the disk, write raw
data on tape, and monitor the beam. The Bevatron has approximately 1
sec of beam every 6 sec. During a beam pulse the computer is devoted
entirely to acquiring data, saving raw data in core, on the disk, and
on tape. After a beam pulse, the 13-disk histograms are updated, then
the display programs are read into core memory and the display is
updated. The system is designed to be capable of accepting over 1000
events per beam burst, and it has met this requirement.
_Simple Data-Analysis Programs_: These compute displays (linear, log,
isometric, and contour--all double precision), read out the sealers,
monitor the real-time clock, allow resorting of raw data from tape,
and generate tapes for remote plotting.
3. Lessons from Operating Experience
The system now functions as originally intended and does its job very
satisfactorily.
The experimenter relies heavily on the main computer center for data
processing.
In assembling this system now, one would buy the disk already
interfaced by the computer manufacturer; furthermore the manufacturer
now offers programs that would greatly reduce the programming costs.
A memory scope would eliminate the need for a disk-to-CRT display
interface.
Less-expensive magnetic tapes are now available. However, it would be
better to buy the tape already interfaced by the manufacturer of the
computer.
The added cost of buying a 16-or 18-bit computer would have been
almost completely offset by savings in the cost of programming.
The addition of a fast printer (e.g., Inktronic $5600) would have
paid for itself in time saved during programming but would not be of
much use in experiment.
4. Costs
The costs of the PDP-8 are given in Table 2.
TABLE 2 System Costs PDP-8--1967
---------------------------------------------------
Costs Man-Months
---------------------------------------------------
CPU, 4k, 12-bit $18,000 -
Data disk 8,300 -
Disk interface 2,500 6
CRT display control 1,000 1
CRT 700 -
Mag tape (556 BPI, 30 IPS) 8,800 0
Mag tape interface 2,000 6
Misc. interfaces 2,700 12
8 scalers (on loan) - -
ADC (4096-channel, 35-µsec 2,500 -
dead time)
8-parameter input to ADC 1,500 -
(analog multiplexer)
Remote console (switch panel) 300 -
Time-of-day clock 1,000 -
--------
$49,300
Systems programs 12
Data-handling programs 12
Engineering diagnosis, debugging 9
----
58
---------------------------------------------------
[Illustration: FIGURE 5 Block diagram of EMR 6130 data-acquisition
system at Columbia University.]
D. A MEDIUM-SIZED ON-LINE COMPUTER SYSTEM
1. Introduction
An EMR 6130 computer system has been installed and is being prepared
for use with Columbia University's Neutron Velocity Spectrometer
data-acquisition and analysis system. The spectrometer is
characterized by high data rates and many events per burst. At
present, peak arrival rates are approximately 10^6 events per second,
with 40-60 events per burst and a burst rate of 70 Hz. The arrival
distribution is random; therefore, 10 percent of the interarrival
intervals are 100 nsec long, and 1 percent are 10 nsec long. In the
future, peak arrival rates of 10^7 events/sec and 400-600 events per
burst are possible, with a burst rate of 300 Hz. With an appropriate
time-of-flight "front end," the 6130 will be able to handle the
anticipated faster rates.
The EMR 6130 is a 16-bit, 775-nsec computer. The memory has a
multibus structure which permits each bus to communicate
simultaneously with a separate memory module. Up to four memory buses
may be purchased. The Columbia system has two memory buses. If a
high-speed buffered data channel is used, block transfer may occur at
memory cycle speeds. With two buses, data may be stored in two memory
modules at rates up to twice memory speed. Alternatively, one bus,
channel, and one or more memory modules may be dedicated to data
acquisition, while the central processor and standard peripheral
devices, using the second bus, simultaneously operate in the
remaining memory modules.
2. Description of the System
A block diagram of the Columbia system is given in Figure 5. The
system has three 8k core modules. Memory bus 1 is dedicated to a
high-speed channel serving the time-of-flight acquisition system.
Memory bus 2 serves both the central processor and a second
high-speed channel. Low-speed input-output devices, such as the
operator's console, teletype, card reader, and plotter communicate
directly through the processor. The high-speed input-output devices,
namely, a magnetic tape unit, line printer, fixed head disk, and
interactive CRT display, communicate through the channel.
The box designated as "time-of-flight system" represents special-purpose
electronics, including a 50-mHz clock, time-quantizing circuits
which "clock" an input event from one of the detectors to the nearest
clock pulse following its arrival, a 50-mHz counter, and a 16-word
derandomizing buffer capable of storing a new word of data (i.e.,
arrival time) every 20 nsec. The number of channels, nominally
16,000, is limited not by the front end but by the amount of core
available for histogram storage in the 6130 system. (For the high
data rates anticipated in the future, the time-of-flight clock speed
and derandomizing buffer data acceptance rate will be increased
to 100 mHz. At the same time, an accumulating buffer of several
hundred words capacity, with a 20-mHz data acceptance rate, will be
added to empty the derandomizing buffer and store temporarily the
time-of-arrival data prior to its transmission to the 6130 system.)
[Illustration: FIGURE 6 Diagram illustrating mode of utilization of
core memory in the Columbia System shown in Figure 5.]
