> The DDM-110 does start when a Yamaha RX-5 sends MIDI clock to the A-190.
> but the weird thing is: the Korg plays when the yamaha is
> stopped...and stops when the Yamaha plays.

I don't think it's a problem of the clock polarity but the start/stop
polarity.
Maybe you have to invert the start/stop signal that starts/stops the Korg
(e.g. with an A-165).

Best wishes
Dieter Doepfer