HC55120CM INTERSIL [Intersil Corporation], HC55120CM Datasheet - Page 23

• Manufacturer Part Number: HC55120CM
• Description: Low Power Universal SLIC Family
• Manufacturer: INTERSIL [Intersil Corporation]

2-Wire Loopback Test
Most of the SLICs in the UniSLIC14 family feature 2-Wire
loopback testing. This loopback function is only activated
when the subscriber is on hook and the logic command to
the SLIC is in the Test Active State. (Note: if the subscriber is
off hook and in the Test Active State, the function performed
is the Line Voltage Measurement.)
During the 2-wire loopback test, a 2k internal resistor is
switched across the tip and ring terminals of the SLIC. This
allows the SHD function and the 4-wire to 4-wire AC
transmission, right up to the subscriber loop, to be tested.
Together with the 4-wire loopback test in the Open Circuit
logic state, this 2-wire loopback test allows the complete
network (including SLIC) to be tested up to the subscriber
loop.
Pulse Metering
The HC55121, HC55142 and the HC55150 are designed to
support pulse metering. They offer solutions to the following
pulse metering design issues:
1) Providing adequate signal gain and current drive to the
subscriber metering equipment to overcome the attenuation
of this (12kHz, 16kHz) out of band signal.
2) Attenuating the pulse metering transhybrid signal without
severely attenuating the voice band signal to avoid clipping
in the CODEC/Filter.
3) Tailoring the overload levels in the SLIC to avoid clipping
of the combined voiceband and pulse metering signal.
4) Having the provision of silent polarity reversal as a backup
in the case where the loop attenuates the out of band signal
too much for it to be detected by the subscriber’s metering
equipment.
Adequate Signal Gain
Adequate signal gain and current drive to the subscriber’s
metering equipment is made easier by the network shown in
Figure 23. The pulse metering signal is supplied to a
dedicated high impedance input pin called SPM. The circuit
in Figure 23 shows the connection of a network that sets the
2-wire impedance (Z
to be approximately 200 . If the line impedance (Z
to 200 at the pulse metering frequencies, then the 4-Wire
HC55120, HC55121, HC55130, HC55131, HC55140, HC55141, HC55142, HC55143, HC55150, HC55151
RING
TIP
2-WIRE LOOPBACK
FIGURE 22. 4-WIRE AND 2-WIRE LOOPBACK TESTS
INTERNAL
600
UniSLIC14
TR
), at the pulse metering frequencies,
4-23
4-WIRE LOOPBACK
V
V
PTG
TX
RX
+
-
CODEC/FILTER
DUAL SUPPLY
L
) is equal
to 2-wire gain (V
input signal requirements of the pulse metering signal.
Note: The automatic pulse metering 2-wire impedance
matching is independent of the programmed 2-wire
impedance matching at voiceband frequencies.
Calculation of the pulse metering gain is achieved by
replacing V
following the same process through to Equation 21. The
UniSLIC14 sets the 2-wire input impedance of the SLIC
(Z
results are shown in Equation 40.
Avoiding Clipping in the CODEC/Filter
The amplitude of the returning pulse metering signal is often
very large and could easily over drive the input to the
CODEC/Filter. By using the same method discussed in
section “Transhybrid Balance”, most if not all of the pulse
metering signal can be canceled out before it reaches the
input to the CODEC/Filter. This connection is shown in
Figure 23.
Overload Levels and Silent Polarity Reversal
The pulse metering signal and voice are simultaneously
transmitted, and therefore require additional overhead to
prevent distortion of the signal. Reference section “Off hook
Overhead Voltage” to account for the additional pulse
metering signal requirements.
Most of the SLICs in the UniSLIC14 family feature full
polarity reversal. Full polarity reversal means that the SLIC
can: transmit, determine the status of the line (on hook and
off hook) and provide “silent” polarity reversal. Reference
Equation 39 to program the polarity reversal time.
A
4-2
FIGURE 23. PULSE METERING WITH TRANSHYBRID
TR
1/80K
), including the protection resistors, equal to 200 . The
=
------------- = 8
SPM
V
TR
I
X
Z
T
RX
BALANCE
500K
500K
–
/500k in Equation 15 with SPM/125k and
TR
-------------------------
Z
L
500K
SETS 2-WIRE
IMPEDANCE
AT 12-16kHz
5
+ Z
EQUAL TO
Z
/ SPM) is equal 4. Thereby lowering the
L
200
I
TR
X
500K
UniSLIC14
+
=
-
A = 1
–
125K
8
---------------------------
200 + 200
200
V
PTG
V
SPM
TX
RX
R1
= 0
=
–
4
R2
12/16kHz
PULSE METERING
INPUT SIGNAL
(EQ. 40)