3. How the System is Used
During the time-of-flight experiment, memory is utilized as follows
(see Figure 6). The channel dedicated to data acquisition writes on
alternate bursts, into two buffer regions, of approximately 100 words
each, in the top of memory module 3. The remaining parts of memory
module 3 and all module 2 will be devoted to histogram storage (i.e.,
time-of-flight channels). Module 1 will contain a stripped-down
monitor program and all data-handling programs, including buffer
regions for the external devices other than the time-of-flight front
end. Programs will be capable of referring to all module 1 or 2 in
full concurrency with data acquisition. Reference to module 3 will
also overlap data acquisition, except for a period of high input data
rate of 100-to 200-µsec duration per burst. With the type of memory
allocation described, the system will permit the use of all standard
I/O devices, concurrent with the essential operations of input data
buffering and histogram generation. Thus, new data may be stored on,
or old data retrieved from, the disk or magnetic tape; either new or
old data may be displayed on the CRT; and the same or other data may
be output with the plotter or line printer. Control information will
be input from the teletype, the operator's console, or from
special-purpose switches. The importance to the physicist is that
hard copy output is immediately available during data acquisition and
may be used to monitor, or modify, the experiment.
Subsequent to the input data increase, a high-speed memory
incrementing channel will be used to input time-of-flight data
directly to the histogram area. With this channel the buffer area in
module 3 will no longer be required. Histogram data will be stored in
all modules 2 and 3, and no program intervention will be required for
histogram generation.
Between data-acquisition runs, the system will be used for data
analysis.
4. Present Status
The computer, with two memory modules and one channel and bus, was
delivered in July 1968. The remaining memory module channel and bus
were delivered in the fall of 1968, the CRT arrived in June 1969,
and the line printer (which was not purchased from EMR) came shortly
afterward. The first time-of-flight run with this system was scheduled
for December 1969. During the period from delivery to the first run,
one full-time programmer and approximately half the time of one
physicist were devoted to the debugging of manufacturer-supplied
programs and the writing of the on-line programs required for the run.
It has been hoped that the system would be used extensively for the
analysis of previously acquired data, beginning shortly after
delivery; however, very little such use has proved possible,
essentially because of the unreliability of the 100-cpm card reader
supplied by EMR. The lack of a line printer was also a factor. A more
reliable reader has been purchased. The delivery of a line printer
should rectify the second need.
The development of high-speed, buffered, time-of-flight front ends
has been a continuing interest at Columbia. It is therefore difficult
to estimate the precise costs of the time-of-flight system developed
for use with the 6130. A rough estimate of the design and development
time is approximately 3 engineer man years.
5. Lessons from Development and Testing Experience
Columbia chose to order the EMR 6130, even though at that time (1966)
it was not in production, because it seemed a very powerful machine
which matched the needs of the system planners. The alternate
possibility open was to order a larger, much more expensive machine
of proven capability. As it turned out, difficulties in the
development of the 6130 caused a delay of over a year in the delivery
of the main frame and of over two years in the delivery of the CRT
display. (When these delays became apparent, EMR loaned Columbia a
24-bit computer and also a small display for use during the interim
period.) The EMR 6130 is perhaps the most powerful 16-bit computer
available today, in spite of one or two changes in the original
specifications, but in order to get it Columbia apparently traded
time for money.
6. Cost
The costs of the Columbia University EMR 6130 system are given in
Table 3.
TABLE 3 Cost of Columbia University EMR 6130 System
(Prices from EMR Except where Indicated)
----------------------------------------------------------
1 Central processor with 16k core memory $65,500
1 Additional 8k core memory 17,850
1 Teletype, model 35 1,930
2 Word/byte buffered channels 10,300
1 Additional memory bus and control 4,300
1 Additional cabinet assembly 5,000
1 Card reader, 100 cpm 4,100
1 Magnetic disk and control 16,800
1 Tape transport and control 26,000
1 8 levels, priority interrupt 1,000
1 CRT display, including vector generator and
light pen 27,750
1 300-lpm line printer[B] 18,000
--------
$198,530
----------------------------------------------------------
[B] Purchased from printer manufacturer with 6130 interface.
E. A LARGE SYSTEM BASED ON A SINGLE COMPUTER (THE YALE-IBM
NUCLEAR-DATA-ACQUISITION SYSTEM)
1. Introduction
Since early 1966, Yale and IBM Research have been engaged in a joint
study in the application of computers to nuclear-data acquisition.
The main goal was the production of an integrated hardware-software
system which is fully under the control of the experimenter in the
sense that he can define his entire data acquisition and analysis
process with a Fortran program.
The joint study may be divided into four areas: (1) Development of a
suitable general and powerful data-acquisition interface and control
unit (front end) with a set of compatible nuclear instrumentation
modules (scalers, ADC's, and general-purpose input registers). (2)
Development of a suitable display system. (3) Development of a
data-acquisition language (as an extension to Fortran) and the
necessary library routines to support this language. (4) Development
of a general-purpose multiprogramming system for the selected
computer (the IBM System/360, Model 44) into which the
data-acquisition system could be incorporated.
The first three areas became operational in July 1967, within three
months after delivery of the computer, using the standard batch
programming system for the 360/44 as a basis. Development continues
on the multiprogramming system, which has now reached a state where
users inexperienced in using the system can compile and execute
Fortran programs, but the data-acquisition components are not yet
operational.
2. Description of the System
Nuclear data are input by means of a general-purpose
nuclear-data-acquisition interface and control unit, organized around
the concept of an event, an occurrence in the real world which causes
the outputs of a group of instruments selected by the user to be read
into computer memory. Sixteen independent events are provided for,
with each of which may be associated any or all of 30 different
instruments (scalers, ADC's, or general-purpose monitor registers) by
means of a diode plug board matrix. The instruments themselves, also
designed and built by IBM, are modular and completely interchangeable
and enable the experimenter to configure his experiment in any way
desired, i.e., to determine not only which instruments are to be read
but also in what order. Exclusion logic is provided to prevent
processing of certain events if and when other defined events occur
simultaneously (Figure 7).
