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32-bit PCI expansion slots on a motherboard

The 32-bit Peripheral Component Interconnect bus or in practice commonly known as 32-bit PCI bus is a local bus standard is used for attaching peripheral devices to a computer. Devices such as integrated circuits that are fitted onto the motherboard, or expansion cards that fit into slots.

A bus is a subsystem that transfers data or power between computer components inside a computer or between computers. Unlike a point-to-point connection, a bus can logically connect several peripherals over the same set of wires. Each bus defines its set of connectors to physically plug devices, cards or cables together.

The 32-bit PCI bus has replaced the ISA and VESA Local Bus and provides direct access to system memory for connected devices, but uses a bridge to connect to the front side bus and therefore to the CPU.

32-bit PCI Slot Voltage Visual Comparison

The PCI bus originally operated at 33 MHz using a 32-bit-wide path. Revisions to the standard include increasing the speed from 33 MHz to 66 MHz and doubling the bit count to 64. Currently, PCI-X provides for 64-bit transfers at a speed
of 133 MHz which equates to 1-GBps (gigabyte per second)!

32-bit PCI cards use 47 pins to connect (49 pins for a mastering card, which can control the PCI bus without CPU intervention). The PCI bus is able to work with so few pins because of hardware
multiplexing, which means that the device sends more than one
signal over a single pin. Also, PCI supports devices that use
either 5 volts or 3.3 volts.

32-bit PCI Golfinger Voltage Visual Comparison

Conventional PCI bus specifications

• 33.33 MHz clock with synchronous transfers
• Peak transfer rate of 133 MB per second for 32-bit bus width (33.33 MHz × 32 bits ÷ 8 bits/byte = 133 MB/s)
• 32-bit bus width
• 32-bit address space (4 gigabytes)
• 32-bit port space (now deprecated)
• 256-byte configuration space
• 3.3- or 5-volt signaling
• Reflected-wave switching

Conventional PCI variants

• PCI 2.2 allows for 66 MHz signaling (requires 3.3 volt signaling) (peak transfer rate of 533 MB/s)
• PCI-X changes the protocol slightly and increases the data rate to 133 MHz (peak transfer rate of 1066 MB/s)
• PCI-X 2.0 specifies a 266 MHz rate (peak transfer rate of 2133 MB/s) and also 533 MHz rate, expands the configuration space to 4096 bytes, adds a 16-bit bus variant and allows
  for 1.5 volt signaling
• Mini PCI is a new form factor of PCI 2.2 for use mainly inside laptops
• Cardbus is a PCMCIA form factor for 32-bit, 33 MHz PCI
• Compact PCI, uses Eurocard-sized modules plugged into a PCI backplane.
• PC/104-Plus is an industrial bus that utilizes the PCI signal lines with different connectors.
• Advanced Telecommunications Computing Architecture (ATCA or AdvancedTCA) is a next-generation bus for the telecommunications industry

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