The First Generation Computers

Estelle Sidler

The First Generation Computers Do you remember this computer? It is the Bendix G-15 General Purpose Digital Computer, a First Generation computer introduced in 1956. Another picture (66k). And another (105k). You can download larger versions of the following pictures on this page by clicking on them. Why this interest […]

The First Generation Computers

Do you remember this computer?

It is the Bendix G-15 General Purpose Digital Computer, a First Generation
computer introduced in 1956.

Another picture
(66k). And another
(105k). You can download larger versions of the following pictures on this
page by clicking on them.

 Our G-15, front view

 Our G-15, front view

Why this interest in the Bendix G-15?

Against the odds, the
Western Australian branch
The Australian Computer Museum Inc
has rescued one from the scrap heap.
That’s it, over on the right.

It is in pretty good condition, considering its age, and we hope one day
we can get it working again. We also have various programming, operating and
technical manuals, and schematics. They have been scanned and you can download
them here.

This web site started life in 1998 as a sort of begging letter, seeking more
information about the maintenance procedures. We have since been told that there
was no formal maintenance manual and that our documentation is complete so far
as maintaining the machine is concerned. Still, if you can help with
some of the
other items we are missing
or add anything at all to our store of knowledge
about the Bendix G-15, please get in touch with me, David Green at
email address.

First Generation Computers.

The first generation of computers is said by some to have started in 1946 with
the first ‘computer’ to use electronic valves (ie. vacuum tubes).
Others would say it started in May 1949 with the introduction of
the first stored program computer. Whichever, the distinguishing feature of the
first generation computers was the use of electronic valves.

My personal take on this is that ENIAC was the World’s first electronic
and that the era of the first generation computers began in
1946 because that was the year when people consciously set out to build
stored program computers (many won’t agree, and I don’t intend to debate it).
The first past the post, as it were, was the EDSAC in 1949. The period closed
about 1958 with the introduction of transistors and the general adoption of
ferrite core memories.

OECD figures indicate that by the end of 1958 about 2,500 first generation
computers were installed world-wide. (Compare this with the number of
PCs shipped world-wide in just the third quarter of 2006, quoted as 59.1
million units by research company Gartner).

Two key events took place in the summer of 1946 at the Moore School of
Electrical Engineering at the University of Pennsylvania. One was the
completion of the ENIAC. The other was the delivery of a course of lectures
on “The Theory and Techniques of Electronic Digital Computers”. In particular,
they described the need to store the instructions to manipulate data in the
computer along with the data. The design features worked out by John von Neumann
and his colleagues and described in these lectures laid the foundation for the
development of the first generation of computers.
That just left the technical problems!

 Bendix G-15, side panel open

 Bendix G-15, side panel open

One of the projects to commence in 1946 was the construction of the IAS
computer at the Institute of Advanced Study at Princeton. The IAS computer used
a random access electrostatic storage system and parallel binary arithmetic.
It was very fast when compared with the delay line computers, with their
sequential memories and serial arithmetic.

The Princeton group was liberal with information about their computer and
before long many universities around the world were building their own, close
copies. One of these was the SILLIAC at Sydney University in Australia.

I have written an emulator for SILLIAC. You can find it
here, along with
a link to a copy of the SILLIAC Programming Manual.

First Generation Technologies

In 1946 there was no ‘best’ way of storing instructions and data in a computer
memory. There were four competing technologies for providing computer memory:
electrostatic storage tubes, acoustic delay lines (mercury or nickel),
magnetic drums (and disks?), and magnetic core storage.

A high-speed electrostatic store was the heart of several early
computers, including the computer at the Institute for Advanced Studies in
Princeton. Professor F. C. Williams and Dr. T. Kilburn, who invented this type
of store, described it in Proc.I.E.E. 96, Pt.III, 40 (March, 1949). A simple
account of the Williams tube is given

The great advantage of this type of “memory” is that, by suitably controlling
the deflector plates of the cathode ray tube, it is possible to redirect the
beam almost instantaneously to any part of the screen: random access memory.

Acoustic delay lines are based on the principle that
electricity travels at the speed of light while mechanical vibrations travel at
about the speed of sound. So data can be stored as a string of mechanical pulses
circulating in a loop, through a delay line with its output connected
electrically back to its input. Of course, converting electric pulses to
mechanical pulses and back again uses up energy, and travel through the delay
line distorts the pulses, so the output has to be amplified and reshaped before
it is fed back to the start of the tube.

 Bendix G-15, side panel and side door open

 Bendix G-15, side panel and side door open

The sequence of bits flowing through the delay line is just a continuously
repeating stream of pulses and spaces, so a separate source of regular clock
pulses is needed to determine the boundaries between words in the stream and to
regulate the use of the stream.

Delay lines have some obvious drawbacks. One is that the match between their length
and the speed of the pulses is critical, yet both are dependent on temperature.
This required precision engineering on the one hand and careful temperature control
on the other. Another is a programming consideration. The data is available only
at the instant it leaves the delay line. If it is not used then, it is not available
again until all the other pulses have made their way through the line. This made for
very entertaining programming!

