IBM RS/6000 - Manual (Page 42)

2.2.6 The Secondary PCI Bus
The secondary PCI bus is generated through the use of a PCI-to-PCI bridge chip.
This component regenerates a second PCI bus interface from the primary bus and
uses it to drive additional PCI expansion card slots. The PCI-to-PCI bridge handles
all the arbitration for devices on the secondary bus. Due to arbitration restrictions
on the primary PCI bus, the SCSI interface resides on this secondary PCI bus.
Secondary bus latency adds, on average, two PCI clocks (60 nSec at 33 MHz) of
overhead on the first cycle of all SCSI operations. For this reason, PIO (Parallel
I/O) operations will incur this overhead for each cycle, while burst data transfers will
incur the latency for the first cycle of the block transfer only.
2.2.7 The EISA Bus
The EISA bus is generated through a bridge from the primary PCI bus. The bridge
drives several expansion card slots. Since EISA is a proper super-set of the ISA
bus specification, all ISA adapters are completely supported in these EISA
expansion card slots.
Note that 8-bit “belly hanger” ISA cards are not supported in either EISA or
full-specification ISA card connectors. These special cards require unique
8-bit-only slots that are not supported by the PCI-based RS/6000 servers.
The EISA bus contains the integrated native I/O support chip. This chip
incorporates a floppy disk controller, two full function UARTs (Universal
Asynchronous Receiver/Transmitters, support for RS232 drivers) and a
bi-directional parallel port.
2.2.8 The X-Bus
The X-Bus is an 8-bit ISA subset bus used to attach several common subsystems
to the PCI-based RS/6000 servers. The following components are attached to this
bus:
The Keyboard/Mouse controller, which is common to all Power Personal
Systems products
Real-Time Clock and Non-volatile RAM
The functions of Real-Time Clock (RTC) and Non-volatile RAM (NVRAM) are
integrated into a single component in the PCI-based RS/6000 servers. This
component also supplies the logic required to perform the function of powering
the system on at a designed time. The component is operated from a lithium
battery on the board so that all time-keeping functions continue to work while
system power is turned off.
Mini-Support Processor (I C Controller)
The Mini-Support Processor is a minicontroller that is imbedded into the I/O
planar of the PCI-based RS/6000 servers. It allows the PowerPC processor
access to VPD, Operator panels and other I C bus attached devices.
18 Introduction to PCI-Based RS/6000 Servers