The cathode-ray-tube display unit provides a 1024 x 1024 point plotting
oscilloscope with seven levels of intensity, character-generation
hardware, a light pen, and a programmed function keyboard, by means
of which the user can call in programs by pushing buttons. Such
programs can perform any function from changing displays gains to
curve-fitting. They may be system-supplied or user-written and may
be (and usually are) written Fortran. A parallel, high-resolution
photographic system permits computer assembly of publication quality
illustrations.
[Illustration: FIGURE 7 Block diagram of the IBM 360/44 system at
Yale.]
The data-acquisition and display-programming system is composed of a
group of subroutines which may be called from Fortran programs for
performing the various processes in data acquisition and display. For
this purpose, a considerable number of additional statements have
been added to the Fortran language. These statements perform such
functions as defining multidimensional pulse-height analyzers in the
computer memory, performing pulse-height analysis using incoming data
as channel numbers, and defining separate programs to process each
of the 16 classes of input events. All the special statements that
make up the new data-acquisition language are implemented by means
of a preprocessor which converts them into Fortran coding, which the
standard 360/44 Fortran compiler then processes.
3. Software System
The general-purpose multiprogramming operating system is a multilevel
priority system designed to provide access to the system
simultaneously by an, in principle, unlimited number of users, each
with unique priority. Unlimited means that there is no arbitrary
restriction on the number of users; the nth user can always get
access if the facilities his particular job requires are not already
in use. Two types of user are recognized by the system: The basic
unit of execution is the logical user, or task. Each logical user has
a unique priority level. Switching between users is carried out as a
response to I/O, timer, or external interrupts, at which time the
highest priority user in a position to execute gains control of the
central processor. The basic unit of memory protection is the
physical user, composed of one or more logical users engaged in a
common cause. Physical users correspond to real people doing
independent work simultaneously. By dividing his work up among a
group of logical users, a person may take advantage of the parallel
processing capabilities of the system in a natural way. Since logical
users within a physical user are not memory-protected against each
other, they may communicate rapidly, at full machine speed.
Communication between different physical users is also possible, via
real or simulated I/O devices. While this system by no means
guarantees execution time to any but the highest priority user, it is
adequate in a single-experiment environment. The assumption is that
the experimenter, who has actual control of the computer at all
times, loads his logical users in the order in which he requires
their priorities. Following this, other users load their jobs,
getting whatever memory and I/O facilities remain. The amount of
processor time available to the other users varies inversely with the
experimenter's counting rate and the amount of processing he does on
his data. In most experiments, the experimenter uses significantly
less than 50 percent of the processor time simply because those
experiments requiring the sophistication of the computer also have
rather low counting rates. _A priori_, it is estimated that the
simple priority algorithm described above is not only adequate but
pays a dividend in terms of reduced system overhead time as compared
to a more elaborate algorithm. It also guarantees that no data will
be lost due to the lower priority users being in the machine.
In general the new system will provide all the facilities of 44PS
within the multiprogramming framework, including execution of the
Fortran compiler simultaneously with data acquisition.
TABLE 4
--------------------------------------------------------------
The Basic System
--------------------------------------------------------------
CPU:
with 128k-byte memory, 1-µsec registers,
external interrupts, floating point, one
high-speed multiplex channel, one low-speed
multiplex channel, and one single-disk
storage drive $291,280.00
-----------
Standard I/O gear:
one 2403-V tape drive and control, 2701
data adapter for front end, 1442 card
reader/punch, 1443 printer, 1827 data
control unit, four disk cartridges 164,259.00
----------
Data-acquisition and display subsystem[C]:
Display system (RPQ on 1827) with
function keyboard and light pen 19,000.00
2972-VII Scientific Interface and Control
Unit (front end) 40,000.00
6 LeCroy model 157M general-purpose registers
and ADC interface to 2972, as designed for
Maryland 5,950.00
2 LeCroy model 150B scaler banks (each contains
eight 15-bit scalers with separate inhibit,
strobe, and reset, as modified for Maryland) 3,900.00
2 LeCroy interfaces to connect model 150B
to 2972 2,000.00
4 Northern Scientific 4096-channel ADC's 8,800.00
----------
Data-acquisition and display subtotal 79,650.00
===========
TOTAL $535,189.00
--------------------------------------------------------------
[C] As previously stated, the data-acquisition and display subsystem
intalled at Yale is the laboratory prototype of the IBM equipment,
for which Yale paid only a nominal sum.
TABLE 5
-------------------------------------------------------------
Additional Items Needed to Make a System Identical to the
Yale System
-------------------------------------------------------------
CPU and peripherals:
additional high-speed multiplex channel with
extra subchannel, high-speed (1/4 µsec)
general registers, additional single-disk
storage drive, memory protect, additional
tape drive (240IV),[D] 1627 (Calcomp) plotter
and adapter, keypunch, six additional disk
cartridges $114,316.00
Data-acquisition subsystem
2 additional Northern Scientific 4k ADC's 4,400.00
2 additional LeCroy 157M registers 1,400.00
----------
Costs of extras 120,116.00
Grand total for basic system 535,189.00
----------
Cost to copy the Yale system TOTAL $655,305.00
-------------------------------------------------------------
[D] About $20,000 can be saved on tape drives by using the slowest
ones (30k bytes/sec) rather than the 120k bytes/sec units shown here.
