# PCI in a PCI Express x16 slot?

## Dralnu

Are PCI slots backwards compatable, or not?

----------

## d2_racing

If I trust my instinct...no

----------

## SiberianSniper

no, they're not, but there are adaptors, for example http://www.magma.com/

----------

## Dralnu

I didn't think they would.  Issue with too much power or what?

----------

## crazycat

pci is parallel, wile express has serial architecture. PCI-express also needs less wires. They are only software compatible.

----------

## Dralnu

 *crazycat wrote:*   

> pci is parallel, wile express has serial architecture. PCI-express also needs less wires. They are only software compatible.

 

Diffrence being?

----------

## d2_racing

When the data is send :

serial output : 11101010101010101010101010

parallel output :

1

1

1

0

1

0

1

0

1

0

1

0

1

0

1

0

1

0

1

0

1

0

1

0

1

0

So you send data from multiple chanel instead of one.

----------

## crazycat

Parallel(PCI) is actually better because it sends more at once, BUT it's speed is limited since you have to send and syncronise accross a lot of wires, since the length of wires is sometimes different. With serial, you just have a pair of wires, but since you don't have to syncronise it you can just send it at a very high frequency. Anyway, with pci express you theoretically have less complexity/wires so it should be cheaper to build mainboards with it. Also the whole pci bus sits on the same "lane", like 2 ide drives on master/slave. So the data which is send to one device is send to all of them, so with each new PCI device you have less brandwith. With pci-express, it has 16 or 20 or even more "lanes" each of which is a dedicated data link and most gpu's can use up to 16 of such lanes at once. I dont remember it exactly, but i think each lane is 2times as fast as pci(32bit/33mhz) bus, which ,if i remember it correctly, has only 66mb/sek max brandwidth.

----------

## Dralnu

So the x16 is how many serial lines it has running then? Kinda like parrallel, but using serial connections?

----------

## think4urs11

 *Dralnu wrote:*   

> So the x16 is how many serial lines it has running then? Kinda like parrallel, but using serial connections?

 

as you might have guessed already ... 16, each capable for 250MB/s so in total 4GB/s compared to PCI 32bit/33.3Mhz with 133MB/s total (peak)

----------

## John R. Graham

PCI Express is scalable serial.  Each serial channel (called a lane in PCIe patois) is 2.5Gbits/sec.  The x16 means that 16, 2.5Gbits/sec lanes can be engaged in that slot for an aggregate throughput of 40Gbits/sec (5Gbytes/sec) theoretical bandwidth (overhead limits it to about 4Gbytes/sec).  There are also 32 lane slots for up to 8Gbytes/sec actual throughput.

This is far faster than conventional parallel PCI (32 bits wide, 33 million transfers/sec:  0.13Gbytes/sec.

Simple peripherals (parallel port, ethernet card) can use a single lane and benefit from the cost reduction of smaller connectors.  But, PCIe is why you don't see AGP connectors on new motherboards: it's so fast that you don't need AGP.

- John

----------

## Dralnu

Eh, some mobos DO still have AGP.

Thanks for all the info. I think I may look up AGP myself this time. Maybe this should be renamed and thrown somewhere for future refrence for others? (Maybe Docs?)

----------

## John R. Graham

 *Dralnu wrote:*   

> Eh, some mobos DO still have AGP.

 Yeah, but, not the newest, fastest ones.  AGP started out as being an extension of the system bus, but, as that sped up over time, it became more and more of an architectural problem.  It ended across a bridge, just like PCI.

- John

----------

