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MH88400-1

Line Interface Circuit

Mitel Networks 

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.

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.

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.

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.

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.

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.

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

2-13

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