Punch cards (or "punched cards"), also known as Hollerith cards or IBM cards, are paper cards where holes may be punched by hand or machine to represent computer data and instructions. They were a widely-used means of inputting data into early computers. The cards were fed into a card reader connected to a computer, which converted the sequence of holes to digital information.
For example, an early computer programmer would write a program by hand, then convert the program to several punched cards using a punch card machine. The programmer would then take the stack of cards to a computer and feed the cards into a card reader to input the program. Pictured is an example of a woman using a punch card machine to create a punch card.
How did punch cards work?
Using a punch card machine like that shown in the picture above, data can be entered into the card by punching holes on each column, representing one character. Below is an example of a punch card.
Once a card is completed, or the Return key is pressed, the card technically "stores" that information. Because each card only holds so much data, if you write a program using punch cards (one card for each line of code), it requires a stack of punch cards.
To load the program or read punch card data, each card is inserted in a punch card reader to input data from the card into a computer. As the card is inserted, the punch card reader starts on the top-left side of the card, reading vertically from top to bottom. After the card reader has read a column, it moves to the next column. As the reader read the information, it would be written to a computers memory. After all cards were loaded into memory, the computer would be instructed to execute the code. If information was outputted (printed), it would be outputted as punch cards.
The largest punch card program was from the 1950s SAGE air defense system, which used 62,500 punched cards (around 5 MB of data). In the picture below, a woman stands next to the punch cards used in this program.
One of the biggest fears of users dealing with punch cards was dropping the punch cards. If these cards were dropped or got out of order, it could take days or weeks to get the program back in order. In some cases, it wouldn't be possible to put the program back into order.
How can a human read a punch card?
Most of the later punch cards printed at the top of the card what each card contained. For these cards, you could examine the top of the card to see what was stored on the card. If an error was noticed on the card, it would be re-printed. If no data was printed at the top of the card, the human would need to know what number represented and manually translate each column. If you are familiar with modern computers, this would be similar to knowing that binary 01101000 and 01101001 are equal to 104 and 105, which in ASCII put together spells hi.
History of the punch card
Punch cards are known to be used as early as 1725 for controlling textile looms. For example, Joseph Marie Jacquard used punch cards to create a self-portrait woven in silk. The cards were later used to store and search for information in 1832 by Semen Korsakov. Later in 1890, Herman Hollerith developed a method for machines to record and store information on punch cards to be used for the US census. He later formed the company we know as IBM.
Why were punch cards used?
Early computers could not store files like today's computers. If you wanted to create a data file or a program, the only way to use that data with other computers was to use a punch card. After magnetic media was created and began to be cheaper, punch cards stopped being used.
Are punch cards still used?
Punch cards were the primary method of storing and retrieving data in the early 1900s. Other storage devices started replacing punch cards in the 1960s, and today, they are rarely used or found.
Are punch cards input devices?
No. The cards by themselves are not input devices. However, the punch card reader is considered an input device because it takes data from the punch card and sends it to the computer.