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

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
I
ACK
I ,
clears its
output buffer, sets default conditions, and continues scanning
(only if the keyboard was previously enabled).
Default Disable (Hex F5)
This command is similar to Set Default, except the keyboard stops
scanning and awaits further instructions.
Enable (Hex F4)
Upon receipt of this command, the keyboard responds with
I
ACK
I ,
clears its output buffer, and starts scanning.
4-6 Keyboard