[Coco] WICO Command Trackball

[Torsten Dittel]

> Makes one wonder if it could have been used to drive hobby servos.

I don't think so. The idea behind this PWM mode is the following:

Imagine you want to controll a cursor (e.g. a mouse arrow) on the screen
and you would have normally connected a digital joystick (4 switches = 9
positions including center). The cursor would move in one of the 8
directions (with a constant velocity depending on the sample rate of the
joystick) or rest (center position). Now you can replace the joystick
with the trackball + the WICO adaptor in PWM (pulse width modulation)
mode. If you move the track ball fast (max. velocity), the adaptor
(SC87152P) will output a constant signal for the corresponding direction
like holding the digital joystick to one direction. If you move it
slower, the PWM takes place. You get the impression of different cursor
movement velocities and you can even move the cursor within other angels
than just quantized to 45° steps like with a joystick. The paper is
explaining this better and worth reading.

I just had a closer look to the PCB and it's indeed corresponding with
this schematic from the patent file:

http://trs-80.cc/WICO.png

Pin 6 (VPP) of the chip seems to be to connected to ground and not to
+5V, but I might be wrong with that impression (would have to dig out my
multimeter or have a closer look). The resistance networks (55 in the
picture) are custom made ceramic parts labeled "LTI427 TRS-80". All
other resistances (53 & 54) are 3.3K.

Regards,
Torsten