DEC VT220 Technical Manual page 156

7.5.5.1 Commands There are two kinds of commands, commands
that control keyboard transmission characteristics and commands
that control keyboard peripherals. The low bit of the command is
the TYPE flag. The TYPE flag is clear if the command is a
transmission command, and set if the command is a peripheral
command.
Transmission Commands
Mode set
Autorepeat rate set
Peripheral Commands
Flow control
Indicator
Audio
Keyboard ID
Reinitiate keyboard
Some autorepeat control
Jump to test mode
Reinstate defaults
The high order bit of every command is the PARAMS flag. If there
are any parameters to follow, the PARAMS flag is clear. If there
are no parameters to follow, the PARAMS flag is set.
7.5.5.2 Parameters The high order bit of every parameter is
the PARAMS flag. This flag is clear if there are parameters to
follow, and set on the last parameter. The remaining seven bits of
the parameter are for data.
7.5.5.3 Peripheral Commands The following two commands can
turn the data flow from the keyboard off and on.
• Inhibit keyboard transmission -- shuts off or locks the
keyboard and turns on the keyboard locked LED. After
receiving the inhibit command, the keyboard sends a
special command to the system central processor. If the
system central processor receives this code without
requesting it, this indicates that noise on the line was
interpreted as the inhibit command. The central processor
then responds immediately with the resume keyboard
transmission command.
• Resume keyboard transmission -- turns on or unlocks the
keyboard and turns off the keyboard locked LED. If any
keystrokes are lost, the keyboard responds with an error
code.
Each keyboard LED can be turned on and off.
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