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Table Of Contents  CertiGuide to A+ (Core Hardware)
 9  Chapter 0111:  Peripheral Devices

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IV  Ports & Connectors
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V  Keyboards

Keyboards are the most commonly used input devices for PCs. Given this, it makes perfect sense that keyboards have not seen any major technological developments in a long time J.

Keyboards typically interface with the PC by means of a DIN-5 connector (AT motherboards) or a PS/2 connector (ATX motherboards). Some newer keyboards are using wireless (or USB) transmission methods as well. However the underlying technology that makes a keyboard work has remained unchanged for many years. There are basically two ways in which a keystroke is converted into binary data to be sent to the processor.

  • Switched

  • Capacitive

The mechanical switch keyboard was the original design for keyboards in AT and XT based PCs. Each key on the keyboard had its own individual switch, which is made up of two small metal plates that would come into contact with each other when a key was pressed. When the plates touched a signal was sent to the processor telling of which key was pressed.

When the key was released, a small spring would push the key back up into place. This action gives switch keyboards their familiar clicking noise that could become very annoying for the user or any of their neighbors. The original models of switched keyboards also had a design flaw caused by the springing action of the key. This caused a single keystroke to send multiple signals to the processor.

This was solved by an electronic technique called debouncing. With this technique the keyboard controller would constantly scan the keyboard for keystrokes. Keystrokes would only be processed if a key were depressed for two or more scans. This anomaly was ultimately eliminated with the advent of a newer keyboard technology known as capacitive keyboards.

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