Service manual

3-10 AlphaServer 4000/4100 Service Manual
3.3 Maintenance Bus (I
2
C Bus)
The I
2
C bus (referred to as the “I squared C bus”) is a small internal
maintenance bus used to monitor system conditions scanned by the power
control module, write the fault display, store error state, and track
configuration information in the system. Although all system modules (not I/O
modules) sit on the maintenance bus, only the I
2
C controller accesses it.
Everything written or read on the I
2
C bus is done by the controller. The block
diagram below notes differences between the AlphaServer 4000 and 4100 with
respect to the I
2
C bus.
Figure 3-4 I
2
C Bus Block Diagram
PKW0421F-97
Memory
Pairs
CPU 0
IOD 1
IOD 2
PCI 1
PCI 2
PCI 0
PCI 3
IOD 0
IOD 3
PCM
Registers
Mother-
board
OCP
Controller
ICBus
2
ICBus
Controller
2
XBUS
EISA
PCI0
IOD0
MEMs
CPUs
1
(4100 only)
(4100 only)
(4000 only)
(4100 only)
(4100 only)
2
3