Keyboard Encoding And Usage - IBM XT 5160 Technical Reference

Data Stream
Data transmissions from the keyboard consist of a 9-bit data
stream sent serially over the 'data' line. A logical, 1 is sent at an
active (high) level. The following table shows the'functions of
the bits.
Bit Function
1 Start bit (always 1)
2 Data bit 0 (least-significant)
3
Data bit 1
4
Data bit 2
5
Data bit 3
6
Data bit 4
7
Data bit
5
8 Data bit
6
9
Data bit 7 (most-significant)
Keyboard Data Output
When the keyboard is ready to send data, it first checks the status
of the keyboard 'clock' line. If the line is active (high), the
keyboard issues a request-to-send (RTS) by making the 'clock'
line inactive (low). The system must respond with a clear-to-send
(CTS), generated by allowing the 'data' line to become active,
within 250 microseconds after RTS, or data will be stored in the
keyboard buffer. After receiving CTS, the keyboard begins
sending the 9 serial bits. The leading edge of the first clock pulse
will follow CTS by 60 to 120 microseconds. During each clock
cycle, the keyboard clock is active for 25 to 50 microseconds.
Each data bit is valid from 2.5 microseconds before the leading
edge until 2.5 microseconds after the trailing edge of each
keyboard clock cycle.
Keyboard Encoding and Usage
The keyboard routine, provided by IBM in the ROM BIOS, is
responsible for converting the keyboard scan codes into what will
be termed Extended ASCII. The extended ASCII codes returned
by the ROM routine are mapped to the US English keyboard
layout. Some operating systems may make provisions for
alternate keyboard layouts by providing an interrupt replacer,
lOl/102-Key Keyboard
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