Punched card

The signature medium of computing's stone age, punch cards (or "Hollerith" cards) are now long obsolete outside of a few legacy systems.

The punched card actually predates computers considerably, originating in 1801 as a control device for Jacquard looms. Such cards were also used as an input method for the primitive calculating machines of the late 19th century.

The version patented by Herman Hollerith and used with mechanical tabulating machines in the 1890 U.S. Census was a piece of cardboard about 90 mm by 215 mm. There is a widespread myth that it was designed to fit in the currency trays used for that era's larger dollar bills, but recent investigations have falsified this. Hollerith went on to found IBM, which originated as a tabulating-machine manufacturer, but later married the punched card to its early computers, encoding binary information as patterns of small rectangular holes.

The method is quite simple. On a piece of cardboard, successive positions either have a hole punched through them, or are left intact. The bits of paper off cuts 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), thereby allowing one column of the card to represent a digit or other character. The most common standard card format 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); 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.

The card readers used an electrical 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.

One of the key advantages of this system is that a computer was not required to encode information onto the cards--a simple typewriter-like card punch was all that was needed. When the time came to transfer the information thus encoded into the computer, the process could occur at very high speed, thus making best use of expensive computer time.

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. 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.

Some of the above material is based on FOLDOC.