Dual T1/E1 Line Interface Unit
—
LXT331
Datasheet
21
3.2
Line Protection
On the receive side, 1 k
series resistors protect the receiver against current surges coupled into
the device. Due to the high receiver impedance (40 k
typical) the resistors do not affect the
receiver sensitivity. On the transmit side, Schottky diodes D1-D4 protect the output driver. While
not mandatory for normal operation, these protection elements are strongly recommended to
improve the design
’
s robustness.
3.2.1
1.544 Mbps T1 Applications
Figure 12 on page 23
shows a typical host mode T1 application. The serial interface pins are
grouped at the top. Host mode is selected by applying clock (MCLK or TCLK
)
to the SPE pin.
When the TRSTE pin (shown at lower left) is pulled Low, the LXT331 operates normally. Pulling
this pin High causes all outputs to go to a high impedance state.
Figure 12 on page 23
also shows a dual framer that recovers the clock from PMRK and NMRK.
The DFM and DPM monitor output signals are available to drive optional external circuits. The
transmitter power supply pins are tied to the common +5 VDC bus. Note that 68
μ
F decoupling
capacitors are installed. The power supply for the remaining (non-driver) circuitry includes 1.0
μ
F
and 0.1
μ
F decoupling capacitors. Note that all VCC pins must be within ±0.3 V of each other.
The line interface circuitry is identical for both LIU ports. The precision resistors, in line with the
transmit transformer, provide optimal return loss. The recommended transformer/resistor
combinations (on the transmit side) are listed in
Table 7
. 1:1 transformers are used on the receive
side.
3.2.2
2.048 Mbps E1 Coax Applications
Figure 11
shows the line interface for a typical 2.048 Mbps E1/CEPT coaxial (75
) application.
The LEN code should be set to 000 for coax. With 9.1
Rt resistors in line with 1:2 output
transformers, the LXT331 produces 2.37 V peak pulses as required for coax applications.
For E1 75
Coaxial Applications:
001
1:2
Rt = 14.3
10dB
000
1:2
Rt = 9.1
18dB
Table 7. Transmit Transformer Combinations
LEN
Xfmr Ratio
1
Rt Value
2
Rtn Loss
3
For T1/DSX-1 100
Twisted-Pair Applications:
1. Transformer turns ratio accuracy is ± 2%.
2. Rt values are ± 1%.
3. Typical return loss, 51kHz - 3.072 MHz band, with a
capacitor in parallel with the primary side of the transformer.