+ AA6YQ comments below
The add-on controller is new to me, but it has a standalone config utility. Before I changed the DXV config rotor settings from Yaesu to HyGain I used the standalone utility to change the protocol from something called RS232G (IIRC) to DCU-1.
The utility help file said that would use a subset of the DCU-1 protocol.
At any rate, that is what I think, but I'm not yet familiar enough with it to be confident about what caused what behavior.
+ In this case, at least, all's well that ends well.