Technical information

CHAPTER 2
Hardware Design
12
The DOS Compatibility Card assembly contains three printed circuits cards: the
processor card, the PDS adapter card, and the sound expansion card. The processor
card contains the processor and memory system, the video display system, and the
I/O system. The PDS adapter card contains the PDS bus demultiplexer and controller.
The sound expansion card contains the sound generation ICs. The individual ICs in
each system are
■
processor and memory system:
80486 microprocessor
84031 memory controller
84035 data path controller
■
video display system:
82C450 VGA controller
MU9C9760 SynDAC (video DAC, CLUT, and clock synthesizer)
■
I/O system:
8242 keyboard and mouse controller
Pretzel Logic I/O interface controller
■
PDS adapter card:
74F245 bus buffers (8)
AJA bus controller
■
sound expansion card:
CT2501 (combination bus interface, codec, and audio mixer)
YMF262 digitally controlled FM synthesizer
YAC512 audio DAC for the YMF262
All the ICs in the DOS Compatibility Card are commercially available parts except the
Pretzel Logic IC and the AJA IC, which are Apple custom parts. The individual ICs are
described later, in the sections that describe each of the systems.
The block diagram in Figure 2-1 shows the main components of the processor card and
the PDS adapter card.
The processor card has a high-speed processor bus linking the 80486 microprocessor to
the RAM SIMM by way of the 84031 memory controller. The processor card also has an
I/O bus: the XD (data) bus. The XD bus is used for devices on the card: the keyboard
controller, the game controller, the VGA controller, and the sound card. The 84031
memory controller acts as the I/O bus controller that isolates the XD bus from the
processor bus. The XD bus is 8 bits wide and operates synchronously with the processor
bus at a fraction of its speed. The 84035 data path controller provides additional PC/AT
compatible I/O ports that are accessible through the I/O controller.