The following registers are used for the proportional controllers:

   POT0DAT  - port 1 data (vertical/horizontal)
   POT1DAT  - port 2 data (vertical/horizontal)

   Bit positions:

   Bits 15-8   POT0Y value or POT1Y value
   Bits 7-0    POT0X value or POT1X value

All counts are reset to zero when  POTGO  is written with bit zero high.
Counts are normally read one frame after the scan circuitry is enabled.