XRT73L03 Datasheet, PDF (31/62 Pages) Exar Corporation – 3 CHANNEL E3/DS3/STS-1 LINE INTERFCE UNIT

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PRELIMINARY
3 CHANNEL E3/DS3/STS-1 LINE INTERFACE UNIT XRT73L03
REV. P1.0.13
1. In this mode the Transmit Logic Block ignores the
TNData input pin.
2. If the Transmit Section of a given channel is config-
ured to accept Single-Rail data from the Terminal
Equipment, the B3ZS/HDB3 Encoder must be
enabled.
Figure 11 illustrates the behavior of the TPData and
TxClk(n) signals when the Transmit Logic Block has
been configured to accept Single-Rail data from the
Terminal Equipment.
FIGURE 11. THE BEHAVIOR OF THE TPDATA AND TXCLK INPUT SIGNALS WHILE THE TRANSMIT LOGIC BLOCK IS
ACCEPTING SINGLE-RAIL DATA FROM THE TERMINAL EQUIPMENT
Data
TPData
TxClk
1
1
0
2.2 THE TRANSMIT CLOCK DUTY CYCLE ADJUST CIR-
CUITRY
The on-chip Pulse-Shaping circuitry in the Transmit
Section of each Channel of the XRT73L03 generates
pulses of the appropriate shapes and width to meet
the applicable pulse template requirements. The
widths of these output pulses are defined by the width
of the half-period pulses in the TxClk(n) signal.
However, if the widths of the pulses in the TxClk(n)
clock signal are allowed to vary significantly, this
could jeopardize the chip's ability to generate Trans-
mit Output pulses of the appropriate width, thereby
not meeting the Pulse Template requirement specifi-
cation. Consequently, the chip's ability to generate
compliant pulses could depend upon the duty cycle of
the clock signal applied to the TxClk(n) input pin.
The Transmit Clock Duty Cycle Adjust Circuitry ac-
cepts clock pulses via the TxClk(n) input pin at duty
cycles ranging from 30% to 70% and converts them
to a 50% duty cycle.
2.3 THE HDB3/B3ZS ENCODER BLOCK
The purpose of the HDB3/B3ZS Encoder Block is to
aid in the Clock Recovery process at the Remote Ter-
minal Equipment by ensuring an upper limit on the
number of consecutive zeros that can exist in the line
signal.
2.3.1 B3ZS Encoding
If the XRT73L03 has been configured to operate in
the DS3 or SONET STS-1 Modes, the HDB3/B3ZS
Encoder blocks operate in the B3ZS Mode. When the
Encoder is operating in this mode it parses through
and searches the Transmit Binary Data Stream from
the Transmit Logic Block for the occurrence of three
(3) consecutive zeros (e.g., "000"). If the B3ZS En-
coder finds an occurrence of three consecutive zeros,
then it substitutes these three "0’s" with either a "00V"
or a "B0V" pattern.
"B" represents a Bipolar pulse that is compliant with
the Alternating Polarity requirements of the AMI (Al-
ternate Mark Inversion) line code.
"V" represents a Bipolar Violation (e.g., a Bipolar
pulse that violates the Alternating Polarity require-
ments of the AMI line code).
The B3ZS Encoder decides whether to substitute
with either the "00V" or the "B0V" pattern in order to
insure that an odd number of Bipolar pulses exist be-
tween any two consecutive violation pulses.
Figure 12 illustrates the B3ZS Encoder at work with
two separate strings of three or more consecutive ze-
ros.
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