Preliminary Information
Functional Description
The MH88422 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.
MH88422
The RVLC outputs pulses when the external monitor
phone is dial pulsing or when the MH88422 is dial
pulsing via the LC mode.
In addition, when the MH88422 is on-hook, 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
MH88422 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 MH88422. It also
converts the receive ground referenced signal at VR
(Receive) of the MH88422 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. There is
an internal cancellation circuit which prevents the V
R
signal appearing 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).
The MH88422 is suitable to drive a COMBO II 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 similar to a resistance of
approximately 300Ω (loop current dependent).
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. The MH88422 will not seize the line when
powered down.
Line Impedance
The MH88422 provides a fixed Tip-ring impedance
which conforms to the following PTT requirements.
MH88422-1 Zin = 200 + 820 // 115nF
Germany
MH88422-2
Zin = 600Ω
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 flows due to the
external monitor phone or the MH88422 being in the
off-hook mode.
MH88422-3 Zin = 370 + 620 // 310nF
UK
2-23
MITEL [ MITEL NETWORKS CORPORATION ]
