XRT72L13 Datasheet, PDF (292/370 Pages) Exar Corporation – M13 MULTIPLEXER/CLEAR CHANNEL DS3 FRAMER IC

XRT72L13
M13 MULTIPLEXER/CLEAR CHANNEL DS3 FRAMER IC
REV. P1.0.6
áç
PRELIMINARY
FIGURE 87. ILLUSTRATION OF AMI LINE CODE
Data 1 0 1 1 0 0 0 1 0 1 1 1 1 0 1 1 0 1 1 0 0 1 1 0 0 0 0 1
TxPOS
TxNEG
Line Signal
NOTE: One of the main reasons that the AMI Line Code
has been chosen for driving transformer-coupled media is
that this line code introduces no dc component; thereby
minimizing dc distortion in the line.
4.2.5.1.1.2 The B3ZS Line Code
The Transmit DS3 Framer and the associated LIU IC
combine the data and timing information (originating
from the TxLineClk signal) into the line signal that is
transmitted to the far-end receiver. The far-end re-
ceiver has the task of recovering this data and timing
information from the incoming DS3 data stream.
Many clock and data recovery schemes rely on the
use of Phase Locked Loop technology. Phase-
Locked-Loop (PLL) technology for clock recovery re-
lies on transitions in the line signal, in order to main-
tain "lock" with the incoming DS3 data stream. How-
ever, PLL-based clock recovery scheme, are vulnera-
ble to the occurrence of a long stream of consecutive
zeros (e.g., the absence of transitions). This scenario
can cause the PLL to lose "lock" with the incoming
DS3 data, thereby causing the "clock" and data re-
covery process of the receiver to fail. Therefore,
some approach is needed to insure that such a long
string of consecutive zeros can never happen. One
such technique is B3ZS encoding. B3ZS (or Bipolar
3 Zero Substitution) is a form of AMI line coding that
implements the following rule.
In general the B3ZS line code behaves just like AMI;
with the exception of the case when a long string of
consecutive zeros occur on the line. Any string of 3
consecutive zeros will be replaced with either a "00V"
or a "B0V" where "B" refers to a Bipolar pulse (e.g., a
pulse with a polarity that is compliant with the AMI
coding rule). And "V" refers to a Bipolar Violation
pulse (e.g., a pulse with a polarity that violates the al-
ternating polarity scheme of AMI.) The decision be-
tween inserting an "00V" or a "B0V" is made to insure
that an odd number of Bipolar (B) pulses exist be-
tween any two Bipolar Violation (V) pulses. Figure 88
presents a timing diagram that illustrates examples of
B3ZS encoding.
FIGURE 88. ILLUSTRATION OF TWO EXAMPLES OF B3ZS ENCODING
Data 1 0 1 1 0 0 0 1 0 1 1 1 1 0 1 1 0 1 1 0 0 1 1 1 0 0 0 1
TxPOS
TxNEG
00 V
Line Signal
B 0V
The user chooses between AMI or B3ZS line coding
by writing to bit 4 of the "I/O Control" Register (Ad-
dress = 0x01), as shown below.
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