4. Costs
To the laboratory interested in developing a system of the magnitude
of the Yale system, but not a copy, it must be reiterated that
neither Yale's out-of-pocket costs nor the cost of copying the system
represents the total cost of development. IBM's development costs are
not known, but they may be assumed to be very large.
From IBM's viewpoint, the ADC and scaler project is the least
successful part of the whole project. Although those instruments are
technically excellent, IBM is either unwilling or unable to sell them
at a price competitive with the costs of front-end and interface
equipment available from the traditional nuclear instrument
manufacturers. However, ADC's and scalers available from the
traditional sources can easily be interfaced to the front end (whose
price is in keeping with its power and versatility). The University
of Maryland has followed this procedure.
We therefore present the cost of copying the Yale system by some
other laboratory. In Tables 4 and 5 following the example of
Maryland, we have not selected IBM ADC's and scalers but rather less
expensive components from traditional manufacturers, together with
suitable adapters available commercially. The prices shown are to be
considered strictly reference numbers and in no way constitute price
quotations.
5. General Comments on Experience with the System
Starting by producing an operational data-acquisition software system
running within the standard batch programming system for the 360/44
enabled the system to become operational within three months of the
delivery of the computer. This not only enabled it to do useful work
almost immediately but also enabled important experience to be gained
which is being applied to the development of the multiprogrammed
version.
One of the main lessons so far is that a batch-oriented system barely
begins to tap the real-time potentials of a computer such as the
360/44. In a batch system, whatever analysis is needed during data
acquisition must be somehow tied to the processing of events. If this
is not possible, it is necessary to stop data acquisition in order to
do analysis even though, on a millisecond time scale, plenty of CPU
time is available during acquisition. Multiprogramming software is
necessary in order to utilize this available time. This means that
multiprogramming not only makes the machine available to several
people at a time, but, more important, it makes large amounts of
parallel processing power available to the experimenter.
It has also been shown quite conclusively that the ability of the
physicist to program his own experiment (in Fortran) gives him
enormous power, power which simply would not be available on a
suitable time scale if he has to queue up for the services of a
system programmer.
While the generalized event structure gives the experimenter
considerable ability to deal with complex experimental situations, it
has an overhead associated with it which limits it to about 5000
events per second. This is, of course, adequate for all experiments
that demand such an event structure. For simple pulse-height
analysis, it is unnecessary overhead, but it can be "turned off" in a
trivial way, by simply defining the completion of filling of the
buffer as an event and calling a special pulse-height-analysis
program to process the entire buffer, bypassing the event sorting.
This allows for close to 100,000 pulse-height analyses per second.
There are, however, few situations that justify using a computer as
powerful as the 360/44 in a manner just described (i.e., doing
nothing but simple addition 100,000 times per second). Therefore,
such simple experiments will shortly be handled by means of a link
between an existing multichannel analyzer and the computer.
The system does not suffer from having the front end directly
connected to the 360/44. The data channel on the 360/44 is
sufficiently sophisticated so that it performs all the functions that
one might relegate to a small CPU placed between the front end and
the 360/44, without any interference with the program currently
running in the CPU. The one application described above, which does
warrant a separate processor, is handled best by attaching the
processor to a separate input port rather than by placing it between
the front end and the computer. This enables it to do its intended
job without acting as a bottleneck in jobs requiring the power of the
front end. It also, incidentally, will function as a completely
separate data input terminal if two simultaneous terminals should
ever be required.
The particular display system employed has worked very well. Because
the display list is in the main memory of the computer, programming
of light-pen and other manipulative actions is extremely easy, but at
the price of large amounts of memory being tied up. It is clear that
the system cannot support two such terminals if they are to be truly
independent of each other. It is equally clear that the display is as
useful in data analysis as it is in data acquisition. A second
display terminal is therefore being added. The selected unit (built
by Computer Displays, Inc.) is oriented around the Tektronix 611
storage oscilloscope. It provides both alphameric and graphic
display, as well as an interactive device (a cursor moved by means of
a joystick) for a price of $8000 to $12,000 plus the cost of
interfacing to the computer.
F. MULTIPLE-COMPUTER SYSTEMS
1. Introduction
At the Rutgers-Bell (RB) nuclear physics laboratory, work has been done
with two different two-CPU systems. The first of these represented
essentially two duplicate processors (Figure 8), and the second, now in
the process of implementation, two processors of different size and
capability (Figure 9). While full data are not yet available on the
actual performance of the second system, an outline of the experience
to date will be given.
[Illustration: FIGURE 8 The two-central-processor system of
Rutgers-Bell.]
2. Two Equivalent Processors
The initial success of the original RB SDS 910 data-acquisition
system was soon tempered by a result of its popularity: during most
experiments the computer was unavailable for program development or
data analysis. Since most experiments required the use of displays
and light pens in at least one stage of data analysis, the computer
center could not handle the work.
[Illustration: FIGURE 9 The new Rutgers-Bell Sigma 2-Sigma 5 system.]
The solution adopted was to acquire another computer with the same
instruction set (an SDS 925) and to provide switches such that the
line printer, card reader, and plotter could be run from either
computer. No provision was made for direct transfer of data from one
computer to the other.
3. Lessons from Operating Experience
In practice this system worked out quite well. There was complete
interchangeability of programs from the 910 to the 925, which
differed only in being five times faster. Normally the switchable
peripherals were run from the 925; when the group taking data wished
to print or plot current spectra, they consulted with the 925 users,
then used the peripherals with little more difficulty than
permanently attached units would have involved.
