A Riser card operates as an extension to the onboard PCI slots (some riser cards support AGP slots as well). Some have just 1 PCI slot (for low-profile 1U applications) and other have 2 or 3 PCI slots (for ITX or 2U applications). The latter require 3 IRQ signals but because of the PCI specifications, only one IRQ signal is transferred in each PCI slot. Therefore, another two additional IRQ signals must be routed from other onboard PCI slots to the riser card. There are three types of PCI riser card used in this way:

• Active Riser Card - PCI bridge chip on the riser card itself will generate IRQ signals
• Passive Riser Card - Paddle boards are used to transfer IRQ signals from other motherboard PCI slots.
• OEM Riser Card - Special design for certain motherboards that used PCI reserve pins for IRQ signals transfer.

In addition, the orientation of the Riser is not suitable for some cases, because when cards are added they are parallel with the motherboard, whilst the fittings in the case expect the cards to be at right angles. A cable based 2 slot riser to cope with this is being investigated and there has been talk of a 3 slot riser but nothing confirmed at this time.