ACT apricot - Mouseportoperation

KEYBOARD
MOUSE PORT OPERATION
The interface to the mouse is basically a hardware
multiplexing arrangement, the data line for transmitting
from the Keyboard to the System Unit is also used by the
Mouse. This data line is also shared with the reset switch
which overrides any data transmission.
The mouse port provides 12 volts continuously to the
mouse and has two control lines, used to implement the
hand-shaking protocol and the data line used to transmit
data to the Keyboard from the mouse.
The two control lines are Bus Request and Bus Grant. Bus
Request is asserted by
the mouse when it has data to
transmit, then if the Apricot CPU has sent a mouse enable
command to the Keyboard and the Keyboard has finished
transmitting the current character, the Keyboard will assert
the Bus Grant line thus permitting the mouse to transmit
data. This data is sent to the CPU along the same wire as
transmitted data from the Keyboard. The mouse must not
transmit data while Bus Grant is not asserted as this will
corrupt
any data currently being sent by the Keyboard.
The Keyboard also has a Mouse Disable command. This
immediately sets the Bus Grant line inactive and prevents
further
bus requests being granted. The Keyboard does not
restart transmission until the mouse sets the Bus Request
line inactive. After power-up the mouse is initially disabled.
The Keyboard Mouse Enable command allows the
assertion of the Bus Request line to be recognised by the
Keyboard and the bus granted as in the above description.
The data from the mouse port is preceded by a prefix code
(sent by
the mouse ) this is an implicit X-off character and
the System Unit must not send more than four further
characters to the Keyboard. At the end of the data
transmission from the mouse port, may resume if an X -off
had not previously been received from the Keyboard.