User manual

8
about the left and right “mouse” buttons.
Field Size (bits) Range Meaning
X 8 127 Amount of horizontal finger motion
Y 8 127 Amount of vertical finger motion
Left 1 0 or 1 State of left physical button or tap/drag gesture
Right 1 0 or 1 State of right physical button
Because the Relative packet is designed to be compatible with the existing mouse protocol, the exact
contents of the Relative packet vary from one protocol to another. See the later sections of this Guide for
details. (For example, in the PS/2 protocol, the packet actually reports 9-bit deltas, plus a third “middle” button
which is not supported by current products. Also, positive Y values correspond to upward motion in the PS/2
protocol, but to downward motion in the Serial and ADB protocols.)
In Relative mode, placing the finger on the pad does not automatically cause packets to be sent. However,
moving the finger in any direction produces a sequence of packets that describe the motion. Pressing or
releasing a mouse button causes the Touch Pad to send a packet reporting this change in the state of the
buttons. Tapping the finger quickly on the pad also simulates a brief click of the left mouse button, and the
“tap-and-a-half” drag gesture simulates an extended motion with the left button held down. When there are no
finger motions or button state changes to report, the Touch Pad ceases to transmit packets and remains silent
until the next motion or button activity.
Packet rate
The Touch Pad reports approximately 40 packets per second by default. By setting the Rate bit of the mode
byte , the host can double the packet rate to approximately 80 packets per second. The higher packet rate is
referable because it leads to the smoothest cursor motion.
The PS/2 protocol allows synchronous, bidirectional bit-serial communication between the host and the
pointing device. Either side may transmit a command or data byte at any time, although only one side can
transmit at one time. During initialization, the host sends command bytes to the device. Some commands are
followed by argument bytes.
The device acknowledges each command and argument byte with an ACK ($FA) byte, possibly followed by
one or more data bytes. If the host has enabled “Stream mode” transmission, then the device may send
spontaneous data packets to the host describing finger motions and button state changes.
Touch Pads integrated into KVM drawer typically use the PS/2 protocol.
Electrical interface
The PS/2 protocol includes two signal wires as well as +5V power and ground. The signal wires, CLK and