XRT72L56 Datasheet, PDF (417/486 Pages) Exar Corporation – SIX CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER

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PRELIMINARY
XRT72L56
SIX CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER
REV. P1.1.2
FIGURE 181. 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.
6.2.5.1.1.2 The HDB3 Line Code
The Transmit E3 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 remote receiver. The remote re-
ceiver has the task of recovering this data and timing
information from the incoming E3 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 relies on transi-
tions in the line signal, in order to maintain lock with
the incoming E3 data stream. However, PLL-based
clock recovery scheme, are vulnerable to the occur-
rence 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 E3 data, thereby
causing the clock and data recovery 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 HDB3 en-
coding. HDB3 (or High Density Bipolar - 3) is a form
of AMI line coding that implements the following rule.
In general the HDB3 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 4
consecutive zeros will be replaced with either a
"000V" or a "B00V" 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 vio-
lates the alternating polarity scheme of AMI.) The de-
cision between inserting an "000V" or a "B00V" is
made to insure that an odd number of Bipolar (B)
pulses exist between any two Bipolar Violation (V)
pulses. Figure 182 presents a timing diagram that il-
lustrates examples of HDB3 encoding.
FIGURE 182. ILLUSTRATION OF TWO EXAMPLES OF HDB3 ENCODING
Data 1 0 1 1 0 0 0 0 0 1 1 1 1 0 1 1 0 1 1 0 0 1 1 0 0 0 0 1
TxPOS
TxNEG
TxLineClk
Line Signal
0 00V
B0 0V
The user chooses between AMI or HDB3 line coding
by writing to bit 4 of the I/O Control Register (Address
= 0x01), as shown below.
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