MITEL MH88400-1, MH88400-2, MH88400-3 Datasheet

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MH88400

Line Interface Circuit

Preliminary Information

Features

• FAX and Modem interf ace

• Suitable for V.29 FAX & V.22 bis Da ta

• Allows caller i dent ificatio n

• Variants match German, UK, 600

Ω & 900Ω

network im pedan ces

• Isolation circuitry conforms to international PTT requirements

• Transformerless 2-4 W ire conversi on

• Loop star t oper ation

• Pulse and DT MF operati on

• Accommo date s extern al mo nitor ph one

• Line state dete ction out put s:

-loop current/ringing outputs

-monitor phon e sw itch h ook

• Single +5 V ope ration

Applications

Interface to Central Office for:

•DAA

• Modem

•FAX

• Answering M achi ne

• Terminal Equipm ent

ISSUE 1 April 1995

Ordering Informa tion

MH88400-1 14 Pin DIL Package MH88400-2 14 Pin DIL Package MH88400-3 14 Pin DIL Package

0°C to 70°C

Descript io n

The Mitel MH88400 Line Interface Circuit provides a complete audio and signalling link between audio equipment and central office. The functions provided by the MH88400 include 2-4 Wire conversion, loop seizure, external monitor phone switch hook status and ringing voltage and loop current detection. The device is fabricated as a thick film hybrid which incorporates various technologies for optimum circuit design, high voltage isolation and very high reliability.

Variants are provided to meet German (-1) and UK (-3) and general 600Ω (-2) line impedance (see section on Line Impedances).

Isolation Barrier

Transformer

Isolation

Opto-

TIP

RING

Active

Termination

TXIN

RLS

PHS

Audio Input

Buffer

Input

Buffer

NETWORK CONNECTIONS

Isolation

Opto-

Isolation

Opto-

Isolation

Opto-

Isolation

Opto-

Isolation

Power Supply

Audio

Buffer

Logic Input

Buffer

Audio

Buffer

Loop

Ring &

Buffer

Switch Hook

Buffer

USER CONNECTIONS

VDD

AGND

RVLC

SHK

Figure 1 - Functional Block Diagram

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MH88400 Preliminary Information

VDD

AGND

RVLC

SHK

VX VR

1 2 3 4 5

6 78

TIP

RLS

PHS

TXIN RING

Figure 2 - Pin Connections

Pin Description

Pin # Name Description

1VDDPositive Power Su pp ly Voltage. +5V. 2AGNDAnal og Ground. 4-Wire Ground. Norm all y conne cted to System Ground, 3LC

RVLC

Loop Control (Input). A logic low activates internal circuitry whi ch provides a dc termination across Tip and Ring. Used for seizing the line and dial pulsing

Ring Voltage and Current Detect (Output). A logic low indicates that loop current is detected. The loop current can be due to the external monitor phone or the MH88400 in the off- hook mode. The RVLC outputs pulses when the external monit or phone is dial pulsing or when the MH88400 is dial pulses via the LC input. In addition, when the MH88400 is in the on-hook mode, a pulsing output indicates th at ringing voltag e is across the Tip and Ring leads; the pulsing outputs frequency is twice the ringing frequency.

5SHK

Switch Hook Detect (Output). This is an optional out put whi ch can be used with the PHS input and an external phone. When loop current flows from PHS to TIP, SHK

goes

to logic low.

6VXTransmit (Output). 4-Wire ground(AGND) referenced audi o output, biased at 2.5V.

Outputs for both off-hook and on-hook.

7VRRecei ve (In pu t). 4-Wire ground (A GND) referenced audio input , biased at 2.5V. LC

must be activated low and loop current must be flowing. 8NCNo Connection . This pin is not connected internally 9 RING Ring Lead. Connects to the “Ring” lead of the central office through a relay contact. The

central office “Tip” and “Ring” leads may be interchanged

10 TXIN Transmit (Input). Connects to the “Ring” lead of the central offi ce through a coupling

capacitor.

11 TF Tip Feed. Connects to the “Tip” lead of the central office through an internal resistor and

an optional external resistor.

12 PHS Monitor Phone S ense (Inpu t). This is an optional inp ut which can be used with the

output and an external phone. When loop current flows from PHS to TIP, SHK goes

SHK

to logic low.

13 RLS Ring Loop Sense (Input). Sense node for ringing voltage detector and the loop current

detector.

14 TIP Tip Lead. Connects to the “Tip” lead of the central office through an optional relay

contact. The central office “Tip” and “Ring” leads may be interchanged.

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Preliminary Information MH88400

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 Terminat ion

The DC loop termination c ircuitry provides the loop with an active DC load termination when a logic low is applied to the LC termination is simpler to a resistance of approximately 300Ω (loop current dependant). Internal optically isolat ed 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.

(Loop Control) input. The

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.

In addition, when the MH88400 is in the on-hook mode, a pulsing out put 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 s pecified 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

The RVLC outputs pulses when the external monitor phone is dial pulsing or when t he MH88400 is dial pulsing v ia the LC

mode.

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