General Description
The VSC7127 and VSC7129 contain six cascaded Port Bypass Circuits (PBCs), the FibreTimer™ configurable Repeater/Retimer (CRU) and a Signal Detect Unit (SDU). These parts are typically used in distributing Fibre Channel signals to an array of disk drives in an FC-AL loop as illustrated in Figure 1. The first VSC7127’s CRU is configured as a Repeater to attenuate jitter, the second VSC7127’s CRU is bypassed to reduce power and the third VSC7127’s CRU is configured as a retimer so that the output of the device is a jitter compliance point.
Each PBC is a multiplexer that is controlled by the corresponding SELx line which, if HIGH, selects the external input or, if LOW, selects the output of the previous PBC. For the VSC712xR, when MODE is LOW and SEL5 is HIGH, the CRU is a sophisticated repeater which has low latency, no peaking and attenuates jitter even at low frequencies. When MODE is HIGH and SEL5 is HIGH, the CRU is a retimer which eliminates jitter transfer but has increased latency due to an elasticity buffer which adds/drops Fibre Channel fill words in order to accomodate the difference between the baud rate of the incoming data and the local REFCLK. When SEL5 is LOW, the CRU is bypassed and powered down. The SDU monitors the analog levels of the IO+/- input and monitors the output of the CRU digitally to indicate whether valid data is present.
The VSC7127/VSC7129 are similar to the VSC7124 which does not contain the FibreTimer™ cell or CMU.

FEATUREs
• ANSI X3T11 Fibre Channel Compliant
• 1.0625Gb/s Operation
• Features the FibreTimer™ Configurable Clock
   Recovery Unit (CRU): Repeater, Retimer or Bypassed
• Six Port Bypass Circuits (PBC)
• Analog/Digital Signal Detect (SDU)
• On-Chip Transmit Termination
• 3.3V, 700mW Power Dissipation
• Compatible with HDMP-0451 (VSC7127) or HDMP-0452 (VSC7129)
• 44-Pin, 10mm PQFP Package