August 1956 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
The radar system I worked on in the USAF used two early memory types
described in this 1956 article. In fact, the radar was designed
during that era, so it is no surprise. Our IFF (Identification Friend
or Foe) secondary radar had a whopping 1 kilobyte of magnetic core
memory in its processor circuitry. It consisted of 1024 tiny toroids
mounted in a square matrix with four hair-width enamel coated wires
running through them as x and y magnetization current lines, sense,
and inhibit functions. If my memory serves me (pun intended) after
three decades away from it, the TTL circuitry (no microprocessor)
stored range values to calculate speed and direction from sample
to sample. The other memory type was a mercury acoustic delay line
contraption having a piezoelectric transducer at one end to launch
an electrical pulse along its length and another transducer at the
other end to convert back to an electrical pulse. It was used to
cancel out stationary targets (clutter) as part of the MTI (moving
target indication) circuitry in our precision approach radar (PAR).
The Electronic Mind - How it Remembers
By H. H. Fantel
Man's machines are doing the work of his muscles ... now, electronics
explores ways to take the load off his mind
Close-up of magnetic core memory consisting
of small ferrite rings retaining information imparted through
pulses from the wire network.
When I was about five years old, I always carried a piece of
string in my pocket. I used to tie knots in it to remind myself
of things I shouldn't forget. Without knowing it at the time, I
had, in effect, developed a "machine mind." To be sure, it wasn't
capable of very fine mental distinctions. But it could tell the
difference between yes and no. A knot meant "yes" (= there is something
I ought to do). No knot meant "no" (= relax and do nothing).
Today's digital computers use essentially the same system. Instead
of a string, there is a wire. Instead of a knot, there is a voltage
pulse. Otherwise, it's the same. The pulse means: go ahead and do
something. No pulse means: rest.
In the language of computer engineers, each "pulse" or each "no-pulse"
is called a "bit," because each represents one specific bit of information.
Together, and arranged in logical sequence, billions of bits make
up the complex patterns which direct the computers toward the solution
of a problem. Yet everything the machine needs to know is broken
down into simple yes-or-no propositions. "Pulse" or "no-pulse" is
all that any part of the machine ever needs to distinguish, all
it ever needs to remember. The computer's memory is therefore a
device for storing electrical pulses in predetermined patterns.
Over-all view of the basic components of
a Univac computer. Behind the control desk in foreground, the
central computer mechanism occupies cabinet at rear. The two
round objects showing through open center door are mercury delay
lines, part of the machine's "internal memory." The array of
tape recorders at right serves as "external memory." Automatic
typewriter in center spells out answers to problems.
Human concepts expressed through words and
numbers must be broken down into simple yes-or-no "bits," somewhat
like playing "20 Questions," except that here the elementary
items of information run into billions. This machine converts
numerical data from keyboard into monosyllabic computer code
recorded on tape. The information is then retained in the computer's
External and Internal Memory. Most electronic
computers have two basic types of memory - external and internal.
As the names imply, the external memory is a sort of auxiliary,
tacked onto the central computer mechanism. The internal memory
is located right within the works of the computer.
The external memory acts as a go-between for the machine and
its human masters. It remembers the instructions given to the computer'
by human mathematicians. By setting up the external memory, scientists
can "program" the computer to do a given job. This memory also acts
as a sort of "time transformer" to match the slow speed of human
beings to the high speed of the machine. Scientists might take days
to punch a set of instructions into the external memory. Later,
the memory can reel out this row of instructions fast enough to
keep up with a computer when it zooms through the whole problem.
Physically, the external memory takes the form of punched cards,
magnetic tape, or punched paper tape.
Punched cards and paper tape have long been in use with conventional
office equipment as data storage media - punched-card tabulating
equipment, for example, has become commonplace in American business,
and the paper-tape-driven automatic typewriter has relieved many
a typist of repetitive labor. The data storage principle in each
of these cases is extremely simple: information is recorded on the
card or tape in the form of punched holes. A hole means an electric
pulse; no hole, no pulse. Because of the wide use of these storage
devices, some computers have been equipped with punched-card and
punched-tape readers, and a number of machines have been developed
to convert these media automatically to magnetic tape.
