DigiLink cables are required for two different uses:- For connection of HD I/O's to HD Core and HD Process cards.- For connection of a HD I/O to another HD I/O via the Expansion Port.Additional notes, not related to the DigiLink cables: The Legacy Port on the back of the of the HD I/O uses the "legacy" interface cable that came with your legacy interface. The Legacy Port supports any of the following configurations, at 48 kHz or lower sample rate only:- 888|24 I/O - connect 1 to a single 192 I/O or 96 I/O, connect 2 using a "Y" cable- 882|20 I/O - connect 1 to a single 192 I/O or 96 I/O, connect 2 using a "Y" cable- 1622 I/O - connect 1 to a single 192 I/O or 96 I/O, all 16 channels using the 1622's single cable- 24-bit ADAT Bridge - connect 8 channels to a single 192 I/O or 96 I/O, all 16 channels using a "Y" cable