XRT73LC00A_08 Datasheet, PDF (33/63 Pages) Exar Corporation – E3/DS3/STS-1 LINE INTERFACE UNIT

XRT73LC00A
E3/DS3/STS-1 LINE INTERFACE UNIT
REV. 1.0.1
2.4.2 Disabling the Transmit Line Build-Out Circuit
If the Transmit Line Build-Out circuit is disabled, then the XRT73LC00A outputs partially-shaped pulses onto
the line via the TTIP and TRING output pins.
Disable the Transmit Line Build-Out circuit in the XRT73LC00A by doing the following:
• If the XRT73LC00A is operating in the Hardware Mode, set the TXLEV input pin (pin 1) to “High”
• If the XRT73LC00A is operating in the HOST Mode, set the TXLEV bit-field to “1” as illustrated below.
COMMAND REGISTER CR1 (ADDRESS = 0X01)
D4
D3
D2
D1
D0
TXOFF TAOS TXCLKINV TXLEV TXBIN
0
X
X
1
X
2.4.3 Design Guideline for Setting the Transmit Line Build-Out Circuit
The setting ofTXLEV input pin or bit-field should be based upon the overall cable length between the
Transmitting Terminal and the Digital Cross Connect system where the pulse template measurements are
made.
If the cable length between the Transmitting Terminal and the DSX-3 or STSX-1 is less than 225 feet, it is
advisable to enable the Transmit Line Build-Out circuit by setting the TXLEV input pin or bit-field to "0".
NOTE: In this case the XRT73LC00A outputs shaped (e.g., not square-wave) pulses onto the line via the TTIP and TRING
output pins. The shape of this output pulse is such that it complies with the pulse template requirements even
when subjected to cable loss ranging from 0 to 225 feet.
If the cable length between the Transmitting
Terminal and the DSX-3 or STSX-1 is greater than 225 feet, it is advisable to disable the Transmit Line Build-
Out circuit by setting the TXLEV input pin or bit-field to "1".
NOTE: In this case the XRT73LC00A outputs partially-shaped pulses onto the line via the TTIP and TRING output pins.
The cable loss that these pulses experience over long cable lengths (e.g., greater than 225 feet) causes these
pulses to be properly shaped and comply with the appropriate pulse template requirement.
2.4.4 The Transmit Line Build-Out Circuit and E3 Applications
The ITU-T G.703 Pulse Template Requirements for E3 states that the E3 transmit output pulse should be
measured at the Secondary Side of the Transmit Output Transformer for Pulse Template compliance. There is
no Digital Cross Connect System pulse template requirement for E3 and the Transmit Line Build-Out circuit in
the XRT73LC00A is disabled whenever it is operating in the E3 Mode.
2.5 Interfacing the Transmit Section of the XRT73LC00A to the Line
The E3, DS3 and SONET STS-1 specification documents all state that line signals transmitted over coaxial
cable are to be terminated with 75 Ohms. Therefore, interface the Transmit Section of the XRT73LC00A, as
illustrated in Figure 14 which shows two 31.6 Ohm resistors in series with the primary side of the transformer.
These two 31.6Ohm resistors closely match the 75Ohm load termination resistor thereby minimizing Transmit
Return Loss.
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