Track-ball
General description
The track–ball assembly is used as an I/O device for military consoles, it consists of two main parts:
- Trackball
- Buttons
- Interface
Principle of operation
Track-ball, as the name suggests, is made up of a rigid “ball” that can rotate on itself. The speed and direction of the ball is obtained using two encoders, one on the X axis and the other on the Y axis.
The direction is derived from the phase between the two encoder signals.
One of the two encoder signals is used as a clock and the other identifies the direction, the speed of the pulses over time gives the module of the displacement over time.
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Technical Specifications
The three keys plus the x and y movement values are “encapsulated” in a standard PS2 mouse message.
Messages are managed according to the so-called make-break technique, i.e. one code per key pressed and one code per key released. This technique allows to manage the keys on the variations without sending continuous messages to the processor.
The implemented interface responds to the PS2 protocol and consists of:
- Clock
- Date
In practice, the interface goes under the generic name of “synchronous serial interface” with the addition of having bidirectional data with open collector type electrical levels through which, in addition to data exchange, it is possible to detect the device connection in real time.
In order to guarantee the functioning of the device, there is an 8-bit uC inside it which has the task of managing the encoders and the input keys.
The “formatted” message is sent on the PS2 interface (other interfaces like RS232 or RS422 or other type like USB… can be impeded).
The following environmental requirements form a development baseline:
- Operating Temperature -20/70°C
- Storage temperature -35/80°C
- Humidity 90% non-condensing
- Vibrations 2g/11mS
- Shock 30g 11mS
- EMI, in accordance with CE regulations
