HC55171_03 Datasheet, PDF (8/20 Pages) Intersil Corporation – 5 REN Ringing SLIC for ISDN Modem/TA and WLL

HC55171
Circuit Operation and Design Information
Introduction
The HC55171 is a high voltage Subscriber Line Interface
Circuit (SLIC) specifically designed for through SLIC
ringing applications. Through SLIC ringing applications are
broadly defined as any application that requires ringing
capability but does not have the standard wired central
office interface. The most common implementation of the
ringing SLIC is in the analog pots port. The analog pots port
provides the ringing function as well as interface
compatibility with answering and fax machines.
Subscriber Line Interface Basics
The basic SLIC provides DC loop current to power the handset,
supports full duplex analog transmission between the handset
and CODEC, matches the impedance of the SLIC to the
impedance of the handset and performs loop supervision
functions to detect when the handset is off hook. The ringing
SLIC adds through the SLIC ringing capability to this suite of
features. The analog interfaces of the SLIC are categorized as
the 2-wire interface (high voltage DC, differential AC) and the 4-
wire interface (low voltage DC, single ended AC).
DC Loop Current
The Tip and Ring terminals of the subscriber line circuit are
biased at negative potentials with respect to ground. The Tip
terminal DC potential is slightly negative with respect to ground,
and the ring terminal DC potential is slightly positive with
respect to the battery voltage (resulting in a large negative
voltage). The HC55171 typical Tip DC voltage is -4V and the
typical ring DC voltage is defined as V_BAT + 4V. For example,
when the battery voltage is -24V the ring voltage is -20V.
To clearly comprehend the Tip and Ring interface it is helpful
to understand that the handset and the SLIC constitute a DC
and AC current loop as shown in Figure 1. The loop is often
referred to as the subscriber loop.
LOOP
CURRENT
TIP
RING
SLIC
FIGURE 1. SUBSCRIBER LOOP
When the handset is on hook (idle) the phone is an open
circuit load and the DC loop current is zero. The SLIC can
still provide AC transmission in this condition, which
supports caller id services. The DC resistance of the off hook
handset is typically 400Ω . Since the Tip DC voltage is more
positive than the ring DC voltage, DC loop current flows from
Tip to Ring when the handset is off hook. The SLIC is
designed with feedback to limit the maximum loop current
when the handset is off hook.
Full Duplex Analog Transmission
Familiarity with the signal paths of the SLIC is critical in
understanding the full duplex transmission capability of the
device. The analog interfaces of the SLIC are categorized as
2-wire interfaces and 4-wire interfaces.
The 2-wire interface of the SLIC consists of the bidirectional
Tip and Ring terminals of the device. A differential
transmitter drives AC signals out of the Tip and Ring
terminals to the handset. A differential receiver across Tip
and Ring receives AC signals from the handset. The
differential receiver is connected across sense resistors that
are in the Tip and Ring signal paths. The differential
transmitter and receiver concept is depicted in Figure 2.
DIFFERENTIAL
TRANSMITTER
-1
-
+
-
+
-
+
DIFFERENTIAL
RECEIVER
FIGURE 2. DIFFERENTIAL TRANSMIT / RECEIVE CONCEPT
Since the receiver is connected across the transmit signal
path, one may deduce that in addition to receiving signals
from the handset, the receiver will detect part of the transmit
signal. Indeed this does occur and is the reason that all SLIC
circuits require a hybrid balance or echo cancellation function.
The 4-wire interface of the SLIC consists of the receive
(VRX) and transmit (OUT1) terminals. The 4-wire interfaces
are single ended signal paths. The receiver is a dedicated
input port and the transmitter is a dedicated output port. The
4-wire receive input of the SLIC drives the 2-wire differential
transmitter and the 2-wire differential receiver drives the 4-
wire transmit output.
The complete signal path for voice signals includes two digital
data busses, a CODEC and a SLIC. There is a receive data
bus and transmit data bus, each with an independent 3-wire
serial interface. The CODEC contains a coder and decoder.
The coder converts the SLIC analog transmit output to digital
data for the transmit data bus. The receive digital data bus is
converted to analog data and drives the SLIC receive input.
The CODECs use logarithmic compression schemes to extend
the resolution of the 8-bit data to 14 bits. The accepted
compression schemes are A-law (Intersil CODEC - CD22357A)
and µ-law (Intersil CODEC - CD22354A). The complete signal
path from the handset to the CODEC is shown in Figure 3.
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