I did some googling and there’s nothing about HSCI . Heidenhain are really secretive about their stuff ,no wiring diagrams ,only the basic information. I guess it works as a business model.
i think at this stage there are 2 options to choose from.
1. Go with Heidenhain drives and TNC Controller ,probably very expensive
2. Strip everything out and install another servo-drive combo and go with Linux+Mesa ,probably very time consuming