A further advantage of the switchable peripherals, in addition to the
cost saving, was that the experiments associated with the 910 could
proceed while the peripherals were being serviced. The 910 is
exceedingly reliable, averaging less that one main frame failure per
year, and the 925 is nearly as reliable. The vast majority of service
calls have been occasioned by the peripherals and have competed with
data analysis but not with accelerator utilization.
In addition to the switched peripherals, both computers were equipped
with two magnetic tape transports, electric typewriter, and
high-speed paper-tape reader and punch. While these units were also
subject to downtime, the paper-tape system and the typewriter could
be exchanged between the 910 and 925. Only the magnetic-tape
transports required the use of the 910 CPU during servicing, and the
presence of two transports has usually meant that the second one
could carry the load until the weekly accelerator maintenance period.
While the reliability record of the central processors has been
excellent, that of many of the peripherals has not. Here is an
excellent justification for renting computing equipment: if units do
not work well, they can be returned. For a time, a low-cost card
reader (100 cards per minute) built by NCR for SDS was used. It was
unacceptable in reliability and was replaced by the Univac reader
which came with the 925. Another unit returned was a cartridge
magnetic-tape system built by SDS. The Ampex TM-4 magnetic-tape
transports on both the 910 and 925 have been consistently poor in
reliability, but no other unit has been available to replace them. A
manufacturer's name does not seem to be a guarantee of good or bad
quality--the line printer, also made by NCR, has been excellent both
in reliability and print quality.
4. Limitations on a Twin-Computer System
While the two-computer system generally rated high in user
satisfaction, considerations of performance have led to the design of
a larger and more powerful system with totally new components. The
925, without wired multiplication or floating-point operations, was
too slow for theoretical computation or for many types of data
analysis such as those using Monte Carlo methods. Interactive methods
of analysis, using a display and light pen, have been found very
effective in the cases where the 925 could accommodate them but have
not been available through either the Bell Laboratories or Rutgers
computer centers.
A further limitation on the earlier system was that only one person
could use the 925 at a time. The generation of a display involved the
full time of the CPU, and while multiprogramming might have been able
to divert some CPU time, the 8k memory size did not permit it.
Data acquisition on the 910 was limited in array size to the capacity
of the core memory. For multiparameter experiments, three, six, or
even twelve 4096-channel arrays have been stored in core, but the
advantages of live display available with core storage have
discouraged anyone from handling large arrays by logging raw data on
magnetic tape for analysis later. Memory expansion would have been
desirable, but the necessity of making the expansion on both the 910
and the 925 effectively doubled the cost.
Limited flexibility, then, is a major drawback of this type of
system. As long as only two users needed to be accommodated, and each
could adapt to exactly half of the total core storage, it was
satisfactory and provided redundant facilities to guard against
experiment downtime due to computer failures.
5. New Directions
In ordering a new computer powerful enough to handle most of the
nuclear physics laboratory's data analysis and theoretical computing
tasks, cost ruled out the acquisition of a pair of program-compatible
computers. It was recognized that desirable features of the original
system would have to be obtained in new ways. Accessibility of the
system for programming could be improved by running a simple
time-sharing monitor on it. Reliability could be enhanced by avoiding
bargain peripherals and using only items of demonstrated high quality
and by the capability of running the peripherals on either computer.
The use of a separate CPU for data collection still seemed
particularly desirable, however. A combination of a large (by present
standards) computer with a powerful small computer as a front end was
designed. It includes a display disk for refreshing displays without
CPU attention, as well as for storing data arrays too large to be
kept in core. The computers selected were a 32k, 32-bit SDS Sigma 5
and a 12k, 16-bit Sigma 2.
The new system, with separate and nonequivalent computers, will have
advantages over the old system in data analysis and general
computation, because these will be done on the larger computer,
either in time sharing or batch mode. Time sharing should enhance the
flexibility of the system by making it easier to generate and debug
new programs, in addition to improving the accessibility.
For the data-collection computer, RB will lose the advantage of a
separate computer on which complete debugging of programs may be
done. This loss can be tolerated since the fraction of the load
carried by the Sigma 2 will be less than that carried by the 910 in
the old system. In the old system, very few distinct data input or
display programs were written. A few subroutines and their calling
parameters sufficed for all needs for six years; the logic and I/O
operations unique to each experiment were written in Fortran by the
experimenters.
In the new system, the Sigma 2 will be concerned with the operations
used in the data acquisition and formating of displays; most of the
rest can be left in the Sigma 5, with routines sent over to the Sigma
2 under the time-sharing system. If the user should prefer, he can
operate the Sigma 2 directly and make use of the Sigma 5 only for
data storage.
Until very recently, program development on the Sigma 2 has been slow
because it lacked means of getting program listings quickly. We have
now developed an assembler for the Sigma 2 which runs on the Sigma 5.
The availability of card reader input and line printer output has
greatly speeded Sigma 2 software development. The loading of Sigma 2
programs is also much more convenient, since they can be stored on
the Sigma 5 disk and loaded exactly as Sigma 5 programs. It seems
highly desirable to have an assembler for any small data-acquisition
computer capable of running on another machine; the means of
transporting the object code to the small computer is of less
importance.
