Punch card

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 generation of computing, from the 1920s into the 1950s, punch cards were the primary medium for data storage and processing. Eventually the punch card would be phased out and replaced with huge floppys for media storage, for loading data, in the late 1970's, early 1980's.

IBM punch card format

The IBM card format, which became standard, held 80 columns of 12 punch locations each, representing 80 characters. Originally only numeric information was coded with 1 or 2 punchs per column: digits (digit) and signs (zone – sometimes overpunching the Least Significant Digit). Later, codes were introduced for upper-case letters and special characters. A column with 2 punches (zone + digit) was a letter; 3 punches (zone + digit + 8) was a special character. The introduction of EBCDIC in 1964 allowed columns with as many as 6 punches (zones + digit). The punch cards were 7 and 3/8 inches long by 3 and 1/4 inches high and were 0.007 inch thick with one of the upper corners cut at an angle.

Related Topics:
Least Significant Digit - EBCDIC - 1964

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Corner cut

A major reason for the corner cut was so the punch card could not be inserted backwards or upside down. If the punch card was inserted backwards or upside down it hit a small plastic pin in the machine called the corner cut pin. This would engage a micro switch and halt the machine operation until the card was inserted properly with the corner cut on the correct side of the punch card as used in that system. Stopping the machine meant the machine would not continue to sort or validate.

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Many computer installations used cards with the opposite corner cut (sometimes no corner cut) as "job separators", so that an operator could stack several job decks in the card reader at the same time and be able to quickly separate the decks manually when he removed them from the stacker. These cards were prepunched (e.g., a JCL command to start a new job) in large quantities in advance. This was especially useful when the main computer did not read the cards directly, but instead read their images from magnetic tape that was prepared offline by card to tape converters or smaller computers.

Related Topics:
JCL - Magnetic tape - Card to tape converter

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