MH88631
Functional Description
The MH88631-1, 2, 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 for 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 Transmit audio circuitry via
TXIN+ and TXIN-. The transmit 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 transmit signal
appears at the Vx output pin.
Relay K1 is the loop seize which applies active line
termination and also provides biasing of the current
modulator circuitry. Activating and deactivating K1
provides dial pulsing. Q1 is the current modulator
output transistor, referred to as the loop driver
transistor.
Outgoing analog signals 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 Rx 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 balance network reduces power loss and
signal reflections. The network balance input,
NETBAL, of the interface’s 2-wire to 4-wire convertor
circuitry provides selection of a 600Ω balance, used
when feeding channel banks or when performing
external tests on the trunk circuit, or the AT&T
compromise. When the NETBAL input pin is
grounded, the interface balance against the AT&T
Preliminary Information
compromise network consisting of 350Ω plus 1kΩ
shunted by a 0.21µF capacitor. This is typical of
North American C.O. connections.
The Tip and Ring also provide the balanced input to
the signal detection circuitry which generates 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 -48 VDC
supply Tip and Ring through biasing resistors R1 and
R2. Upon detection of TG or RG, the system then
pulls RC1 low, closing K1, then pulls RC2 low which
opens K2 to remove the -48 volts supply from Tip
and Ring.
In the Ground Start signalling environment, initiating
a call to the C.O. is performed by the following
sequence of events. The system provides a logic low
on the ring ground input pin of the interface. This
activates the circuitry which grounds the ring lead
through a current limiting resistance. The C.O.
recognizes 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 circuitry provides the signalling
status outputs. The system, controlling the
interfaces, monitors these active low logic outputs.
RV is the Ringing Voltage detect outputs. When the
C.O. applies ringing voltage to the termination, the
trunk interface provides a 50ms debounced output at
RV during the ringing burst period. This output will
remain low for approximately 50ms after C.O.
removes the ringing Voltage. Ringing voltage above
40 VRMS at 20Hz 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 provides a means of determining call original
or other hand shaking functions. The high
impedance detection circuitry of the interface will
detect both Tip and Ring Ground voltages above
approximately -15.3V of true ground.
RC is the Reverse Loop Current detect output and
FC is the Forward Loop Current detect output.
2-228
MITEL [ MITEL NETWORKS CORPORATION ]
