Keypad

A telephone keypad using the ITU E.161 standard.

Numeric keypad, integrated with a computer keyboard

A calculator

A keypad is a block or pad of buttons set with an arrangement of digits, symbols, or alphabetical letters. Pads mostly containing numbers and used with computers are numeric keypads. Keypads are found on devices which require mainly numeric input such as calculators, television remotes, push-button telephones, vending machines, ATMs, point of sale terminals, combination locks, safes, and digital door locks. Many devices follow the E.161 standard for their arrangement.

Uses and functions

A computer keyboard usually has a small numeric keypad on the side, in addition to the other number keys on the top, but with a calculator-style arrangement of buttons that allow more efficient entry of numerical data. This number pad (commonly abbreviated to numpad) is usually positioned on the right side of the keyboard because most people are right-handed.

Many laptop computers have special function keys that turn part of the alphabetical keyboard into a numerical keypad as there is insufficient space to allow a separate keypad to be built into the laptop's chassis. Separate external plug-in keypads can be purchased.

Keypads for the entry of PINs and for product selection appear on many devices including ATMs, vending machines, point of sale payment devices, time clocks, combination locks and digital door locks.

1984 advertisement for keypad operated through shop window glass

In 1984, the first projected capacitance keypad was used to sense through the shop window of a travel agency.

Key layout

The first key-activated mechanical calculators and many cash registers used "parallel" keys with one column of 0 to 9 for each position the machine could use. A smaller, 10-key input first started on the Standard Adding Machine in 1901. The calculator had the digit keys arranged in one row, with zero on the left, and 9 on the right. The modern four-row arrangement debuted with the Sundstrand Adding Machine in 1911.

There is no standard for the layout of the four arithmetic operations, the decimal point, equal sign or other more advanced mathematical functions on the keypad of a calculator.

The invention of the push-button telephone keypad is attributed to John E. Karlin, an industrial psychologist at Bell Labs in Murray Hill, NJ. On a telephone keypad, the numbers 1 through 9 are arranged from left to right, top to bottom with 0 in a row below 789 and in the center. Telephone keypads also have the special buttons labelled * (star) and # (octothorpe, number sign, "pound", "hex" or "hash") on either side of the zero key. The keys on a telephone may also bear letters which have had several auxiliary uses, such as remembering area codes or whole telephone numbers.

Keypad circuit layout

Tri-state multiplexing allows more keys to be detected with less I/O connections.

A 16 key keypad can be connected to the host electronics through 16 separate connecting leads, plus a ground lead. However, by using x/y multiplexing, these 17 leads can be reduced to just 8.

A 4 x 4 x/y array is usually used.( see diagram), 4 I/O lines being outputs and the other 4 being inputs. A circuit is completed between an output and an input when a key is depressed. This creates a unique signal that is detectable by the host electronics. The 8 I/O lines could be reduced even further to 5, if a different method of multiplexing was used.

The diagram shows that 8 leads could, in fact, be used to detect 28 separate keys if tri-state multiplexing is used instead of x/y multiplexing. If a diode is used, in series with each key, then 56 keys are detectable .

Tri-state multiplexing enables n²-n keys to be detected with just n I/O lines.

Multi-key operation.

Keypads are usually operated, one key at a time, but two or more keys can, if required, be operated simultaneously. The diagram shows (figures A and B) that 2 keys of an x/y matrix keypad, can be pressed and accurately "read" at the same time. Errors occur, however, with some key selections, when 3 keys are pressed simultaneously.

Although single keys and pairs of keys can be pressed, there are potential problems with 3 keys.

Figure C illustrates the problem. There is a true signal path from terminal 1, through the brown switch to terminal 4. However, the signal also routes through the blue and green switches to terminal 3, falsely indicating that all four switches are closed. This same error would occur if the black switch was closed and the brown switch was open. The false indication that switch 1/3 is closed is called "Ghosting". This error only occurs when three corners of a rectangular set of switches are closed

If necessary, these errors can be eliminated by placing a diode in series with each key switch, as shown in Figure D. The "false" signal path is blocked by the diode in series with the blue switch.

Origin of the order difference

Although calculator keypads pre-date telephone keypads by nearly thirty years, the top-to-bottom order for telephones was the result of research studies conducted by a Bell Labs Human Factors group led by John Karlin. They tested a variety of layouts including a Facit like the two-row arrangement, buttons in a circle, buttons in an arc, and rows of three buttons. The definitive study was published in 1960: "Human Factor Engineering Studies of the Design and Use of Pushbutton Telephone Sets" by R. L. Deininger. This study concluded that the adopted layout was best, and that the calculator layout was about 3% slower than the adopted telephone keypad.

Despite the conclusions obtained in the study, there are several popular theories and folk histories explaining the inverse order of telephone and calculator keypads.

• One popular theory suggests that the reason is similar to that given for the QWERTY layout, the unfamiliar ordering slowed users to accommodate the slow switches of the late 1950s and early 1960s.
• Another explanation proposed is that at the time of the introduction of the telephone keypad, telephone numbers in the U.S. were commonly given out using alphabetical characters for the first two digits. Thus 555-1234 would be given out as KL5-1234. These alpha sequences were mapped to words. "27" was given out as "CRestview", "26" as "ATwood", etc. By placing the "1" key in the upper left, the alphabet was arranged in the normal left-to-right descending order for English characters. Additionally, on a rotary telephone, the "1" hole was at the top, albeit at the top right.

See also

• Arrow keys
• Digital door lock
• Keyboard (computing)
• Keyboard technology
• Mobile phone
• Push-button telephone
• Silicone rubber keypad
• Telephone keypad

External links

• Interfacing Matrix Keypad to 8051 Controller