XRT7250 datasheet(196/463 Pages) EXAR

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DS3/E3 FRAMER IC

XRT7250

REV. 1.1.1

177

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.

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.

FIGURE 68. ILLUSTRATION OF AMI LINE CODE

Data

TxPOS

TxNEG

Line Signal

FIGURE 69. ILLUSTRATION OF TWO EXAMPLES OF B3ZS ENCODING

Data

TxPOS

TxNEG

0 0

Line Signal

0 V

XRT7250 Datasheet(PDF) 196 Page - Exar Corporation

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