The reliability of the new equipment has been excellent. Only the
card reader has had any downtime of consequence, and modifications
seem to have resolved its problems. The Sigma 5 main frame has had no
failures in 12 months, and the Sigma 2 has had only one in the past
year. If this record continues, the loss of the redundancy inherent
in the old 910/925 system will not have any serious effects.
6. Computer-Independent Data Bus System
One component of the new system is taking on an increasingly
important role, although it had not been a part of the original
planning. That is the computer-independent data bus consisting of
system controller, bin controller, and register units. Only the
system controller is specific to a particular computer; moreover the
same system controller design could be used on both the Sigma 2 and
Sigma 5 by restricting the data path to 16 bits. The register units
are used to interface external devices to the computer quite cheaply;
a typical register used here to interface an existing Calcomp plotter
to the new computers costs about $300 in parts and labor. Similar
units are used to interface the Sigma 5 to the Sigma 2 and to the
910, to drive a temporary core-resident display on the Sigma 5, to
read pushbutton inputs, and to read ADC's. The display disk
controller now under construction uses these registers to furnish
control information, although the data go directly to and from the
core.
At the present time, the registers are read and written under
program interrupt control, but the design is not limited to
program-controlled operation. By substituting a controller designed
to operate automatically (directly to core or to the I/O processor)
speeds approaching 1 or 2 µsec per word transferred could be
obtained. Such interfaces have been built for various computers using
the European CAMAC bus system, which is conceptually similar.
The system is highly modular and is built into NIM bins with modified
back connectors. Exchange of modular units has been very helpful in
debugging the system, and presumably it will also be helpful in case
of failures in operation. This is a much more satisfactory situation
than that which was obtained with the ADC interface on which RB
collaborated with Brookhaven. The latter unit was built with
computer-type construction: commercial logic cards and wire-wrapped
back panel. Debugging of that unit was exceedingly laborious because
of the lack of modularity in its components.
The computer-independent bus system has not been expensive in
manpower. It has required about 9 man-months in design and debugging
and somewhat less time in construction. The registers cost about
$300, as mentioned, and the controllers $1500 to $2000 depending on
the need for cable drivers.
7. Costs
The costs of the RB multiple-computer system are given in Table 6.
The figures are approximate and not the result of detailed
accounting.
TABLE 6 Systems Costs of Rutgers-Bell Multiple-Computer System
---------------------------------------------------------------------
a. Original 910/925 system
Systems/programming 1 man-year
Interface design 910 6 man-months
925 3 man-months
Interface construction 910 $ 15,000 including display
925 $ 11,000
Computer costs 910 $ 3,500/month; bought
with rental allowance
for $58,000
925 $ 4,800/month; including
line printer
and card reader
Maintenance and updating 910 1 man-year, over 6 years
b. New system (including some components not yet acquired)
Planning and expediting 1 - 1/2 man-years
Systems programming 3 man-years to date,
2 more expected
ADC interface
Engineering design $ 20,000
Construction and test 9,000 spent outside
Parts 11,000
Time spent locally 9 man-months
Data bus system
Parts $ 5,000
Design and debug 9 man-months
Construction 6 man-months
Display disk system
Disk with 64 heads
and amplifiers $ 24,000
Interface to Sigma 5,
Sigma 2, and displays $ 10,000 (estimated,
since design is
not complete)
4 14-in. displays with analog
rotators and light pens $ 12,000
Three teletypes $ 5,000
Miscellaneous technical work 1 man-year
Computer costs
Sigma 2 $ 50,000
Sigma 5 $440,000 purchase
equivalent, but
part is leased
TOTALS FOR NEW SYSTEM
CASH COSTS $586,000
TIME 8 man-years professional
3 man-years technician
---------------------------------------------------------------------
G. A PROCESS-CONTROL SYSTEM: THE BROOKHAVEN MULTIPLE SPECTROMETER
CONTROL SYSTEM (MSCS)
1. Introduction
In 1965, a system based on an SDS 920 computer was put into operation
at the Brookhaven National Laboratory to control data-acquisition
processes involving eight neutron spectrometers and one x-ray
spectrometer. The neutron spectrometers are located on the floor
surrounding the High Flux Beam Reactor (HFBR); the x-ray spectrometer
was placed in the same building in order to facilitate linking it to
the computer. The system can control the execution of experiments on
all nine sets of apparatus simultaneously, yet each experimenter
feels that he is working essentially independently of all other
users. The system controls all angular rotations of crystals and
counters, all detector counting, the data displays, the input and
output operations, and automatic error responses.
It can also perform most of the calculations necessary for real-time
guidance of the course of the experiments. For example, the
experimenter can mount a crystal on a goniometer at approximately the
correct angular orientation, then he can specify to the computer
where several peaks should be found, whereupon the computer will
direct the execution of a trial experiment to find where the peaks
do, in fact, occur, executing least-squares calculations in the
process, after which the error in crystal orientation is known and
the angular scales are automatically corrected. In another example,
the computer is given as input information the crystal constants
(unit cell) and the zone orientation of the crystal on the goniometer
and is asked to produce a scanning of a given part of reciprocal
space. The computer then calculates where to look, turns to a correct
angle to check the intensity of a central peak, and performs the
other necessary steps, making many decisions as it controls the
execution of the entire experiment.
2. Description of System
When it was first assembled, the system included only two teletypes,
both located near the computer. Early in 1969, a communications network
was added to permit the installation of a local, assigned typewriter at
each of the nine spectrometer stations, as well as three assignable
remote teletypes located in the Chemistry and Physics buildings. This
network incorporates a Varian 620i computer. It permits any ordinary
operation to be carried out from any of the 12 remote stations, except
program loading, which still must be done via the high-speed paper-tape
reader at the computer.
