XRT73L00A Datasheet, PDF (27/53 Pages) Exar Corporation – E3/DS3/STS-1 LINE INTERFACE UNIT

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XRT73L00A
E3/DS3/STS-1 LINE INTERFACE UNIT
REV. 2.0.1
The XRT73L00A Transmit Clock Duty Cycle Adjust
circuit alleviates the need to supply a signal with a
50% duty cycle to the TCLK input pin.
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 XRT73L00A is configured to operate in the DS3
or SONET STS-1 Modes, then the HDB3/B3ZS En-
coder block operates 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 (“000”). If the B3ZS Encoder
FIGURE 14. AN EXAMPLE OF B3ZS ENCODING
finds an occurrence of three consecutive zeros, 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 and “V” represents
a bipolar Violation (e.g., a bipolar pulse that violates
the Alternating Polarity requirements of the AMI line
code).
The B3ZS Encoder decides whether to substitute
with either a "00V" or a "B0V" pattern to insure that
an odd number of bipolar pulses exist between any
two consecutive violation pulses.
Figure 14 illustrates the B3ZS Encoder at work with
two separate strings of three (or more) consecutive
zeros.
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 XRT73L00A is configured to operate in the E3
Mode, then the HDB3/B3ZS Encoder block operates
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 (“0000”). If
the HDB3 Encoder finds an occurrence of four con-
FIGURE 15. AN EXAMPLE OF HDB3 ENCODING
secutive zeros, then it substitutes these four “0’s” with
either a “000V” or a “B00V” pattern to insure that an
odd number of bipolar pulses exist between any two
consecutive violation pulses.
Figure 15 illustrates the HDB3 Encoder at work with
two separate strings of four (or more) consecutive ze-
ros.
TClk
TPOS
SR data
1
0
11
000001111011011001100001100100
Encoded
PDATA
1001000100101001001000100001000100
Encoded
NDATA
0010000001010010010001010010100000
Line signal
000 V
B0 0V
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