Resend (Hex Fe); No-Operation (Nop) (Hex Fd Through F7); Set Default (Hex F6); Default Disable (Hex F5) - IBM AT 5170 Technical Reference

system accepts the ACK before executing the command. The
system signals acceptance of the ACK by raising the clock and
data for a minimum of 500 microseconds. The keyboard is
disabled from the time it receives the Reset command until the
ACK is accepted or until another command overrides the previous
one. Following acceptance of the ACK, the keyboard begins the
reset operation, which is similar to a power-on reset. The
keyboard clears the output buffer and sets up default values for
typematic and delay rates.
Resend (Hex FE)
The system can send this command when it detects an error in
any transmission from the keyboard. It can be sent only after a
keyboard transmission and before the system enables the
interface to allow the next keyboard output. Upon receipt of
Resend, the keyboard sends the previous output again unless the
previous output was Resend. In this case, the keyboard
will
resend the last byte before the Resend command.
No-Operation (NOP) (Hex FD through F7)
These commands are reserved and are effectively no-operation or
NOP. The system does not use these codes. If sent, the keyboard
will acknowledge the command and continue in its prior scanning
state. No other operation will occur.
Set Default (Hex F6)
The Set Default command resets all conditions to the power-on
default state. The keyboard responds with ACK, clears its output
buffer, sets default conditions, and continues scanning (only if the
keyboard was previously enabled).
Default Disable (Hex FS)
This command is similar to Set Default, except the keyboard stops
scanning and awaits further instructions.
4-6 Keyboard