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The PCIe slots in a drawer are organized into two I/O domains. Each I/O domain supports up to eight features and is driven through a PCIe switch card. Two PCIe switch cards always provide a backup path for each other through the passive connection in the PCIe+ I/O drawer backplane. During a PCIe fanout card or cable failure, 16 I/O cards in two domains can be driven through a single PCIe switch card. 

The two switch cards are interconnected through the PCIe+ I/O drawer board (Redundant I/O Interconnect, or RII). In addition, switch cards in same PCIe+ I/O drawer are connected to PCIe fanouts across clusters in CPC drawer for higher availability. 

The RII design provides a failover capability during a PCIe fanout card failure. Both domains in one of these PCIe+ I/O drawers are activated with two fanouts. The flexible service processors (FSPs) are used for system control. 

The domains and their related I/O slots are shown in Figure 4-2 on page 123. 

Each I/O domain supports up to eight features (FICON, OSA, Crypto, and so on.) All I/O cards connect to the PCIe switch card through the backplane board. The I/O domains and slots are listed in Table 4-1 on page 125. 

