GENERAL PRINCIPLES
To use punch-card machines, we first convert the original information into patterns of holes in cards. Then we feed the cards into the machines. Electrical impulses read the pattern of holes and convert them into a pattern of timed electrical currents. Actually, the reading of a hole in a column of a punch card is done by a brush of several strands of copper wire pressed against a metal roller ([Fig. 1]). The machine feeds the card (the bottom edge first, where the 9’s are printed) with very careful timing over the roller; and, when the punched hole is between the brush and the roller, an electrical circuit belonging to that column of the card is completed. The machine responds according to its general design and its wiring for the particular problem: it punches new cards, or it prints new marks, or it puts information into new storage places. Clerks, however, move the cards from one machine to another. They wait on the machines, keep the card feeds full, and empty the card hoppers as they fill up. A human error of putting the wrong block of cards into a machine may from time to time cause a little trouble, especially in sorting. Actually, in a year, billions of punch cards are handled precisely.
Fig. 1. Reading of punch cards.
The punch card is a masterpiece of engineering and standardization. Its exact thickness matches the knife-blade edges that feed the cards into slots in the machines, and matches the channels whereby these cards travel through the machines. The standard card is 7⅜ inches long and 3¼ inches wide, and it has a standard thickness of 0.0065 inch and other standard properties with respect to stiffness, finish, etc.
Fig. 2. Scheme of standard punch card.
(Note: Positions 11 and 12 are not usually marked by printed numbers or letters.)
The standard IBM punch card of today has 80 columns and 12 positions for punching in each column ([Fig. 2]). A single punched hole in each of the positions known as 0 to 9 stands for each of the digits 0 to 9 respectively. The remaining 2 single punch positions available in any column are usually called the 11 position and 12 position (though sometimes called the numerical X position and Y position). These two positions do not behave arithmetically as 11 and 12. Actually, in the space between one card and the next card as they are fed through the machines, more positions occur. For example, there may be 4 more: a 10 position preceding the 9, and a 13, a 14, and a 15 position following the 12. The 16 positions in total correspond to a full turn, 360°, of the roller under the brush, and to a complete cycle in the machine; and a single position corresponds to ¹/₁₆ of 360°, or 22½°. In some machines, the total number of positions may be 20. A pair of punches stands for each of the letters of the alphabet, according to the scheme shown.
| A | 12-1 | J | 11-1 | Unused | 0-1 |
| B | 12-2 | K | 11-2 | S | 0-2 |
| C | 12-3 | L | 11-3 | T | 0-3 |
| D | 12-4 | M | 11-4 | U | 0-4 |
| E | 12-5 | N | 11-5 | V | 0-5 |
| F | 12-6 | O | 11-6 | W | 0-6 |
| G | 12-7 | P | 11-7 | X | 0-7 |
| H | 12-8 | Q | 11-8 | Y | 0-8 |
| I | 12-9 | R | 11-9 | Z | 0-9 |
For example, the word MASON is shown punched in [Fig. 3].
Fig. 3. Alphabetic punching.
Fig. 4. Single-panel plugboard.
To increase the versatility of the machines and provide them with instructions, many of them have plugboards ([Fig. 4]). These are standard interchangeable boards filled with prongs on one side and holes or terminals called hubs on the other side. The side with the prongs connects to the ends of electrical circuits in the punch-card machine, which are brought together in one place for the purpose. On the other side of the board, using plugwires, we can connect the hubs to each other in different ways to produce different results. The single-panel plugboard is 10 inches long and 5¾ inches wide. It contains 660 hubs in front and 660 corresponding prongs in the back. A double-panel plugboard or a triple-panel plugboard applies to some machines. In less time than it takes to describe it, we can take one wired-up plugboard out of a machine and put in a new wired-up plugboard and thus change completely the instructions under which the machine operates. Many of the machines have a number of different switches that we must also change, when going from one kind of problem to another.
The numbers that are stored or sorted in punch-card machines may be of any size up to 80 digits, one in each column of the punch card. In doing arithmetic (adding, subtracting, multiplying, and dividing), however, the largest number of digits is usually 10. Beyond 10 digits, we can work out tricks in many cases.