Computer Hardware: Character Codes


Interactive Content! Computer character codes are usually based on the ASCII (American Standard Code for Information Interchange). Originally this was a seven-bit code, but now provides an extended character set of 255 characters using eight bits. Computer keyboards are connected to the CPU via a serial interface whereby the individual codes are transmitted as binary strings. The keyboard encoder generates a unique code for each key press. Key presses are listed to enable comparisons to be made. Note the difference between upper and lower case characters (only one bit changed).

Within the character set there are special characters which are not displayed, i.e. tabbing, line feeds and backspacing. The keyboard encoder is based on a matrix of rows and columns. At each intersection there is a switch, which is made for that key press. The generated binary code is produced from a look-up table. Speed is unimportant and transmission rates are limited to those of the fastest typist.

Within the encoder circuitry it is normal practice to continually scan the rows and columns on the keyboard for a user key press. These scans take place thousands of times between individual key presses. In this example some keys are inactive, i.e. function keys. This is because they are based on combinations of key codes and would only serve to confuse if included.


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