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The punch card (or "Hollerith" card) is a recording medium for holding information for use by automated data processing machines. Made of stiff cardboard, the punch card represents information by the presence or
absence of holes in predefined positions on the card. In the first and second generations of computing, from the 1950s into the 1970s, punch cards were a primary medium for data storage and processing, but are now long obsolete outside of a few
legacy systems.
Origins
The punched card actually predates computers considerably, originated by
Joseph Jacquard in 1801 as a
control device for the Jacquard looms. Such cards were also used as an
input method for the primitive calculating machines of the late 19th
century.
The version by Herman Hollerith, patented on June 8, 1887 and used with mechanical tabulating machines
in the 1890 U.S. Census, was a piece of cardboard about 90
mm by 215 mm, with round holes. This was the same size as the dollar bill of the time, so that storage cabinets designed for money could be used
for his cards. The early applications of punched cards all used specifically-designed card layouts. It wasn't until around 1928
that punched cards and machines were made "general purpose". In that year, punched cards were made a standard size, corresponding
to the US currency of the day.
To compensate for the cyclical nature of the Census Bureau's demand for his machines, Hollerith founded the Tabulating
Machine Company (1896) which was bought by Thomas J. Watson SR., founder of IBM in 1914. IBM manufactured a wide variety of business machines and eventually married the punched card to its early
computers, encoding binary information as patterns of
small rectangular holes.
The IBM punching format with rectangular holes eventually won out over the Univac
90-character format, which used 45 columns (2 characters in each) of 12 round holes. IBM (Hollerith) punched cards are made of
smooth stock, .007 of an inch thick. There are about 143 cards to the inch thickness; a group of such cards is called a
deck.
Functional details
The method is quite simple: On a piece of light-weight cardboard, successive positions either have a hole punched through them
or are left intact. The rectangular bits of paper punched out are called chads. Thus, each punch location on the card represents a single binary digit (or "bit"). Each column on the card contained
several punch positions (multiple bits).
The IBM card format, which became standard, held 80 columns of 12 punch locations each, representing 80 characters (since 12
bits is more than enough for representing a character, not all combinations were used.) originally coded: 1 punch (digit[0-9])
was a digit, 2 punches (zone[12,11,0] + digit[1-9]) was a letter, 3 punches (zone[12,11,0] + digit[1-7] + 8) was a special
character, later the introduction of EBCDIC allowed columns with as many as 6 punches
(zones[12,11,0,8,9] + digit[1-7]).
The IBM cards could be used in 2 different modes. In the "text mode", a column represented a digit or other character. As these cards had 12 bits in a column, while
computers only used 6-bit or 8-bit characters internally, not every combination of holes was legal. On the other hand, in the
"binary mode" every column was treated as a simple bitfield, and every combination of holes was permitted (one column was then
read into 2 bytes). In this "binary mode", cards could be made in which every possible punch position had a hole: these were
called "lace cards." Some other computers (e.g., the IBM 700/7000 series) "binary mode" treated every row as two 36-bit
words, in columns 1-72, ignoring the last 8 columns.
Other coding schemes, sizes of card, and hole shapes were tried at various times. Often
the text was also printed at the top of the card, allowing humans to read the text as well, if the cards were produced by a
card-punch machine (called a "key-punch"), which was like a large, very noisy typewriter. There were also cards with all the
punch positions perforated so programming or data could be punched out manually, one hole at a time, with a device like a blunt
pin with its wire bent into a finger-ring on the other end.
In applications requiring storage of multi-character data, such as words or large numbers, the card columns were used in
groups known as fields.
Electromechanical equipment (called unit record equipment) for punching, sorting and printing the cards was manufactured. These
operations did not require the use of a computer. For applications in which accuracy was critical, the punching might be checked
by use of a verifier.
The card readers used an electrical (metal "brush") or, later, optical sensor to detect which positions on the card contained
a hole. They had high-speed mechanical feeders to feed hundreds of cards through in a very short time.
Advantages
One of the key advantages of this system is that a computer was not required to encode information onto the cards -- the
typewriter-like card-punch machine was all that was needed -- and "key-punch operators" (who did nothing but punch cards
full-time on such machines) were in great demand.
Quality control was often having two different operators key the same data, with the 2nd using a card-verifier instead of a
card-punch. If a card failed verification, the card-verifier would stop, letting the operator replace the card with a corrected
one.
When the time came to transfer the information thus encoded into the computer, the process could occur at very high speed
(either by the computer itself or by a separate device that "read" the cards and "wrote" the data onto magnetic tapes (or, later,
on removable hard disks) that could then be mounted on the computer), thus making best use of expensive computer time.
Obsolescence
Punched-card systems fell out of favor in the mid to late 1970s, as disk and
tape storage became cost effective, and interactive terminals meant that users could edit their work with the computer directly
rather than requiring the intermediate step of the punched cards.
However, their influence lives on through many standard conventions and file formats. The terminals that replaced the punched
cards displayed 80 columns of text, for compatibility with existing software. Many programs still operate on the convention of 80
text columns, although strict adherence to that is fading as newer systems employ graphical user interfaces with variable-width type fonts.
Hanging chads
The term for the punched card area which is removed during a punch is chad. One notorious problem with a punched card
system of tabulation is the incomplete punch; this can lead to a smaller hole than expected, or to a mere slit on the card, or to
a mere dimple on the card. Thus a chad which is still attached to the card is a hanging chad. This technical problem
actually influenced the 2000 U.S.
presidential election; in the state of Florida, voting machines which used punched cards to tabulate votes generated improperly rendered records of
several hundred votes, spread out over an entire state, which tipped the vote in favor of George W. Bush over Albert Gore, thus influencing
history.
See also
External links
- A review of the book IBM and the
Holocaust (ISBN 0609808990) by
Edwin Black, an interesting take on IBM's involvement with Nazi Germany.
- Various punched card codes
- The Undead - Wired magazine article about modern day use of
punch cards
In part, This article was originally based on material from the Free On-line Dictionary of
Computing and is used with permission under the
GFDL.
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