Punch card
The punch card (or "Hollerith" card) is a 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 generation of computing during the 1960s and 1970s, punch cards were a primary medium for data storage and processing, but are now long obsolete outside of a few legacy systems.
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2 Functional details 3 Advantages 4 Obsolescence 5 See also 6 External links |
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."
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 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.
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.
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.
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. Obsolescence
Punched-card systems fell out of favor in the 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.See also
External links
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.