XRT73LC03A Datasheet, PDF (31/61 Pages) Exar Corporation – 3 CHANNEL DS3/E3/STS-1 LINE INTERFACE UNIT

XRT73LC03A
3 CHANNEL DS3/E3/STS-1 LINE INTERFACE UNIT
REV. 1.0.1
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 16 illustrates the behavior of the TPData_(n)
and TxClk_(n) signals when the Transmit Logic Block
has been configured to accept Single-Rail data from
the Terminal Equipment.
FIGURE 16. THE BEHAVIOR OF THE TPDATA AND TXCLK INPUT SGNALS, WHILE THE TRANSMIT LOGIC BLOCK IS
ACCEPTING SINGLE-RAIL DATA FROM THE TERMINAL EQUIPMENT
Data
1
1
0
TPData
TxClk
2.2 THE TRANSMIT CLOCK DUTY CYCLE ADJUST CIR-
CUITRY
The on-chip Pulse-Shaping circuitry within the Trans-
mit Section of each Channel in the XRT73LC03A
generates pulses of the appropriate shapes and
width to meet the applicable pulse template require-
ments. The widths of these output pulses are defined
by the width of the half-period pulses within the
TxClk_(n) signal.
However, if the widths of the pulses within the
TxClk_(n) clock signal are allowed to vary significant-
ly, this could jeopardize the chip’s ability to generate
Transmit Output pulses of the appropriate width and
thereby not meet the Pulse Template requirement
specification. Consequently, the chip’s ability to gen-
erate 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 within the
line signal.
2.3.1 B3ZS Encoding
If the XRT73LC03A has been configured to operate in
the DS3 or SONET STS-1 Modes, then 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 occur-
rence of three (3) consecutive zeros (e.g., "000"). If
the B3ZS Encoder finds an occurrence of three con-
secutive zeros, then it substitutes these three "0’s",
with either a "00V" or a "B0V" pattern.
Where:
"B" represents a Bipolar pulse that is compliant with
the Alternating Polarity requirements of the AMI (Al-
ternate Mark Inversion) line code; and
"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 17 illustrates the B3ZS Encoder at work with
two separate strings of three (or more) consecutive
zeros
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