Description; Sequencing Key Code Scanning; Keyboard Buffer - IBM AT 5170 Technical Reference

Description
The keyboard is a low-profile, 84-key, detachable unit. A
bidirectional serial interface in the keyboard is used to carry
signals between the keyboard and system unit.
Cabling
The keyboard cable connects to the system board through a 5-pin
DIN connector. The following figure lists the connector pins and
their signals.
DIN Connector Pins Signal Name
1
+KBD ClK
2
+KBD DATA
3
Reserved
4 Ground
5
+5.0 Vdc
Sequencing Key Code Scanning
The keyboard is able to detect all keys that are pressed, and their
scan codes will be sent to the interface in correct sequence,
regardless of the number of keys held down. Keystrokes entered
while the interface is inhibited (when the key lock is on) will be
lost. Keystrokes are stored only when the keyboard is not
serviced by the system.
Keyboard Buffer
The keyboard has a 16-character first-in-first-out (FIFO) buffer
where data is stored until the interface is ready to receive it.
A buffer-overrun condition will occur if more than sixteen codes
are placed in the buffer before the first keyed data is sent. The
,..-...., seventeenth code will be replaced with the overrun code, hex 00.
(The 17th position is reserved for overrun codes).
If
more keys
are pressed before the system allows a keyboard output, the data
will be lost. When the keyboard is allowed to send data, the
Keyboard 4-3