[Illustration: FIGURE 10 The multiple-spectrometer control system at
Brookhaven National Laboratory.]
[Illustration: FIGURE 11 Block diagram of a single-spectrometer
control station of the MSCS shown in Figure 10. [From D. R. Beaucage,
M. A. Kelley, D. Ophir, S. Rankowitz, R. J. Spinrad, and R. Van
Norton, Nucl. Instrum. Methods _40_, 26 (1966).]]
The major parts of the system (Figure 10) are the SDS 920 computer
with a 16k, 24-bit memory, a bulk storage memory section comprising
two magnetic tapes units and one 32,010-word drum, the communication
network, and the nine local control stations (SCS) at the
spectrometers. Each SCS (Figure 11) contains the stepping motors
required for computer control of angular rotations of crystals and
counters, together with shaft rotation encoders (optional,
incremental type) to feed information back to the computer. Each SCS
also includes manual controls, the electronic counters associated
with the radiation detectors, counter displays, a decoding and
control section, and other related equipment.
3. Lessons from Operating Experience
a. The system now does "all things imagined to be necessary."
b. The computer has proved to be remarkably reliable, with a record
of about 40,000 hours of use without a breakdown.
c. A reasonable amount of preventive maintenance is done, mostly
during the one week of four that the reactor is shut down.
d. One person serves as operator and programmer (for simple jobs). He
also transports magnetic tapes to the computing center for off-line
data processing and performs smaller tasks. The average user does not
need to do any programming.
e. Fortunately, the people who have written most of the programs have
remained in attendance and have updated the programs frequently.
Machine-language programming has not proved to be a bad chore because
the system is a fixed-hardware setup.
f. Modes of data collection can easily be changed.
g. The overall performance is excellent. The only problem is an
occasional wiping out of a program due to the fact that there is no
hardware memory-protection feature in the computer. These accidents
are estimated to cost at most a loss of a few percent of the running
time.
4. Costs
The costs in manpower and dollars of the MSCS are given in Table 7.
TABLE 7 Cost in Manpower and Dollars of MSCS
-------------------------------------------------------------
a. Engineering Design and Costs (Professional Only) over 12
Calendar Months
Man-Months Cost
---------- --------
Electronic equipment
Development, design,
construction, and
startup 15 $ 28,000
Mechanical development and
liaison 4 7,000
System coordination,
development, design,
coding, etc. 30 56,000
Parameter generating, data
analysis, and programming 15 28,000
---- --------
TOTAL 64 $119,000
b. Construction Time and Costs (Technical) over
16 Calendar Months
Man-Months Cost
---------- --------
System construction and
interconnection 15 $ 17,000
Debugging and startup 6 7,000
Documentation and
drafting 12 14,000
---- --------
TOTAL 33 $ 38,000
c. Major Components
Cost
----------
Original cost
SDS 920 (16k mem.,
96 interrupts) $147,000
15-kc mag. tape and controller 20,000
1 add'l. 15-kc tape 15,000
Magnetic drum memory 36,000
9 Spectrometer control stations 86,000
Off-line paper-tape pre. unit 5,000
----------
TOTAL $309,000
Replacement: The major components of the MSCS cannot be
replaced by new line units as they are no longer in production.
d. Operating Costs
Normal use Per Year
---------
Computer operator/programmer $ 11,000
Computer maintenance 12,000
Materials 4,000
Misc. (minor system improvements) 10,000
Overhead 5,700
---------
TOTAL $ 42,700
Manpower required
Computer operator/programmer 1
Systems programmer (as required) 1
Maintenance personnel (part-time) 2
Note: All manpower with the exception of the on-call systems
programmer has been costed in a above.
e. MSCS Communications Network
The Communications Network adjunct to the MSCS was started
October 1967, and it became operational early in 1969.
Man- Calendar
Cost Months Months
------- ------ --------
1. Engineering design
and programming $25,000 10 12
2. Components (commercially
available) 50,000 - -
3. Construction 6,000 4.2 7
------------------------
TOTALS $81,000 14.2 19
-------------------------------------------------------------
H. RELATIONSHIP TO A REMOTE COMPUTING CENTER
1. The Small Computer with a Fast Data Link to a Remote General
Computing Facility
Although the use of a small data-acquisition and experiment-control
computer on-line to a remote computing center machine is not uncommon
in high-energy particle physics applications, we know of few such
systems presently operating in low-energy nuclear physics.
For the purposes of this discussion, we define "general computing
facility" to be a relatively large-scale centralized installation
charged with the responsibility of servicing a wide range of
computing needs. The typical university computing center is our model
for such a facility.
In light of the fact that only a few years back the remote computer
on-line to a general computing facility was considered to be the wave
of the future, with plans for such systems under vigorous discussion
at many low-energy physics installations, it is at first sight
surprising that there is so little progress to report at this time.
The Van de Graaff accelerator laboratory at the State University of
New York at Stony Brook was one such facility planning to couple a
PDP-9 on hand to an IBM System 360/67 available at the university
computing center. It is instructive to examine what happened there.
In 1967, with the completion of the new accelerator scheduled within
a year, it was decided that the best way to acquire the desired power
and flexibility in computing support was through a coupled system of
the kind under discussion. Plans were formulated for a high-speed
transmission line to a control unit on a selector channel at the
computer center. Since true time-sharing of the System 360/67 was not
in the offing, a 128k-byte partition of high-speed core storage was
to be permanently dedicated to the needs of experimental physics
(including the particle-physics group), and a high-speed
program-swapping drum and at least one tape drive were to be assigned
to the physics users as well.
