Power-On Routine; Keys; Power-On Reset - IBM AT 5170 Technical Reference

characters in the buffer will be sent as in normal operation, and
new data entered will be detected and sent.
Keys
All keys are classified as make/break, which means when a key is
pressed, the keyboard sends a make code for that key to the
keyboard controller. When the key is released, its break code is
sent (the break code for a key is its make code preceded by hex
PO).
All keys are typematic. When a key is pressed and held down, the
keyboard continues to send the make code for that key until the
key is released. The rate at which the make code is sent is known
as the typematic rate (The typematic rate is described under "Set
Typematic Rate/Delay"). When two or more keys are held
down, only the last key pressed repeats at the typematic rate.
Typematic operation stops when the last key pressed is released,
even if other keys are still held down. When a key is pressed and
held down while the interface is inhibited, only the first make
code is stored in the buffer. This prevents buffer overflow as a
result of typematic action.
Power-On Routine
Power-On Reset
The keyboard logic generates a POR when power is applied to the
keyboard. The POR lasts a minimum of 300 milliseconds and a
maximum of 9 seconds.
Note: The keyboard may issue a false return during the first
200 milliseconds after the +5 Vdc is established at the 90%
level. Therefore, the keyboard interface is disabled for this
period.
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4-4 Keyboard