XRT73L04A Datasheet, PDF (34/65 Pages) Exar Corporation – 4 CHANNEL DS3/E3/STS-1 LINE INTERFACE UNIT

XRT73L04A
4 CHANNEL DS3/E3/STS-1 LINE INTERFACE UNIT
REV. 2.0.3
meet the applicable pulse template requirements.
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 XRT73L04A 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 pars-
es 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.
FIGURE 17. AN EXAMPLE OF B3ZS ENCODING
TClk
TPOS
SR data
1 0 11
000001111011011001100001100100
Encoded
PDATA
1001001000101001001000100001000100
Encoded
NDATA
0010000001010010010001010100100000
Line signal
00 V
B0V
2.3.2 HDB3 Encoding
If the XRT73L04A is configured to operate in the E3
Mode, then the HDB3/B3ZS Encoder blocks operate
in the HDB3 Mode. When the Encoder is operating in
this mode, it parses through and searches the Trans-
mit Data Stream from the Transmit Logic Block for the
occurrence of four (4) consecutive zeros (e.g.,
"0000"). If the HDB3 Encoder finds an occurrence of
four consecutive zeros, then it substitutes these four
"0’s", with either a "000V" or a "B00V" pattern. The
HDB3 Encoder decides whether to substitute with ei-
ther the "000V" or the "B00V" pattern in order to in-
sure that an odd number of bipolar pulses exist be-
tween any two consecutive violation pulses.
Figure 18 illustrates the HDB3 Encoder at work with
two separate strings of four (or more) consecutive ze-
ros.
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