In magnetic tape storage, the most commonly used form of external
storage for the electronic computer, characters are represented
by combinations of tiny magnetized and unmagnetized spots, each
representing a "bit" of information. When data recorded on the tape
is fed into the computer, the magnetic tape reader translates the
combination of magnetized and non-magnetized spots to a series of
electronic pulse/no pulse combinations which, in computer language,
have a definite meaning.
The great advantage of magnetic tape over other external memory
devices is its tremendous capacity. One reel of magnetic tape, eight
inches in diameter and one-half inch wide, can store approximately
2,880,000 characters, with up to 200 characters recorded per linear
inch. The tape will preserve data permanently and will not corrode.
It can be "erased" and used again for new data.
Punched cards are also used to give instructions
to the computer. The pattern of holes is then converted into
a sequence of electric pulses.
The machine uses its internal memory in the same way that human
beings use scratch pads in solving a math problem. It is a place
to put down intermediate results and the auxiliary numbers in the
course of a calculation. Since such figures - in the form of electronic
pulses - must be quickly jotted down, stashed away, or shifted from
one place to another, some very tricky mechanisms were developed
to do all this juggling of "bits." Mainly, they are storage devices
permitting information bits to be quickly put in, quickly taken
out, and accurately placed in a meaningful over-all pattern.
Electrostatic Storage. These storage devices
are cathode-ray tubes resembling picture tubes in television sets.
The screen of such a tube acts like a checkerboard of small capacitors.
Whenever the "writing beam" of the tube hits one of the "squares"
on this checkerboard, its capacitor effect stores up an electric
charge imparted to it by the beam. The guided beam places a pattern
of charged and uncharged areas on the checkerboard screen. Each
area is a "bit," and the arrangement of the bits on the board spells
out a numerical meaning. This pattern of electrostatic charges can
be "read" by another beam scanning the area.
Information stored in such a way can be reached very quickly. It
only takes between five- and ten-millionths of a second to "read
out" a digit of information and shift it to wherever it is needed
for the next logical operation of the computer. Because the computer
must spend much of its operating time searching its memory for data
and instructions, the speed of access to specific information largely
determines the over-all speed of the system.
The storage capacity of a single tube is limited usually to about
1000 bits. Tubes can, of course; be used in combination to form
large-capacity storage units.
Mercury delay line serves the Univac I as
internal memory. Porcupine appearance is due to circuitry on
tank surface, amplifying and re-shaping pulses for endless repetition
in this "talk-to-yourself" mumble-tub.
One disadvantage of electrostatic storage is that the data are
lost from the tube's surface unless they are constantly regenerated
by the writing beam. In other words, the machine has to keep talking
to itself to remember what it is saying. Although this is done entirely
by automatic circuitry, the stored data are lost in the event of
a power failure.
A certain unreliability stems from the fact that beam guidance
is highly critical. A small temporary voltage change on the deflection
plates can direct the beam to the wrong checkerboard area on the
face of the tube, resulting in false information. Frequent adjustment
by engineering personnel is essential for operating reliability.
Electro-Acoustic Delay Line. One of the first
memory systems to gain wide commercial acceptance for electronic
computers was the electro-acoustic delay line. In simplest terms,
a delay line memory stores electronic data by constantly recirculating
the information pulse pattern in the form of sound through a delay
element, usually a tank of mercury. At the precise moment when the
information must be inserted into the calculation, it is picked
up by a "listening" device at the far end of the tank and transferred
to where it is needed. The memory tank can then stop mumbling to
The process may be likened to the short-range human device of
repeating a phone number to oneself from the time it is located
in the phone book until it has been dialed.
Magnetic drum principle is illustrated in
simplified drawing above. Each magnetic head reads or writes
pulses received from the computer onto its own track on the
magnetically coated rotating drum.
