Quad Port Bypass Circuit for 1.0625 Gbit/sec
Fibre Channel Arbitrated Loop Disk Arrays
• Supports ANSI X3T11 1.0625 Gbit/sec
FC-AL Disk Attach for Resiliency
• Fully Differential for Minimum
• Quad PBC’s in Single Package
• TTL Bypass Select
• High Speed, PECL I/O’s Referenced to VDD.
• 0.5W Typical Power Dissipation
• 3.3V Power Supply
• 44-Pin, 10mm PQFP
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 NOR-
MAL 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 con-
trol the SEL function.
Using a VSC7121 in a single loop of a disk array is illustrated in Figure 2: “Disk Array Application”. FC-
AL drives are all expected to be dual loop. The VSC7121 is cascaded in a manner such that all the 7121’s inter-
nal 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.
7121 Block Diagram
1 11 1
0 00 0
The VSC7121 can be cascaded through the IN and OUT pins for arrays of disk drives greater than 4. For
disk arrays with a noninteger multiple of 4 disk drives, the unused PBC’s can be hardwired to bypass with a
external pulldown resistor.
G52110-0, Rev. 4.1
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