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Advance Data Sheet
T7633 Dual T1/E1 3.3 V Short-Haul Terminator
May 1998
44
Lucent Technologies Inc.
Line Interface Unit: Loopbacks
The LIU has independent loopback paths that are activated using LOOPA and LOOPB control bits (register
LIU_REG5, bits 2 to 3) as shown in Table 10. The locations of these loopbacks are illustrated in Figure 5, Block
Diagram of Line Interface Unit: Single Channel, on page 26.
Full Local Loopback (FLLOOP)
A full local loopback (FLLOOP) connects the LIU transmit driver input to the receive analog front-end circuitry. Valid
transmit output data continues to be sent to the network. If the LIU transmitter AIS signal (all-1s signal) is sent to
the network, by setting the XLAIS bit (register LIU_REG5, bit 1), the looped data is not affected. The ALOS alarm
continues to monitor the receive line interface signal (RTIP and RRING) while the DLOS alarm monitors the looped
data.
See Digital Loss of Signal (DLOS) Alarm section on page 28 regarding the behavior of the DLOS alarm upon acti-
vation of FLLOOP.
Remote Loopback (RLOOP)
A remote loopback (RLOOP) connects the recovered clock and retimed data to the LIU transmitter at the framer
interface and sends the data back to the line. The LIU receiver front end, clock/data recovery, encoder/decoder (if
enabled), jitter attenuator (if enabled), and LIU transmitter driver circuitry are all exercised during this loopback. The
transmit clock, transmit data, and the transmit AIS inputs are ignored. Valid receive output data continues to be
sent to RPD-LIU and RND-LIU. This loopback mode is very helpful in isolating failures between systems.
See Loss of LIU Transmit Clock (LOTC) Alarm section on page 35 and LIU Transmitter Driver Monitor (TDM) Alarm
section on page 36 regarding the behavior of the LOTC and TDM alarms upon activation and deactivation of
RLOOP.
Digital Local Loopback (DLLOOP)
A digital local loopback (DLLOOP) connects the transmit clock and data through the encoder/decoder pair to the
receive clock and data output pins. This loopback is operational regardless of whether the encoder/decoder pair is
enabled or disabled. The alarm indication signal can be transmitted (XLAIS = 1) without any effect on the looped
signal.
1. The reset default condition is LOOPA = LOOPB = 0 (no loopback).
2. During the transmit AIS condition, the looped data will be the transmitted data from the framer or system interface and not the all 1s signal.
3. Transmit AIS request is ignored.
Table 10. Loopback Control
Operation
Symbol
—
FLLOOP
2
RLOOP
3
DLLOOP
LOOPA
0
0
1
1
LOOPB
0
1
0
1
Normal
1
Full Local Loopback
Remote Loopback
Digital Local Loopback