Family of drums, ranging from giant roller
to small twirler held by the engineer, make a veritable "memory
lane" for Remington Rand.
Small drum rotor whirls within the Univac
at 16,500 rpm in an atmosphere of helium, which reduces friction
and draws off excess heat.
Magnetic core matrix is a net of tiny ferrite
rings, each storing one "bit" of information.
Wikipedia Magnetic Core Memory photo
A mercury delay line memory channel consists simply of a mercury
tank capped at each end by a transducing crystal, and a closed recirculation
circuit. As pulses are delivered to the memory from the computer's
control circuits, the crystal at one end acts as a sort of loudspeaker
and sends a series of pulse/no-pulse tones through the mercury.
These pulses thus travel through the mercury at the speed of sound,
much lower than the speed of electronic pulses through wire. This
provides the delay needed to hold information for controlled lengths
As the pulses reach the end of the mercury column, they strike
the second transducing crystal - which acts as a microphone - and
produce small electrical voltages that can be amplified and fed
back to the input. In this way, the bits continually go in circles
until they are replaced by other data from the control circuits.
Then the whole cycle starts anew.
Each tank has automatic counting devices, which count the number
of times a message is repeated. This provides accurate timing within
a few thousandths of a second for picking up the message when it
Magnetic Drum Storage. The magnetic drum is
an aluminum cylinder or "drum" coated with a magnetic material and
equipped with a row of read-write heads. As the drum spins at high
speed, each head monitors a narrow "track" around the circumference.
When data are to be stored, the pulse combinations are flashed
to the heads and are recorded in the form of magnetized and unmagnetized
areas on the drum's surface. The operation is very much like recording
data on a broad, endless loop of tape with many channels.
An important advantage of magnetic drums is their capacity; such
drums are made to hold as many as 2,552,000 bits, or about 350,000
characters. Other favorable features are their .low cost and the
fact that they retain stored data indefinitely.
Magnetic Core Memory. In recent years, pure
physical research has opened up new horizons through the discovery
of a new basic material: the ferrites. Their impact on electronics
is a whole story in itself. Here we are concerned only with their
possibilities as memory devices.
Magnetic polarity can be established in ferrites by a current
of sufficient strength. Once established, the magnetic field resists
change until an equally strong current is passed in the other direction.
Thus, for example, the magnetic field of a ferrite with positive
polarity can be reversed by applying a sufficiently strong negative
In computers, this physical fact forms the basis of a memory
system. The ferrites are shaped into doughnut-like rings, called
toroidal cores, which are wired together in checkerboard "matrices."
A pair of wires, one horizontal and one vertical, intersect at each
core. To store an electronic "bit," it is only necessary to apply
a polarizing voltage to the pair of wires meeting at a certain core.
The magnetic field of that particular core will then shift and hold
its new polarity, i.e., it will hold one "bit" of pulse-type information.
None of the other cores will be affected, since the current in any
one wire is not strong enough to reverse the cores' polarity. Only
at the intersection point of the two pulsed wires, i.e., at the
desired core location, do the two currents summate and thus achieve
sufficient strength to evoke magnetic response.
Magnetic core memory is the current favorite among computer engineers.
The reasons are easy to see. To "read out" the information stored
in the core memory requires no more time than it takes to send a
"sensing" pulse along the diagonal lines to pick up the pattern
of magnetic shifts. Six characters can be transferred into or out
of the memory in about 20-millionths of a second. At this rate of
figuring, the answers come up fast.
Here, with the most advanced of memory devices, we are actually
closest to my erstwhile piece of string with the knots tied in it.
We may quite literally think of the pulsing wires as strings and
of the magnetized cores as the knots. The magnetizing pulses "tie"
the knots, and the sensing pulses "feel along" the string to locate
Of course, my string didn't know the meaning of the knots. Only
I did. Neither does the machine mind know the meaning of its memories.
Only the creative mind of a human being is able to translate pulses,
"bits," characters, or even words and numbers into the kind of living
sense from which men build their world.
Posted February 26, 2016