Notes:
+V I/O :
This pin is +5 Volts for 5 Volt boards, +3.3 Volts for 3.3 Volt boards or signal rails on universal boards
 
 
Pins 12 & 13 :
These are Grounds for 5 Volt boards or Open keys for 3.3 Volt boards and for universal boards
 
 
Pins 50 & 51 :
These are Open Keys for 5 Volt boards and for universal boards or Ground for 3.3 Volt boards
 
 
 
 This is the best BUS ever made and is found in all new systems today. The PCI BUS which stands for Peripheral Component Interconnect, will probably be the only BUS in new systems within a couple of years. It's main characteristics are; it's fast, 64 bits wide (can work at 32 bits also) and the best of all, it's microprocessor independent, this means that it can work on any system that has a 32 bit or 64 bit microprocessor (doesn't have to be x86 based). Since this new BUS was introduced most all computer component manufacturers have come out with better and faster cards, taking full advantage of it, this is especially true for video cards and network cards.
This is one new standard in the PC world where IBM did not have anything to do with (unless making a crappy interface like ISA pushed the idea of making a better one), it was completely developed by the two biggest names in the industry today, INTEL and Microsoft (of course)! This new BUS was designed to replace the ISA BUS, not complement it like the VL-BUS. This makes it's design all the more attractive, yet for now most systems do implement both ISA and PCI, we will probably start seeing systems with no backward compatibility for old ISA cards soon.

When they came up with the idea of making a new BUS, one thing that was primordial, was to make the lives of motherboard and chipset designers easier. When designing a system, engineers had to design the board around the processor, but now all systems have the same basic design and all interface on the PCI BUS. This means that the processor, support chips, on board drive controllers, etc., all interconnect together, even add-on cards like network cards and video cards are all linked on the same BUS. This eliminates the need for glue logic, name given to the chips needed to match signals from different integrated circuits. This alone helps bring down the price of motherboards, hence the price of complete systems.
PCI was designed to operate at high speeds (33 MHz and higher), since these high speeds demand better motherboard designs, the PCI specification gave guidelines on how to phisicaly layout the chips on the board. INTEL believed that all devices connected to the PCI BUS should be places as close to each other as possible. Components are placed one inch apart on the PCI Speedway (name given by INTEL), by putting components on alternating sides of the speedway, designers had the room to put components on each side two inches apart. This design also minimizes the length of the BUS and also reduces the capacitive effect that usually limits the speed of all BUS's.

By making this new standard as open as possible, INTEL made sure that it would be very easy for chip makers to interface specialized chips, this made it widely accepted. Since the original design was only for onboard chips, the first computers with PCI buses actually had EISA and VL-BUS expansion slots. But when INTEL came up with the new specifications for the Pentium processor, they developed an all new way of connecting peripherals, they specified an expansion connector and an entire expansion architecture. This new expansion connector was designed to eliminate the need for other connectors (like VL-BUS that needs an ISA BUS to work), even though it was designed to also support these older expansion slots.
Another key feature of the new PCI design is that it's processor independent, this means that the circuits and signals are not processor specific or family specific. This means, as weird as it may sound, that the PCI design is not limited to INTEL processors or even the x86 family. Some other processor companies like DEC (makers of Alpha chips) are considering using this new BUS.

Even though it's processor independent, the BUS speed is dependent of the speed of the processor. This means that for PC's the usual speeds at which the BUS will be, are in the range of 20 to 33 MHz. Another feature is to accommodate cards that can't keep up with these high speeds, they adds 3 flow control signals. These signals are used by cards to let the processor know that it's ready to receive or send data. They can even halt the current flow of data, which means that some cards can have data flows much slower than the actual BUS speed.