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Chapter 5 Input/Output Interfaces
5-28 Compaq Deskpro EN Series of Personal Computers
Desktop and Minitower Form Factors
Third Edition – September 1998
Control of the data and clock signals is shared by the 8042and the keyboard depending on the
originator of the transferred data. Note that the clock signal is always generated by the keyboard.
After the keyboard receives a command from the 8042, the keyboard returns an ACK code. If a
parity error or timeout occurs, a Resend command is sent to the 8042.
Table 5-15 lists and describes commands that can be issued by the 8042 to the keyboard.
Table 5–15. 8042-To-Keyboard Commands
Table 5-15.
8042-To-Keyboard Commands
Command Value Description
Set/Reset Status Indicators EDh Enables LED indicators. Value EDh is followed by an option
byte that specifies the indicator as follows:
Bits <7..3> not used
Bit <2>, Caps Lock (0 = off, 1 = on)
Bit <1>, NUM Lock (0 = off, 1 = on)
Bit <0>, Scroll Lock (0 = off, 1 = on)
Echo EEh Keyboard returns EEh when previously enabled.
Invalid Command EFh/F1h These commands are not acknowledged.
Select Alternate Scan Codes F0h Instructs the keyboard to select another set of scan codes
and sends an option byte after ACK is received:
01h = Mode 1
02h = Mode 2
03h = Mode 3
Read ID F2h Instructs the keyboard to stop scanning and return two
keyboard ID bytes.
Set Typematic Rate/Display F3h Instructs the keyboard to change typematic rate and delay
to specified values:
Bit <7>, Reserved - 0
Bits <6,5>, Delay Time
00 = 250 ms
01 = 500 ms
10 = 750 ms
11 = 1000 ms
Bits <4..0>, Transmission Rate:
00000 = 30.0 ms
00001 = 26.6 ms
00010 = 24.0 ms
00011 = 21.8 ms
:
11111 = 2.0 ms
Enable F4h Instructs keyboard to clear output buffer and last typematic
key and begin key scanning.
Default Disable F5h Resets keyboard to power-on default state and halts
scanning pending next 8042 command.
Set Default F6h Resets keyboard to power-on default state and enable
scanning.
Set Keys - Typematic F7h Clears keyboard buffer and sets default scan code set. [1]
Set Keys - Make/Brake F8h Clears keyboard buffer and sets default scan code set. [1]
Set Keys - Make F9h Clears keyboard buffer and sets default scan code set. [1]
Set Keys - Typematic/Make/Brake FAh Clears keyboard buffer and sets default scan code set. [1]
Set Type Key - Typematic FBh Clears keyboard buffer and prepares to receive key ID. [1]
Set Type Key - Make/Brake FCh Clears keyboard buffer and prepares to receive key ID. [1]
Set Type Key - Make FDh Clears keyboard buffer and prepares to receive key ID. [1]
Resend FEh 8042 detected error in keyboard transmission.
Reset FFh Resets program, runs keyboard BAT, defaults to Mode 2.
Note:
[1] Used in Mode 3 only.