RS8953B/8953SPB
3.0 Circuit Descriptions
HDSL Channel Unit
3.4 Loopbacks
3.4 Loopbacks
The RS8953B provides multiple PCM and HDSL loopbacks, as shown in
Figure 3-15. The output towards which data is looped is called the test direction.
Loopback activation in the test direction does not disrupt the through data path in
the non-test direction. Data path options (refer to Table 4-7) are provided to
replace data in the non-test direction with fixed or PRBS test patterns. Table 3-1
shows the loopback controls which are designated by initials corresponding to
test direction and the channel from which data is looped.
PP_LOOP and HP_LOOP automatically switch both PCM data and PCM
multiframe sync signals to the test direction. In these loopback modes, the PCM
transmit and receive clocks are not automatically switched to the test direction.
The PCM transmit and receive clocks must be properly configured for these
loopback modes to operate.
When performing PH_LOOP, all HDSL channels that have payload data
mapped require that PH_LOOP mode be enabled to complete PCM channel
loopback on the HDSL side. Also, the same tap must be used for the RS8953B
scrambler and descrambler, or both the RS8953B scrambler and descrambler
must be disabled.
When performing HH_LOOP, the scrambler and descrambler in the HDSL
transceivers must be enabled. Different tap must be used in each direction. This is
required to prevent the exact same data to be sent in both directions. If the exact
same data is sent in both directions, the echo canceler in the HDSL transceivers
will consider the data an echo and cancel the data. Also, the same tap must be
used for the RS8953B scrambler and descrambler, or both the RS8953B
scrambler and descrambler must be disabled.
Figure 3-15. PCM and HDSL Loopbacks
PCM Channel
TSER
HDSL Channel 1
PH_LOOP
TDAT1
RDAT1
PP_LOOP
HP_LOOP
HH_LOOP
RSER
HDSL Channel 2
PH_LOOP
TDAT2
RDAT2
HH_LOOP
HDSL Channel 3
PH_LOOP
TDAT3
RDAT3
HH_LOOP
N8953BDSB
Conexant
3-17
CONEXANT [ CONEXANT SYSTEMS, INC ]
