DS2149

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10.1Loopback Modes

10.1.1 Local Loopback (LLB)

When local loopback is enabled (set LLB in CR2, or pull the LLB pin high), inbound data at the receiver

inputs are ignored. TCLK and TPOS/TNEG pass through the jitter attenuator if enabled and are output at

RCLK and RPOS/RNEG. The transmit path is unaffected by LLB, and will continue to transmit data

normally (or AIS if TAIS is enabled).

10.1.2 Analog Loopback (ALB)

When analog loopback (ALB) is enabled (set ALB in CR2, or float the LLB pin), the receiver input pins

are disconnected from the clock and data recovery circuit and replaced by TTIP and TRING. This tests

the entire device including the jitter attenuator, transmitter, and receiver circuits.

10.1.3 Remote Loopback (RLB)

When remote loopback (RLB) is enabled (set RLB in CR2, or pull RLB pin high), inbound data at the

receiver inputs is looped back to the transmitter path. Data passes through the jitter attenuator if enabled.

The B8ZS encoder and decoder are not included in the loopback path. The receive path continues to

operate normally.

10.1.4 Network Loopback

When ENLOOP is enabled (set ENLOOP in CR2, or float the ENLOOP pin), the in-band loop code

detector is enabled. The receiver detects the in-band loop code patterns (00001 = loop up and

001 = loop down) present in the inbound data. The detectors detect both framed and unframed loop codes.

When the loop-up pattern is detected and present for 5 seconds, the device invokes remote loopback.

ENLOOP is dropped when:

1) The in-band loop-down pattern is present for 5 seconds.

2) RLB is activated.

3) ALB is activated.

10.1.5 Dual Loopback

Dual loopback is the simultaneous enabling of RLB and LLB. If the jitter attenuator is enabled and, when

both loopback paths are enabled, the jitter attenuator is placed in the local loopback path.