What actually happened was that as funds became available to the
low-energy physics group to implement its share of the remote link to
the computer center, sentiment shifted to the point of view that the
funds could more usefully be invested in a second PDP-9 installed at
the accelerator, and the second small-to-intermediate class computer
was in fact purchased. The two PDP-9's are coupled only by a
switchable tape drive, with no plans at present for direct
channel-to-channel communication. Plans for a remote link to the
computing center have been completely dropped; any further funds for
computing will be invested in larger high-speed core stores for the
PDP-9's, at least in the foreseeable future.
Conversations with the principals involved in the operation of the
Stony Brook low-energy physics facility fail to yield a clear and
uniform explanation of the change in computing outlook. One cannot
escape the impression that the group was not wildly enthusiastic
about the proposed remote linkup in the first place, and that the
evident immediate benefits to the group of a second PDP-9 on hand for
program debugging and experiment setup while the second machine was
running an experiment were irresistible when compared to the future
promise of a remote link to the IBM 360/67. The physicists were not
anxious to undertake what was expected to be a substantial systems
program development task for the coupled system, being unconvinced
that the result would be worth the effort. While they still wish to
increase the computing power available to them on-site, they have
elected to achieve that end by expanding high-speed core storage on
their machines, at least until true time-sharing becomes available at
the central computing facility.
The coupled system at the University of Manitoba cyclotron is
representative of what was intended at Stony Brook. At that
installation, the PDP-9 is linked to the computing center's IBM
360/65 by a control unit commercially available from DEC for about
$15,000. The unit connects the PDP-9 (or its successor, the PDP-15)
directly to a System 360 selector channel, without requiring an
additional control unit. The maximum data-transfer rate at Manitoba
over a 2000-foot twisted pair cable is 50k bytes/sec. A relatively
unsophisticated set of system programs has been written to control
communication and transfer of data between the two computers.
The only experiment to which the coupled system (as distinct from the
stand-alone use of the PDP-9) has been applied is a p-p
bremsstrahlung measurement, where the data are developed in wire
spark chambers and plastic scintillation counters. Information from
the wire chambers defines proton trajectories, and pulse heights from
the counters determine their energies. The PDP-9 first tries to
reconstruct a vertex from the proton trajectories. If a point of
origin can be determined for the protons to the required accuracy,
the relevant coordinates for the proton trajectories and the pulse
heights are sent to the IBM 360/65 for full kinematic and statistical
analysis of the individual event; otherwise, the event is rejected.
The large computer also prepares displays and plots of physical
interest that are returned to the PDP-9 for display on the local CRT
or output on the local _x-y_ plotter.
The remote computer operates in a multiprogramming rather than in a
time-shared environment, with an assigned partition of 65k bytes.
Because of the well-designed program overlay feature of the 360/65
operating system, the Manitoba group does not find itself restricted
by this relatively small partition. Because of other demands on the
computing center, however, they are restricted in the use of this
partition to 16 hours/day and 5 days/week. The operation of the
coupled system is controlled almost entirely from the PDP-9 teletype,
with 360/65 operator intervention required only for initial loading
of the partition, off-line printout, and, of course, mounting
magnetic tapes at the computing center.
Users of the Manitoba system are pleased with the cooperation and
service they have received from the computing center thus far, and
they are anticipating no difficulties developing as their demands on
the central computing facility increase. But while use of the coupled
system for experiments other than that described is clearly possible
and desirable, no information was available on plans for the future.
The Brookhaven on-line remote network (Brooknet), where a pair of CDC
6600 machines sharing a common one million _word_ extended core
storage unit may be interfaced over a high-speed channel to as many
as 64 remote data-acquisition computers, can be considered an extreme
example of a coupled system. Although the software for Brooknet is
reported to be complete and debugged, the system has not yet begun
routine operation, and the first remote computer intended for
low-energy physics application (a PDP-15) has not yet been delivered.
(The only Brooknet user at present is the Chemistry Department, which
has a remote batch terminal: teletype, card reader, and printer.)
2. Reasons for Lack of Popularity
Why has linking data-acquisition computers directly to computing
centers not proved as popular as the obvious advantage of having
access to an extremely powerful computer would lead one to expect?
There are a number of contributing factors:
1. Since the remote computer can be used only if it is in operating
condition and if the necessary personnel are present, the physicist
stands to lose some of his independence and flexibility of operation
(often not four-shift operation).
2. Most remote computers operate on a multiprogramming basis, hence
prompt interrupts are not available. The waiting time for attention
might typically be several tenths of a second, therefore the computer
in the physics laboratory should be fairly powerful in order to
handle the preliminary processing and buffering. With such a computer
at work the necessity for fairly rapid access to the large remote
machine may entirely disappear, or else the experimenter may be able
to store partly processed data on magnetic tape for subsequent
further reduction off-line at the computing center.
3. The total amount of time available to one user of a shared-time
system per day is always limited. The amount of access time
guaranteed by the computing center may not be sufficient.
4. In some cases there is a question of charges, and the total
expense of involvement with the computing center may be comparable
over a period of several years with the extra cost of buying a
sufficiently large local computer for the laboratory to be able to
handle all the essential on-line calculations. Even though the
calculations may take longer in terms of machine time, they may not
require as much lapsed real time if there are stringent limitations
on computer center access time.