Keyboard Codes and Commands

A host system may send 8-bit commands to the keyboard, while a keyboard may send commands and key codes to the system. System commands may be sent to the keyboard at any time. The keyboard will respond within 20 milliseconds, except when performing the basic assurance test (BAT), or executing a Reset command. The table below shows system commands and their hexadecimal values of a keyboard system. 

Command	Hex
Set/Reset Status Indicators	ED
Echo	EE
Invalid Command	EF
Select Alternate Scan Codes	F0
Invalid Command	F1
Read ID	F2
Set Typematic Rate/Delay	F3
Enable	F4
Default Disable	F5
Set Default	F6
Set All Keys - Typematic - Make/Break - Make -Typematic/Make/Break	F7 F8 F9 FA
Set Key Type - Typematic - Make/Break - Make	FB FC FD
Resend	FE
Reset	FF

Keyboard Commands

Table below shows the commands that the keyboard may send to the system and their hexadecimal values.

Command	Hex
Key Detection Error/Overrun	00
Keyboard ID	83AB
BAT Completion Code	AA
BAT Failure Code	FC
Echo	EE
Acknowledge (ACK)	FA
Resent	FE

Keyboard Codes

Keyboard are available for several languages and settings. The keyboard that is most common for the English language is one with 104 keys. Keys of this keyboard are identified by numbers, and for every key there is a scan code. Several scan codes are available, and the default scan code is Scan Code 2 that will discuss here.

Keyboard scan codes consist of a Make and a Break code. The Make code identifies the key pressed and the Break code identifies the release of a key. For most keys the Break code is F0 followed by the Make code. For example, when the Space bar (key 61) is pressed and released, hexadecimal codes 29, F0 and 29 are transmitted from the keyboard to the system via the data serial line. If this key remains pressed, the Make code (29) is continuously transmitted until it is released. Make codes for Scan Code 2 are shown in the table below.
  

Key Numb	Make Code	ASCII-No Shift	ASCII-Shift	Character-No Shift	Character-Shift
1	0E	96	126	`	~
2	16	49	33	1	!
3	1E	50	64	2	@
4	26	51	35	3	#
5	25	52	36	4	$
6	2E	53	37	5	%
7	36	54	94	6	^
8	3D	55	38	7	&
9	3E	56	42	8	*
10	46	57	40	9	(
11	45	48	41	0	)
12	4E	45	95	-	_
13	55	61	43	=	+
14	66	08	08	BS	BS
15	0D	09	09	Tab	Tab
16	15	81	113	Q	q
17	1D	87	119	W	w
18	24	69	101	E	e
19	2D	82	114	R	r
20	2C	84	116	T	t
21	35	89	121	Y	y
22	3C/td>	85	117	U	u
23	43	73	105	I	i
24	44	79	111	O	o
25	4D	80	112	P	p
26	54	91	123	[	{
27	5B	93	125	]	}
28	5D	92	124	\	|
29	58			Caps	Caps
30	1C	65	97	A	a
31	1B	83	115	S	s
32	23	68	100	D	d
33	2B	70	102	F	f
34	34	71	103	G	g
35	33	72	104	H	h
36	3B	74	106	J	j
37	42	75	107	K	k
38	4B	76	108	L	l
39	4C	59	58	;	:
40	52	39	34	'	"
41	5D
42	5A	13	Enter	Enter	Enter
43	12		Shift	Shift	Shift
44	61		58
45	1A	90	122	Z	z
46	22	88	120	X	x
47	21	67	99	C	c
48	2A	86	118	V	v
49	32	66	98	B	b
50	31	78	110	N	n
51	3A	77	109	M	m
52	41	44	60	,	<
53	49	46	62	.	>
54	4A	47	63	/	?
55	59			Shift	Shift
56	14			Ctrl	Ctrl
57	E01F			Win	Win
58	11			Alt	Alt
59	29	32	32	Space	Space
60	E0/11			Alt	Alt
61	E027			Win	Win
62	E0/14			Ctrl	Ctrl
63	E02F			Menu	Menu

The Make and Break arrangement, makes it possible for the system to identify multiple keys pressed and the order in which they have been pressed. For example, if one presses and holds down the Left-Shift key (key number 44), 12 Hex is continuously sent to the system. While this is happening, if key number 9 (the 8/* key) is pressed and released, 3E, F0 and 3E codes are transmitted. The receiving system identifies this sequence of events as the intention to enter an asterisk (*).

References:

• Digital Design and Implementation with Field Programmable Devices, Zainalabedin Navabi, Kluwer Academic Plubishers, 2005.
• www.acces-is.com (source of image)