The VSC7121 is a Quad Port Bypass Circuit (PBC). Four Fibre Channel PBC's are cascaded into a single part to minimize part count, cost, high frequency routing, and jitter accumulation. Port Bypass Circuits are used to provide resiliency in Fibre Channel Arbitrated Loop (FC-AL) architectures. PBC's are used within FC-AL disk arrays to allow for resiliency and hot swapping of FC-AL drives. A Port Bypass Circuit is a 2:1 Multiplexer with two modes of operation: NORMAL and BYPASS. In NORMAL mode, the disk drive is connected to the loop. Data goes from the 7121's L_SOn pin to the Disk Drive RX input and data from the disk drive TX output goes to the 7121's L_SIn pin. Refer to Figure 2 for disk drive application. In BYPASS mode, the disk drive is either absent or non-functional and data bypasses to the next available disk drive. Normal mode is enabled with a HIGH on the SEL pin and BYPASS mode is enabled by a LOW on the SEL pin. Direct Attach Fibre Channel Disk Drives have an "LRC Interlock" signal defined to control the SEL function. Using a VSC7121 in a single loop of a disk array is illustrated in Figure 2: "Disk Array Application". FCAL drives are all expected to be dual loop. The VSC7121 is cascaded in a manner such that all the 7121's internal PBC's are used in the same loop. For dual loop implementations, two or more VSC7121's should be used. Allocating each VSC7121 to only one of two loops preserves redundancy, prevents a single point of failure and lends itself to on-line maintainability.