XRT72L50 Datasheet, PDF (192/471 Pages) Exar Corporation – SINGLE CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER

XRT72L50
SINGLE CHANNEL DS3/E3 FRAMER WITH HDLC CONTROLLER
REV. 1.2.1
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The Transmit DS3 LIU Interface block can transmit data to the LIU IC or other external circuitry via two different
output modes: Unipolar or Bipolar. If the Unipolar (or Single Rail) mode is selected, then the contents of the
DS3 Frame is output, in a binary (NRZ manner) data stream via the TxPOS pin to the LIU IC. The TxNEG pin
will only be used to denote the frame boundaries. TxNEG will pulse "High" for one bit period, at the start of
each new DS3 frame, and will remain "Low" for the remainder of the frame. Figure 55 presents an illustration
of the TxPOS and TxNEG signals during data transmission while the Transmit DS3 LIU Interface block is
operating in the Unipolar mode. This mode is sometimes referred to as Single Rail mode because the data
pulses only exist in one polarity: positive.
FIGURE 55. THE BEHAVIOR OF TXPOS AND TXNEG SIGNALS DURING DATA TRANSMISSION WHILE THE TRANSMIT
DS3 LIU INTERFACE IS OPERATING IN THE UNIPOLAR MODE
Data _{1 0 1 1 0 0 1 0 0 1 1 1 0 1 0 1 0 0 1}
TxPOS
1 10
01
TxNEG
TxLineClk
Frame Boundary
When the Transmit DS3 LIU Interface block is operating in the Bipolar (or Dual Rail) mode, then the contents of
the DS3 Frame is output via both the TxPOS and TxNEG pins. If the Bipolar mode is chosen, then the DS3
data to the LIU can be transmitted via one of two different line codes: Alternate Mark Inversion (AMI) or Binary
- 3 Zero Substitution (B3ZS). Each one of these line codes will be discussed below. Bipolar mode is
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 DS3 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 DS3 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.
TxLineClk - Transmit Line Clock: The LIU IC uses this signal from the Transmit DS3 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 DS3
Framer.
4.2.5.1 Selecting the various Line Codes
Either the Unipolar Mode or Bipolar Mode can be selected by writing the appropriate value to Bit 3 of the I/O
Control Register (Address = 0x01), as shown below.
I/O Control Register (Address = 0x01)
BIT 7
Disable
TxLOC
BIT 6
LOC
BIT 5
Disable
RxLOC
BIT 4
AMI/ZeroSup
R/W
RO
R/W
R/W
1
0
1
0
BIT 3
Unipolar/
Bipolar
R/W
0
BIT2
BIT 1
TxLine CLK RxLine CLK
Invert
Invert
R/W
R/W
0
0
BIT 0
Reframe
R/W
0
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