Preliminary Information
Functional Description
The MH88400 Line Interface Circuit is a COIC
(Central Office Interface Circuit) used to interface
FAX’s Modems or user defined equipment to
Central Office 2-Wire Analog Trunks.
MH88400
In addition, when the MH88400 is in the on-hook
mode, a pulsing output indicates that ringing voltage
is across the tip and ring leads; the pulsing output
frequency is twice the ringing frequency. See
Figure 3.
Ringing frequency may require external validation.
An RC monostable is usually satisfactory for this
purpose or this may also be achieved using software
applications.
Opto-Isolation
The isolation barrier is designed to meet regulatory
requirements for a reinforced barrier of 3kVac. It
provides full isolation of mains voltages up to 250V
RMS and all telecom voltages. In order that this
barrier is not bypassed a creepage/clearance
distance of 6.4mm minimum must be maintained
between wiring, pcb tracking, etc., connected to
external circuitry on either side of the barrier. To
make this requirement simpler, pins on the
MH88400 connected to opposite sides of the
barrier are on opposite sides of the package.
2-4 Wire Conversion
The 2-4 Wire conversion circuit converts the
balanced full duplex signal at Tip and Ring of the
central office line into a transmit ground referenced
signal at VX (Transmit) of the MH88400. It also
converts the receive ground referenced signal at VR
(Receive) of the MH88400 into a balanced transmit
signal at Tip and Ring of the central office line.
In full duplex transmission, the Tip-Ring signal
consists of an audio signal from the central office as
well as an audio signal due to the VR input.
Consequently, both of these signals will appear at the
VX output. The degree to which the 2-4 wire
conversion circuit minimizes the contribution of the
VR signal at the VX output is specified as transhybrid
loss (THL).
A simple THL cancellation circuit as shown in figure 7
can be used for certain applications to give the
required VX/VR signal separation. The MH88400 is
then suitable to drive a COMBO 2 CODEC or a VLSI
MODEM device.
External Protection Circuit
To meet regulatory high voltage requirement, an
external protection circuit is required. The
protection circuit shown in Figure 3 (Clamp Diode
D1) is recommended.
DC Loop Termination
The DC loop termination circuitry provides the loop
with an active DC load termination when a logic low
is applied to the LC (Loop Control) input. The
termination is simpler to a resistance of
approximately 300Ω (loop current dependant).
Internal optically isolated circuitry is used to switch
the termination in and out the loop. This is used for
both seizing the line as well as generating dial
pulses.
Line Impedance
The MH88400 is suitable to drive a COMBO 2
CODEC or a VLSI modem device. The MH88400
provides a fixed Tip-Ring impedance which conforms
to the following PTT requirements.
MH88400-1 Zin = 200 + 820 // 115nF
Germany FTZ
MH88400-2 600Ω
MH88400-3 Zin = 370 + 620 // 310nF
UK BSI
Supervision Features
The supervision circuitry is capable of detecting
ringing voltage and loop current as well as the
status of an optional external monitor phone. The
RVLC (Ring Voltage Loop Current Detect) output
provides a logic low when loop current due to the
external monitor phone or due to the MH88400
being in the off-hook mode is detected.
The RVLC outputs pulses when the external
monitor phone is dial pulsing or when the MH88400
is dial pulsing via the LC mode.
2-13
MITEL [ MITEL NETWORKS CORPORATION ]
