MITEL MH88422-2 Datasheet



MH88422

Line Interface Circuit

Preliminary Information

Features

• FAX and Modem interf ace (V29/V2 2 bis)

• Variants mat ch Germ an, UK & 600

Ω network

impedance s

• Provides reinforced barrier to interna tional PTT
requiremen ts

• Transformerless 2-4 W ire conversion

• 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:

• -l oop curr ent/ri nging outp uts

• Single +5V operat ion, l ow idle po wer (5 mW)

• Full Duplex

• Suitable for p ower dow n ap plica tions

Applications

Interface to Central Office for:

•DAA

• Modem

•FAX

• Answering M achi ne

• Terminal Equip men t

ISSUE 5 April 1995

Ordering Information

MH88422 -1 14 Pin DIL Pa ckage
MH88422 -2 14 Pin DIL Packag e
MH88422 -3 14 Pin DIL Packag e

Surface Mount Options are available by adding the

Suffix “S”

Description

The Mitel MH88422 Line Interface Circuit provides a
complete audio and signalling link between audio
equipment and central office. The functions provided
by the MH88422 include 2-4 Wire conversion, loop
seizure, ring voltage and loop current detection. The
device is fabricated as 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) line impedance (see section on Line
Impedances), as well as a 600Ω variants (-2).

Isolation Barrier

Power

Supply

Opto-

TIP

RING

Active

Termina tio n

TXIN

TF

RLS

Audio Input

Buffer

Input
Buffer

NETWORK CONNECTIONS

Isolation

Opto-

Isolation

Opto-

Isolation

Opto-

Isolation

Logic Input

Buffer

Audio

Buffer

Audio
Buffer

Loop

Ring &

Buffer

USER CONNECTIONS

Transhybrid

Loss

Cancellat io n

Circuit

VDD
AGND

LC

VR

VX

RVLC

Figure 1 - Functional Block Diagram

2-21

MH88422 Preliminary Information

VDD

AGND

LC

RVLC

NC

VX

VR

1
2
3
4
5

6
78

14

TIP

13

RLS

12

NC

11

TF

10

TXIN
RING

9

NC

Figure 2 - Pin Connections

Pin Description

Pin # Name Description

1VDDPositive Pow er Su pp ly Voltage. +5V.
2AGNDAnalog Ground. 4-Wire Ground. Normally connect ed to System Ground.
3LC

4RVLCRin g Voltage and Current Detect (Output). A logic low indicates that loop current is

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

detected. The loop current can be due to the external monitor phone or the MH88422 in
the off-hook mode. The RVLC output pulses when the external monitor phone is dial
pulsing or when the MH88422 is dial pulsing via the LC

input. In addition, when the
MH88422 is on the on-hook mode, a pulsing output indicates ringing voltage across the
Tip and Ring leads. The pulsing output frequency is twice the ringing frequency.

5NCNo Connection should be mad e to this pin.
6VXTransmit (Outp u t). 4-Wire ground (AGND) referenced audio out put , biased at 2.5V.

Outputs in off-hook mode only.

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

must be activated low and loop current must be flowing .

8NCNo Connection should be mad e to this pin.
9 RING Ring Lead. Connects to the “Ring” lead of the central office through an opt iona l 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 coupli ng

capacitor.

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

an optional external resistor.

12 NC No Connection sho uld be mad e to this pin.
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 opti onal relay

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

2-22

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

The RVLC outputs pulses when the external monitor
phone is dial pulsing or when the MH88422 is dial
pulsing via the LC

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.

mode.

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.

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
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.

(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 flows due to the
external monitor phone or the MH88422 being in the
off-hook m ode .

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
signal appearing at the VX output. The degree to
which the 2-4 wire conver sion circuit min imizes 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

R

2-23

MH88422 Preliminary Information

Absolute Maximum Ratings* - All voltages are with respect to AGND unless otherwise specified.

Parameter Symbol Min Max Units

.

1 DC Supply Voltage V
2 Storage Temperature T
3 DC Loop Voltage V
4 Ringing Voltage V
5 Loop Current I

* Exceeding these values may cause permanent damage. Functional operation under these conditions is not implied

DD

S

Bat

R

Loop

Recommended Operating Conditions.

Parameters Sym Min Typ

1 DC Supply Voltages V
2 Operating Temperature T
3 Ringing Voltage V

‡ Typical figures are at 25°C with nom inal +5V sup ply and are for design aid only

DD

OP

R

4.5 5.0 5.5 V
070°C

‡

Max Units Test Conditions

75 V

Loop Electrical Characteristics*

‡

Characteristics Sym Min Typ

1

Ringing Voltage No Detect

-1 Variant only Detect
Ringing Voltage No Detect

All other Variants Detect

2

Ringing Frequency 15 68 Hz

VR

27

VR

14

Max Units Test Conditions

20 V

7V

V

V

RMS
RMS

RMS
RMS

-0.3 6 V

-55 +80 °C

-100 +100 V

- 120 V

-90

V

RMS

Bat

=-48V

Refer to FT2 1TR2 for test
method

Externally
Adjustable

RMS

mA

3

On-Hook 2-wire Impedance 40k Ω 1kHz

4

Operating Loop Current 15 80 mA

5

Operating Loop Resista nce 2800 Ω V

6

Off-Hook DC -1 Variant
Voltage

-2 Variant

-3 Variant

7

Leakage Current (2-Wire to AGND) 10 µ A 100V

8

Leakage Current on Hook

6.0

6.0

4.0

4.0

6.0

6.0

9.6
15

6.0

7.8

9.0

12.5

V
V
V
V
V
V

910µAV

(Tip to Ring)

9

DC Resistance during dialling

-1 variant only 200 220 Ω I

10

Dial Pulse Distort ion ON

-1 Variant OFF
All other Variant

* Loop Electrical Characteristics are over recommended operating conditions unless otherwise stated.
‡ Typical figures are at 25°C and are for desi gn aid only.
Note 1: All of the above characteristics use a test circuit as per Figure 3.

0

+6

0

+2
+8
+2

+4

+10

+4

ms
ms
ms

=-48V, I

Bat

=25mA

I

Loop

=42mA

I

Loop

=15-20mA

I

Loop

=26mA

I

Loop

=25mA

I

Loop

=42mA

I

Loop

=-50V

Bat

=20-40mA

Loop

DC

Loop

=15mA

2-24