XRT72L50 Datasheet, PDF (297/471 Pages) Exar Corporation – SINGLE CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER

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XRT72L50
SINGLE CHANNEL DS3/E3 FRAMER WITH HDLC CONTROLLER
REV. 1.2.1
TABLE 53: THE RELATIONSHIP BETWEEN THE CONTENT OF BIT 3 (UNIPOLAR/BIPOLAR) WITHIN THE UNI I/O
CONTROL REGISTER AND THE TRANSMIT E3 FRAMER LINE INTERFACE OUTPUT MODE
BIT 3
0
1
TRANSMIT E3 FRAMER LIU INTERFACE OUTPUT MODE
Bipolar Mode: AMI or HDB3 Line Codes are Transmitted and Received
Unipolar (Single Rail) Mode of transmission and reception of E3 data is selected.
NOTES:
1. The default condition is the Bipolar Mode.
2. This selection also effects the operation of the Receive E3 LIU Interface block
5.2.5.1.1
The Bipolar Mode Line Codes
If the Framer is choosen to operate in the Bipolar Mode, then the E3 data-stream can be choosen to be
transmitted via the AMI (Alternate Mark Inversion) or the HDB3 Line Codes. The definition of AMI and HDB3
line codes follow.
5.2.5.1.1.1
The AMI Line Code
AMI or Alternate Mark Inversion, means that consecutive "one's" pulses (or marks) will be of opposite polarity
with respect to each other. The line code involves the use of three different amplitude levels: +1, 0, and -1. +1
and -1 amplitude signals are used to represent one's (or mark) pulses and the "0" amplitude pulses (or the
absence of a pulse) are used to represent zeros (or space) pulses. The general rule for AMI is: if a given mark
pulse is of positive polarity, then the very next mark pulse will be of negative polarity and vice versa. This
alternating-polarity relationship exists between two consecutive mark pulses, independent of the number of
'zeros' that may exist between these two pulses. Figure 111 presents an illustration of the AMI Line Code as
would appear at the TxPOS and TxNEG pins of the Framer, as well as the output signal on the line.
FIGURE 111. 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.
5.2.5.1.1.2
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 receiver 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 transitions 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 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 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 encoding. HDB3 (or High Density Bipolar - 3) is a form of AMI line coding that implements
the following rule.
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