Scancode
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A scancode (or scan code) is the data that most computer keyboards send to a computer to report which keys have been pressed. A number, or sequence of numbers, is assigned to each key on the keyboard.
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[edit] Variants
Mapping key positions by row and column requires less complex computer hardware; therefore, in the past, using software or firmware to translate the scancodes to text characters was less expensive than wiring the keyboard by text character.[1] This cost difference is not as profound as it used to be. However, many types of computers still use their traditional scancodes to maintain backward compatibility.
Some keyboard standards include a scancode for each key being pressed and a different one for each key being released. In addition, many keyboard standards (for example, IBM PC compatible standards) allow the keyboard itself to generate "typematic" repeating keys by having the keyboard itself generate the pressed-key scancode repeatedly while the key is held down, with the release scancode sent once when the key is released.
[edit] Scancode sets
[edit] PC compatibles
Scancodes on IBM PC compatible keyboards are sets of 1 to 3 bytes which are sent by the keyboard. Most character keys have a single byte scancode; keys that perform special functions have 2-byte or 3-byte scancodes, usually beginning with the byte (in hexadecimal) E0, E1, or E2. In addition, a few keys send longer scancodes, effectively emulating a series of keys to make it easier for different types of software to process.
PC-compatibles have used three scancode sets. The most commonly encountered are the "XT" ("set 1") scancodes, used by the IBM PC XT and earlier. These mostly consist of a single byte; the low 7 bits identify the key, and the most significant bit is clear for a key press or set for a key release. Some additional keys have an E0 (or rarely, E1 or E2) prefix. These were initially assigned so that ignoring the E0 prefix (which is in the key-up range and thus would have no effect on an operating system that did not understand them) would produce reasonable results. For example the numeric keypad's Enter key produces a scancode of E0 1C, which corresponds to the regular Enter key's scancode of 1C.
Later additions (such as the Windows keys on many keyboards) have not followed this pattern.
The IBM PC AT introduced the "AT" ("set 2") scancodes, with a different key numbering and where a key release is indicated by an F0 prefix. For backward compatibility, the keyboard controller on the motherboard translates these into XT (set 1) scancodes.[2] This translation can be disabled, allowing the raw scancodes to be seen.[3] Therefore, whether an engineer will encounter AT scancodes or XT scancodes on a modern PC-compatible depends on how the keyboard is being accessed.
The set 2 scancodes are used by most keyboards since the PC AT, including PS/2 keyboards. "Set 3", a subset of the scancodes used by the 122-key keyboard on the IBM 3270 PC, also exists, but is rarely used.
To add to the confusion, the keyboard can be told to send scancode set 1, 2 or 3. The most common mode of operation on a PC compatible computer is for the keyboard to send set 2, which is translated by the keyboard controller into set 1.
| key | set 1 (IBM PC XT) | set 2 (IBM PC AT) | set 3 (IBM 3270 PC) | |||
|---|---|---|---|---|---|---|
| press | release | press | release | press | release | |
| A (normal letter) | 1E |
9E |
1C |
F0 1C |
1C |
F0 1C |
| Return / Enter (main keyboard) | 1C |
9C |
5A |
F0 5A |
5A |
F0 5A |
| Enter (numeric keypad) | E0 1C |
E0 9C |
E0 5A |
E0 F0 5A |
79 |
F0 79 |
| Left Windows key | E0 5B |
E0 DB |
E0 1F |
E0 F0 1F |
8B |
F0 8B |
| Right Windows key | E0 5C |
E0 DC |
E0 27 |
E0 F0 27 |
8C |
F0 8C |
[edit] USB
USB keyboards use a new set of scancodes, mostly specified in the USB standard. All computers that recognize USB keyboards recognize these new scancodes.
[edit] See also
[edit] References
- ^ For example, the traditional 12-key numeric keypad is scanned as 3 columns by 4 rows, requiring a total of 7 connections. Scanning each key separately would require 12 connections.
- ^ Keyboard scancodes (section 10, "Keyboard internal scancodes"), Andries Brouwer. Version 1.2e, 2004-05-20, accessed 2006-11-15.
- ^ Keyboard scancodes (section 11, "The AT keyboard controller"), Andries Brouwer. Version 1.2g, 2009-07-07, accessed 2010-07-18.
[edit] External links
- Keyboard scancodes: written by Andries Brouwer
- Interfacing the AT Keyboard: Detailed information on keyboard scancodes from beyondlogic.org
