TP3404_14 Datasheet, PDF (6/19 Pages) Texas Instruments – TP3404 Quad Digital Adapter for Subscriber Loops (QDASL)

TP3404
NRND
SNOS703 – DECEMBER 2004
FUNCTIONAL DESCRIPTION
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The QDASL contains 4 transceivers, each of which can interoperate with any of the TP340X family of single-
channel DASL transceivers. Each QDASL transceiver has its own independent line transmit and receive section,
timing recovery circuit, scrambler/descrambler and loop activation controller. Functions which are shared by the 4
transceivers include the Microwire control port and the digital interface with time-slot assignment.
BURST MODE OPERATION
For full-duplex operation over a single twisted-pair, burst mode timing is used, with the QDASL end of each line
acting as the loop timing master, and the DASL at the terminal being the timing slave (the QDASL transceivers
cannot operate in loop timing slave mode).
Each burst within a DASL line is initiated by the QDASL Master transmitting a start bit, for burst framing, followed
by the B1, B2 and D channel data from 2 consecutive 8 kHz frames, combined in the format shown in Figure 2.
During transmit bursts the receiver input for that channel is inhibited to avoid disturbing the adaptive circuits. The
slave's receiver is enabled at this time and it synchronizes to the start bit of the burst, which is always an
unscrambled “1” (of the opposite polarity to the last “1” sent in the previous burst). When the slave detects that
36 bits following the start bit have been received, it disables the received input, waits 6 line symbol periods to
match the other end settling guard time, and then begins to transmit its burst back towards the master, which by
this time has enabled its receiver input. The burst repetition rate is thus 4 kHz.
LINE TRANSMIT SECTIONS
Alternate Mark Inversion (AMI) line coding, in which binary “1”s are alternately transmitted as a positive pulse
then a negative pulse, is used on each DASL line because of its spectral efficiency and null DC energy content.
All transmitted bits, excluding the start bit, are scrambled by a 9-bit scrambler to provide good spectral spreading
with a strong timing content. The scrambler feedback polynomial is: X⁹ + X⁵ + 1.
Figure 2. Burst Mode Timing on the Line
Pulse shaping is obtained by means of a Digital to Analog Converter followed by a Continuous Smoothing Filter,
in order to limit RF energy and crosstalk while minimizing Inter-Symbol Interference (ISI). Figure 3 shows the
pulse shape at the Lo output, while a template for the typical power spectrum transmitted to the line with random
data is shown in Figure 4.
Each line-driver output, Lo0–Lo3, is designed to drive a transformer through a capacitor and termination resistor.
A 1:1 transformer, terminated in 100Ω, results in signal amplitude of typically 1.3 Vpk on the line. Over-voltage
protection must be included in each interface circuit.
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