The Datum controller in the DG Nova 3 was a Datum 4091 optioned for the Diablo 30 drive.
Looks like acquiring one of those, or an emulator may be in order to gen up a working system.
|Part number 4091-NDC|
|Datum logo on component side etch|
Clipping about the Datum controllers from Computer World
Bitsavers link to Datum catalog mentioning the controller subsystem.