Gearing up for what promises to be rapid growth in demand for links between the Apple Computer Inc Macintosh and IBM Corp mainframes, Avatar Corp, Hopkinton, Massachusetts is to buy Tri-Data Systems Inc of Santa Clara for an undisclosed cash sum in a deal financed by Hambro International Equity Partners. The new privately-held company will have annual sales of $20m from Tri-Data’s Netway and Avatar’s MacMainFrame 3270 emulation product lines.