VSC7122

The VSC7122 is a Quad Port Bypass Circuit (PBC). Four Fibre Channel PBC's are cascaded into a single art to minimize part count, cost, high frequency routing, and jitter accumulation. Port Bypass Circuits are used o provide resiliency in Fibre Channel Arbitrated Loop (FC-AL) architectures. PBC's are used within FC-AL isk 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 ode, the disk drive is connected to the loop. Data goes from the 7122's L_SOn pin to the Disk Drive RX nput and data from the disk drive TX output goes to the 7122's L_SIn pin. Refer to Figure 2 for disk drive pplication. In BYPASS mode, the disk drive is either absent or non-functional and data bypasses to the next vailable disk drive. Normal mode is enabled with a HIGH on the SEL pin and BYPASS mode is enabled by a OW on the SEL pin. Direct Attach Fibre Channel Disk Drives have an "LRC Interlock" signal defined to control he SEL function.

Using a VSC7122 in a single loop of a disk array is illustrated in Figure 2: "Disk Array Application". FCAL rives are all expected to be dual loop. The VSC7122 is cascaded in a manner such that all the 7122's internal BC's are used in the same loop. For dual loop implementations, two or more VSC7122's should be used. llocating each VSC7122 to only one of two loops preserves redundancy, prevents a single point of failure and ends itself to on-line maintainability.

The VSC7122 is very similar to the VSC7121 except that LSO+ outputs are all full power outputs identical o OUT. This is useful in passive backplanes to provide additional amplitude on long traces.