# PCIe branching - Requirement of Switch

Considering the following scenario:

A designer wants to connect 3 devices on a PCIe x4 finger edge connector, commonly found on mother boards. All 3 devices will be populated on the same PCIe card. One device has 1 port of 2 lanes i.e (1 x2 PCIe port) Remaining two devices have 1 port of 1 lane (1 x1 PCIe port) each.

Scheme 1:

Scheme 2:

I've seen scheme 1 in the datasheets of some PCIe switches. I want to know if a PCIe switch is really required in this case.

What if we omit the switch and just split the PCIe lanes as in scheme 2?

Will it be a legit PCIe network?

If yes then how will bus numbers be assigned?

Kindly ask if there is a need for clarification.

Edit 1:

Quote from PCI_Express_Base_Specification_Revision_3.0 :

The ability to split a Port into two or more Links is optional. An example of this behavior would be a x16 Port that may be able to configure two x8 Links, four x4 Links, or 16 x1 Links.

More Information can be found under heading 4.2.4.10.1. Required and Optional Port Behavior

• About "just split the PCIe lanes?", how do you propose to do that? – Anindo Ghosh Nov 8 '13 at 7:41
• @AnindoGhosh, Splitting lanes as in Scheme 2 or described in spedification. See Edit 1 – microMolvi Nov 8 '13 at 9:55