
1-20
AlphaServer DS20
Service Manual
1.8.2
I/O Subsystem
The I/O subsystem consists of two 64-bit PCI buses. One has an embedded ISA
bridge, three PCI option slots, and a single ISA slot; the other bus has three PCI
option slots.
Figure 1-11 PCI Block Diagram
PCI-1
PCI-0
3 64-bit slots
3 64-bit slots
3&,%XV
PCI-1 Bus
PK1494-98
P Chip
PCI 1
P Chip
PCI 0
1 ISA
Slot
ADAPTEC
Ultra SCSI
Controller
(2 ports)
Crossbar
Switch
8 D Chips
C Chip
PAD Bus (32 bits)
CAP Bus (24 bits)
I/O ASIC
COM1
COM2
Parallel
Port
Floppy
Cypress Chip
(ISA Bridge,
IDE, KBRD,
MSE, USB)