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MH88422
(Rev.:1995)

Line Interface Circuit

Mitel Networks 

Preliminary Information

MH88422

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.

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.

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.

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.

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.

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 VR

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.

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

MH88422-3 Zin = 370 + 620 // 310nF

UK

2-23

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