PC expansion bus systems - PC Based Instrumentation and Control - page 77

meet the tight space requirements of embedded systems, each of the two bus
types (8-bit and 16-bit) offers two bus options, according to whether or not
the P1 and P2 bus connectors extend through the module as stack through
connectors.

Peripheral Component
Interconnect/Interface
(PCI) bus

The Peripheral Component Interconnect/Interface 'PCI' bus was originally
developed as a local bus expansion for the ISA/EISA (PC/AT) bus. The first
version of the PCI bus ran at 33 MHz with a 32-bit bus (133 MBps) but the
current version runs at 66 MHz with a 64-bit bus. The PCI bus operates either
synchronously or asynchronously with the motherboard bus rate. While oper-
ating asynchronously the bus will operate at any frequency up to the maximum
(66 MHz). Flow control is used to allow the bus to operate with slower devices.
The bus is unterminated and the bus clock operates at 133 MHz.
PCI supports full device bus mastering , and provides bus arbitration facilities
through the system chipset. PCI architecture allows bus mastering of multiple
devices on the bus simultaneously, with the arbitration circuitry working to
ensure that no device on the bus (including the processor) locks out any other
device. However, in the event that no other device requires access to the bus,
PCI will allow a bus master to transfer data at the maximum permissible rate.
Note that, with some early motherboards it might be possible that not all of the
available PCI bus slots will be capable of bus mastering. When in doubt it is
wise to check with the motherboard manual.
The PCI standard forms part of the Plug and Play standard developed by
Intel, Microsoft, and many other companies in which the PCI chipset handles
the identification of cards, working in conjunction with the BIOS and operating
to automatically allocate resources for compatible peripheral cards.
The PCI bus uses its own internal interrupt system for dealing with requests
from the cards on the bus. These interrupts are often called '#A', '#B', '#C',
and '#D' to avoid confusion with the normal numbered system IRQs, though
they are sometimes referred to by number. PCI interrupt levels are not generally
seen by the user except in the PCI BIOS setup screen.
PCI interrupts are mapped to the normal system interrupts (usually IRQ9
to IRQ12). This imposes a limit of four interrupts available for PCI devices.
Where more slots are provided (or where a USB controller is present) several
PCI devices may be configured to share an IRQ.
Other variants and extensions of the basic PCI specification include:

PCI-X The latest version 64 bits at 133 MHz

cPCI Compact PCI is PCI in a VME form factor, using either

3U/6U modules and using 2 mm connectors

PC/104-Plus PCI add-on to the PC/104 specification

PISA PCI add-on for the PC/AT bus standard

P2CI PCI on the VME64 P2 connector

PMC PCI on a Mezzanine Card, 'PMC'

PXI cPCI for Instrumentation

IPCI Industrial PCI (another version of cPCI)

Serial PCI PCI based on a serial link

Card Bus 32-bit PCI on the PC Card (PCMCIA) format