MH88630
Functional Description
The MH88630 is a Central Office Loop Start and/or
Ground Start Analog Trunk interface circuit providing
a complete audio and signalling link to the Central
Office.
The trunk interface circuit performs transformerless
2 to 4 wire conversion, between the 2-wire telephone
loop and the 4-wire transmit and receive pairs of a
voice switching system. The 4-wire connection can
be interfaced to a filter/codec, such as the MT896X,
for use in a digital voice switched system.
Voiceband analog signals, coming from the C.O.,
applied differentially across Tip and Ring, pass
through a bridge rectifier and appear at VTR+ and
VTR- where they are actively terminated. Refer to
Fig 3 - Application Circuit. External capacitors C1
and C4 couple the incoming signals into the
balanced input of the receive audio circuit via TXIN+
and TXIN-. The receive gain is adjusted by feedback
resistor R4. For best performance R4 should be
physically located as close as possible to the GSX
pin. The gain adjusted receive signal appears at the
Vx output pin.
Relay K1 is the loop seize relay which applies active
line termination and also provides biasing of the
current modulator circuit. Activating and deactivating
K1 provides dial pulsing. Q1 is the current modulator
output transistor, referred to as the loop drive
transistor.
Outgoing analog signal from the system are provided
to the interface at the VR input where they enter an
amplifier section with the gain controlled by the
selection of the Tx gain feedback resistor R3. For
best performance, R3 should be physically located
as close as possible to the GSR pin. The output of
the amplifier, RXOUT is coupled to RXIN- the current
modulator circuitry, via C3. The balanced input to the
current modulator is completed with the connection
of C2 from RXIN+ to ground. Transmission to the
C.O. is accomplished by modulating the loop with the
outgoing analog signals. To ensure that the
transmitted signals are not coupled to the receive
circuitry, transhybrid loss is maximised. The
impedance matching, performed by the balanced
network reduces power loss and signal reflections.
The network balance input, NETBAL, of the
interface’s 2-wire to 4-wire converter circuitry
provides selection of a 600Ω balance, used when
feeding channel banks or when performing external
Preliminary Information
tests on the trunk circuit, or the AT&T compromise.
With the NETBAL input pin left the interface is
balanced to match to 600Ω. If the NETBAL input pin
is grounded, the interface is balanced against the
AT&T compromise network consisting of 350Ω plus
1kΩ shunted by a 0.21µF capacitor. This is intended
for typical North American C.O. connections.
The Tip and Ring also enter the balanced input of the
signal detection circuitry which provides the
signalling status outputs TG, RC, RV, FC and RG.
For Ground Start signalling, relay K2 and resistor R1
and R2, are required. Activation of K2 is controlled
by the relay control logic input signal RC2. In the idle
state, K2 is closed connecting the -48Vdc supply to
Tip and Ring through biasing resistors R1 and R2.
Upon detection of TG or RG, the system them pulls
RC1+ low closing K1, then pulls RC2 low which
opens K2 to remove the -48V volts supply from Tip
and Ring.
In the Ground Start signalling environment, initiating
a call to the C.O. is preformed by the following
sequence of events. The system provides a logic low
on the ring ground input pin of the interface. This
activates the circuit which grounds the ring lead
through a current limiting resistance. The C.O.
recognises the ground condition and connects the tip
lead to ground. The interface senses this condition
and the tip lead ground detect output switches to a
logic low. The system then applies active line
termination by closing K1 using RC1 and opens K2
using RC2. A call from the C.O. can be performed
similarly. The C.O. can signal to the interface by
pulling either Tip or Ring to ground potential, or by
applying ringing voltage to the ring lead.
Signal Detection Circuitry
The signal detection circuit provides the signalling
status output. The system, controlling the interface,
monitors these active low logic outputs. RV is the
Ringing Voltage detect output. When the C.O.
applies ringing voltage to the termination, the trunk
interface provides a 50ms debounced outputs at RV
during the ringing burst period. This output will
remain low for approximately 50ms after the C.O.
removes the ringing voltage. Ringing voltage above
40 VRMS at 20 Hz will be detected.
TG is the Tip lead Ground detect output and RG is
the Ring lead Ground detect output. The TG and RG
outputs provide a means of determining call
origination or other handshaking functions. The high
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MITEL [ MITEL NETWORKS CORPORATION ]
