DSN2 coordinate files can only be written and read by the TNT program Convert. DSN2 files are binary files which are read using direct record access. The format was designed to allow the rapid location of the atoms of particular residues. It presumes that the entire file will not be read into memory. The assumptions which underlie these choices are no longer valid and one expects that no new graphics programs will be written to read this format.
The file format itself is not documented well. The implementation in TNT was created by reverse-engineering the file as produced by Alwyn Jones' program TOM. It appears that Turbo-Frodo uses a different definition. Turbo-Frodo's DSN2 is not supported by TNT.
If you are interested in the details of TNT's implementation you can write to me and I will send them to you.