2.0 Circuit Description
Bt8370/8375/8376
2.9 Transmit Line Interface Unit
Fully Integrated T1/E1 Framer and Line Interface
2.9.4 Line Driver
The line driver provides current drive to the low-power, bipolar analog signal
from the transmit Digital-to-Analog Converter (DAC). The transmit DAC
converts the coded pulse shape values to properly scaled, analog bipolar signals
which drive the line transformer. The coded pulse shape values can optionally be
filtered during T1 Longhaul by LBO filter options. The internal sensing circuits
limit the drive current to less than 50 mA when a shorted line condition is
detected for more than 48 transmitted pulses. Normal drive levels are restored
when the short is removed. Typically, this is caused by a transmit cable short
circuit or a transient transmission line current surge. The activation sets the
TSHORT bit in Alarm 2 Status register [ALM2; addr 048].
2.9.4.1 Termination
Impedance
If an external termination resistor is used, the TERM bit in TLIU_CR must be set.
The transformer setting must also be set via the TURNS bit in TLIU_CR. An
external termination resistor can be used only if the transformer turns ratio is
1:1.36. The external termination resistor (RTerm) is placed in parallel across
XTIP/XRING, as illustrated in Figure 2-45.
Figure 2-45. External Termination Resistor Placement
1:N
Cable
XTIP
Bt8370
RTerm
RLoad
VOUT
XRING
2-80
Conexant
N8370DSE
CONEXANT [ CONEXANT SYSTEMS, INC ]
