XRT72L56 Datasheet, PDF (416/486 Pages) Exar Corporation – SIX CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER

XRT72L56
SIX CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER
REV. P1.1.2
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PRELIMINARY
sometimes referred to as Dual Rail because the data
pulses occur in two polarities: positive and negative.
The role of the TxPOS, TxNEG and TxLineClk output
pins, for this mode are discussed below.
TxPOS - Transmit Positive Polarity Pulse: The
Transmit E3 LIU Interface block will assert this output
to the LIU IC when it desires for the LIU to generate
and transmit a positive polarity pulse to the remote
terminal equipment.
TxNEG - Transmit Negative Polarity Pulse: The
Transmit E3 LIU Interface block will assert this output
to the LIU IC when it desires for the LIU to generate
and transmit a negative polarity pulse to the remote
terminal equipment.
I/O CONTROL REGISTER (ADDRESS = 0X01)
TxLineClk - Transmit Line Clock: The LIU IC uses
this signal from the Transmit E3 LIU Interface block to
sample the state of its TxPOS and TxNEG inputs.
The results of this sampling dictates the type of pulse
(positive polarity, zero, or negative polarity) that it will
generate and transmit to the remote Receive E3
Framer.
6.2.5.1 Selecting the various Line Codes
The user can select either the Unipolar Mode or Bipo-
lar Mode by writing the appropriate value to Bit 3 of
the I/O Control Register (Address = 0x01), as shown
below.
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
BIT2
BIT 1
BIT 0
Disable TxLOC LOC
Disable AMI/ZeroSup* Unipolar/
RxLOC
Bipolar*
TxLine CLK RxLine CLK
Invert
Invert
Reframe
R/W
RO
R/W
R/W
R/W
R/W
R/W
R/W
1
0
1
0
0
0
0
0
Table 81 relates the value of this bit field to the Trans-
mit E3 LIU Interface Output Mode.
TABLE 81: 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
TRANSMIT E3 FRAMER LIU INTERFACE OUTPUT MODE
0
Bipolar Mode: AMI or HDB3 Line Codes are Transmitted and Received
1
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
6.2.5.1.1 The Bipolar Mode Line Codes
If the Framer is selected to operate in the Bipolar
Mode, then the DS3 data-stream can be transmitted
via the AMI (Alternate Mark Inversion) or the HDB3
Line Codes. The definition of AMI and HDB3 line
codes follow.
6.2.5.1.1.1 The AMI Line Code
AMI or Alternate Mark Inversion, means that consec-
utive "one's" pulses (or marks) will be of opposite po-
larity with respect to each other. The line code in-
volves 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" ampli-
tude 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 po-
larity and vice versa. This alternating-polarity rela-
tionship exists between two consecutive mark pulses,
independent of the number of 'zeros' that may exist
between these two pulses. Figure 181 presents an il-
lustration 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.
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