Data can be transferred in 'normal' mode if Blockwise is disabled. For example if your DNC software does not support it and/or transferring programs to/from CNC memory at lower baud rates.
TNC 2500 supports the Heidenhain Blockwise Protocol for reliable high speed drip feeding of programs that exceed the memory capacity of the machine. Blockwise Protocol is Enabled/Disabled via parameter 5030.
Use a DNC program such as RemoDNC that supports the Blockwise protocol.
See this page for setting up RemoDNC to drip feed large programs using the Heidenhain Blockwise Protocol