XRT72L56 Datasheet, PDF (215/486 Pages) Exar Corporation – SIX CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER

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SIX CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER
PRELIMINARY
XRT72L56
REV. P1.1.2
FIGURE 68. 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. Howev-
er, PLL-based clock recovery scheme, are vulnerable
to the occurrence of a long stream of consecutive ze-
ros (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 recovery
process of the receiver to fail. Therefore, some ap-
proach is needed to insure that such a long string of
consecutive zeros can never happen. One such tech-
nique is B3ZS encoding. B3ZS (or Bipolar 3 Zero
Substitution) is a form of AMI line coding that imple-
ments 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 alternat-
ing polarity scheme of AMI.) The decision between
inserting an 00V or a B0V is made to insure that an
odd number of Bipolar (B) pulses exist between any
two Bipolar Violation (V) pulses. Figure 69 presents a
timing diagram that illustrates examples of B3ZS en-
coding.
FIGURE 69. 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 (Address
= 0x01), as shown below.
196