PCI BUS.. Why so many GNDs and Power Lines?

Hi, i am studing about PCI bus and i was wondering. Why the bus have so many 5V/3.3V and GND pins? Are all of them interconnected? What is the minimum number of pins that i must use to send and receive information through PCI bus with 32 Bits?

Thank you!

'so many ground lines' are for proper grounding in pci card to reduce emi/interfernce.

if it is 32 bits , then use 32bit signal lines (ad0-31) only .
leave all those sigs with names..64 and(32-63).

srizbf
3rdmay2010

If you are used to low speed digital like when you first start out. Then no one every considers the power and ground lines. When you look at a digital design book, you are just dealing with gates and Flip-Flops. You never see power and ground. You make the assumtion that everything is ideal and if you put a 1 on a line then there is just a 1 on the line.

Now move up in frequency.

When you place a 1 on a line, it really takes some time (c) to move down that line. Whit a high enough speed and a short enough line, the start of the line can be at a 1 and the end of the line can be at a 0. Eventually then 1 will make it down the line and then the magic happens, it bounces back torward the start. This can cause all kinds of problems. So to stop this, the lines are made short.

Next problem.

At high frequencies, all components will act like little antenas. So one line will transmit on its antena and the next line will recieve on its antena. This causes problems so gound lines and planes are used through out the design to stop this.

Another problem:
For every signal line, there must be a return path to ground (current loop). If there is only one return path, then every signal line (16 data 32 address and a bunch of signals) will all use the same ground return and in effect every signal is also present on that return. (a mess) So use multiple grounds to spread out the junk.

Last problem:
If their is only one ground return, then all the current that the PCI device uses must pass though that one ground trace. (high heat) As signal lines get smaller and smaller, they can't carry as much current, so multiple traces are use to spread it out and keep things cooler.

Hope this helps

Red

Perfect. Thats completely answered my question. Just to make it clear i should have no ptoblem if i short circuit all the power lines right (gnd with gnd and 5/3.3 with 5/3.3)? About the distance. As far as i know PCI runs at 33Mhz right? The wave lenght would be something like 10m correct? So i wont have any problem if i keep the lines shorter than 10 cm correct?

Thank you!

You got it. Connect them all. Be sure to decouple (caps) the power to ground.
It is best to provide a ground plane and connect all the ground to it. That will help stop those little "radio" transmitters.

As freq goes up, length need to come down. If you try and shove a 1 Hz signal down a 100mm wire on a proto-board, you get 1 Hz at the other end. If you try and put a 97.1MHz signal down the same wire, you get KEGL. (google that)

Let me know how it goes.

Red

Thanks! Thats helped a lots. As soon i got any result i will come back and post here.

Thank you!