The HDR uses a three-slot card cage for all inputs and outputs. Each slot supports a single card for eight channels of I/O, giving you a total of 24 channels I/O simultaneously. Four different tape I/O cards give you analog (+4dBu line level), ADAT optical, Tascam TDIF, and AES/EBU format options. All cards are sold as additional purchases to the HDR and are the same cards that are used in the Mackie Digital 8*Bus (d8b).