Keyboard Data Output; Keyboard Data Input - IBM 5170 Technical Reference

Keyboard Data Output
When the keyboard is ready to send data, it first checks for a
keyboard-inhibit or system request-to-send status on the
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clock
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and
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data
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lines. If the
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clock
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line is low (inhibit status), data is
stored in the keyboard buffer. If the
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clock
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line is high and
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data
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is low (request-to-send), data is stored in the keyboard
buffer, and the keyboard receives system data.
If
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clock
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and
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data
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are both high, the keyboard sends the 0
start bit, 8 data bits, the parity bit and the stop bit. Data will be
valid before the falling edge and beyond the rising edge of
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clock
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During transmission, the keyboard checks the
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clock
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line for a positive level at least every 60 milliseconds. If the
system lowers the
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clock
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line from a positive level after the
keyboard starts sending data, a condition known as
line contention
occurs, and the keyboard stops sending data. If line contention
occurs before the rising edge of the tenth clock (parity bit), the
keyboard buffer returns the
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data
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and
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clock
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lines to a positive
level. If contention does not occur by the tenth clock, the
keyboard completes the transmission.
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Following a transmission, the system can inhibit the keyboard
until the system processes the input or until it requests that a
response be sent.
Keyboard Data Input
When the system is ready to send data to the keyboard, it first
checks if the keyboard is sending data. If the keyboard is sending
but has not reached the tenth clock, the system can override the
keyboard output by forcing the
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clock
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line to a negative level.
If the keyboard transmission is beyond the tenth clock, the system
must receive the transmission.
If the keyboard is not sending, or if the system elects to override
the keyboard's output, the system forces the
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clock
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line to a
negative level for more than 60 microseconds while preparing to
send. When the system is ready to send the start bit (
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data
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line
will be low), it allows the
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clock
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line to go to a positive level.
Keyboard 4-15