Technical information

CHAPTER 2
Hardware Design
30 Sound Expansion Card
Bus Demultiplexing 2
The adapter card contains eight 74F245 buffer ICs. They perform the demultiplexing of
the 64-bit PowerPC 601 bus to the 32-bit bus that is connected to the Pretzel Logic IC.
The buffer ICs are controlled by the AJA IC.
Burst Transfers 2
The Macintosh host computer and the DOS Compatibility Card do not perform burst
transfers in the same way. The memory controller on the Power Macintosh 6100
computer performs burst transfers of 32-byte length (eight longwords), whereas the
card’s Pretzel Logic IC can perform block transfers of 16-byte length (four longwords).
To provide a compatible transfer mechanism between the two, burst transfers on the
Pretzel Logic IC are disabled and the bus controller breaks up burst transfers into four
single-longword-compelled transfers. The Pretzel Logic IC keeps the /ABB signal
asserted during the multiple compelled transfers so that it continues to be bus master.
Sound Expansion Card 2
The DOS Compatibility Card has an expansion connector that provides a subset of the
unbuffered XD bus. That connector is the interface to the sound expansion card.
The sound expansion card provides MPC level 1 and level 2 sound output capability.
The card does not provide sound input capability; instead, the Macintosh host computer
provides sound input and record features. The sound expansion card is compatible
with the Sound Blaster register set and uses the standard ISA bus interface and
8-bit DMA channel.
The sound expansion card is designed around three ICs:
■ CT2501 sound system IC
■ YMF262 FM operator IC
■ TAC512 DAC IC
CT2501 Sound System IC 2
The CT2501 is a single IC that incorporates all the functions of a 16-bit PC sound system
except FM synthesis and output filtering. The CT2501 sound system IC, also known as
the Vibra 16, includes the following features:
■ an 8-bit ISA bus interface including DMA support and interrupt generation
■ FIFO buffers and control logic for digital audio playback and format conversion for
the DAC