A mercury delay line is a tube filled with mercury, with a piezo-electric
crystal at each end. Piezo-electric crystals, such as quartz, have the special
property that they expand or contract when the electrical voltage across the crystal
faces is changed. Conversley, they generate a change in electrical voltage when
they are deformed. So when a series of electrical pulses representing binary data
is applied to the transmitting crystal at one end of the mercury tube, it is
transformed into corresponding mechanical pressure waves. The waves travel through
the mercury until they hit the receiving crystal at the far end of the tube,
where the crystal transforms the mechanical vibrations back into the original
electrical pulses.

Mercury delay lines had been developed for data storage in radar applications.
Although far from ideal, they were an available form of computer memory around
which a computer could be designed. Computers using mercury delay lines included
the ACE computer developed at the National Physical Laboratory, Teddington, and
its successor, the English Electric DEUCE.

A good deal of information about DEUCE (manuals, operating instructions,
program and subroutine codes and so on) is available on the Web and you can find
links to it

Nickel delay lines take the form of a nickel wire. Pulses of current
representing bits of data are passed through a coil surrounding one end of the
wire. They set up pulses of mechanical stress due to the ‘magnetostrictive’ effect.
A receiving coil at the other end of the wire is used to convert these pressure
waves back into electrical pulses. The Elliott 400 series, including the 401,
402, 403 used nickel delay lines. Much later, in 1966, the
Olivetti Programma 101
desk top calculator also used nickel delay lines.

 Bendix G-15, side door fully open

 Bendix G-15, side door fully open

The magnetic drum is a more familiar technology, comparable with modern
magnetic discs. It consisted of a non-magnetic cylinder coated with a
magnetic material, and an array of read/write heads to provide a set of parallel
tracks of data round the circumference of the cylinder as it rotated. Drums had the
same program optimisation problem as delay lines.

Two of the most (commercially) successful computers of the time, the
IBM 650 and the Bendix G-15, used magnetic drums as their main memory.

The Massachusetts Institute of Technology Whirlwind 1 was another early computer
and building started in 1947. However, the most important contribution made by the
MIT group was the development of the magnetic core memory, which they later
installed in Whirlwind. The MIT group made their core memory designs available to
the computer industry and core memories rapidly superceded the other three memory

Where Does the Bendix G-15 Fit In?

Table 1 shows, in chronological order
between 1950 and 1958, the initial operating date of computing systems in the USA.
This is not to suggest that all of these computers were first generation
computers, or that no first generation computers were made after 1958.
It does give a rough guide to the number of first generation computers made.

Bendix introduced their G-15 in 1956. It was not the first Bendix computing
machine. They introduced a model named the
D-12, in 1954.
However, the D-12 was a digital differential analyser and not a
general purpose computer.

We don’t know when the last Bendix G-15 was built, but about three hundred of
the computers were ultimately installed in the USA. Three found their way
to Australia. The one we have was purchased by the Department of Main Roads in
Perth in 1962. It was used in the design of the Mitchell Freeway, the main road
connecting the Northern suburbs to the city.

The G-15 was superceded by the second generation (transistorised)
Bendix G-20.

Table 2
shows the computers installed or on order, in Australia, about
December 1962. The three Bendix G-15s were in Perth (Department of Main Roads),
Sydney (A.W.A. Service Bureau) and Melbourne (E.D.P Pty Ltd).

Close-up of packages in situ

Close-up of packages in situ

Overview of the G-15

The Bendix G-15 was a fairly sophisticated, medium size computer for its day.
It used a magnetic drum for internal memory storage and had 180 tube
packages and 300 germanium diode packages for logical circuitry. Cooling
was by internal forced air.

Storage on the Magnetic Drum comprised 2160 words in twenty channels
of 108 words
each. Average access time was 4.5 milliseconds. In addition, there were
16 words of
fast-access storage in four channels of 4 words each, with average
access time of
0.54 milliseconds; and eight words in registers consisting of 1 one-word
register, 1 one-word arithmetic register, and 3 two-word arithmetic
registers for
double-precision operations.

A 108-word buffer channel on the magnetic drum allowed input-output
to proceed
simultaneously with computation.

Word size was 29 bits, allowing single-precision numbers of seven
decimal digits
plus sign during input-output and twenty nine binary digits internally,
double-precision numbers of fourteen decimal digits plus sign during
fifty eight binary digits internally.

Each machine language instruction specified the address of the
operand and the
address of the next instruction. Double-length arithmetic registers
permitted the
programming of double-precision operations with the same ease as single-precision

A CA155 valve package

A CA155 valve package

An interpreter called Intercom 1000 and a compiler called Algo provided
simpler alternatives to machine language programming. Algo followed the
principles set forth in the international algorithmic language, Algol, and
permitted the programmer to state a problem in algebraic form. The Bendix
Corporation claimed to be the first manufacturer to introduce a programming
system patterned on Algol.

The basic computation times, in milliseconds, were as follows (including the
time required for the computer to read the command prior to its execution). The
time range for multiplication and division represents the range between single
decimal digit precision and maximum precision.

                                Single-Precision       Double-Precision

Addition or Subtraction                     0.54                   0.81

Multiplication or Division          2.43 to 16.7           2.43 to 33.1

External Storage was provided on searchable paper tape (2,500 words
per magazine) and, optionally, on one to four, magnetic tape units with
300,000 words per tape unit reel.

More detail about the Bendix G-15 General
Purpose Digital Computer


You’re visitor:

If you know anything at all about the Bendix G-15 we would like to
hear from you. Please get in touch at
Email address

Last update Thursday, 29 September 2011.

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