- if both pins are low, two relays controlled by them
are open and motor of the appropriate DOF is not
running;
- if one pin is high and another is low, current flows
in one direction and motor is running in appropriate
direction;
- for opposite state of pins, motor is running in
opposite direction;
- 'forbidden combination' is when both pins are high,
because then both relays are active and source is
short circuited;