2-287

zFeatures

Loop Start Trunk Interface

900

Input Impedance

2-4 Wire Conversion

On-Hook Reception

Line State Detection Outputs:

- Forward Loop
- Reverse Loop
- Switch Hook
- Ringing Voltage
·

Relay Driver

Industrial Temperature Range option

DIL and SMT versions

Meets FCC part 68 Leakage Current
Requirements

Applications

Interface to Central Office for:
·

PABX

Key Telephone Systems

Channel Bank

Voice Mail

Terminal Equipment

Digital Loop Carrier

Optical Multiplexer

Description

The Mitel MH88636-4 Central Office Trunk Interface
circuit provides a complete analog and signalling link
between audio switching equipment and a subscriber
Line.

The device is fabricated as a thick film hybrid
technology for optimum circuit design and very high
reliability for both commercial and industrial
temperature changes.

Figure 1 - Functional Block Diagram

Status

Detection

Receive

Gain

Transmit

Gain

2 - 4 Wire

Hybrid

Line

Termination

Loop Relay

Driver

Network
Balance

XLA XLB XLC XLD

LRC

LRD

VRLY

TIP

VCC

VEE

AGND

SHK

RING

Impedance
Matching

ISSUE 2

January 1997

Ordering Information

MH88636AD-4I

28 Pin DIL Package

-40

C to 85

MH88636AD-4

28 Pin DIL Package

MH88636AS-4

28 Pin SMT Package

C to 70

MH88636-4

Central Office Interface Circuit

Preliminary Information

MH88636-4

Preliminary Information

2-288

Figure 2 - Pin Connections

Pin Description

Pin #

Name

Description

XLB

Loop Relay Contact B. Connects to XLA through the Loop Relay contacts (K1) when the
relay is activated.

XLC

Loop Relay Contact C. Connects to XLD through the Loop Relay contacts (K1) when the
relay is activated.

Internal Connection. No connection should be made to this pin.

TIP

Tip lead. Connects to the Tip lead of a Telephone Line.

RING

Ring Lead. Connects to the Ring lead of a Telephone Line.

6 - 11

Internal Connection. No connection should be made to this pin.

Reverse Loop detect. In the on-hook status, a logic 0 output indicates that reverse loop
battery is present. In the off-hook state, a logic 0 output indicates that reverse loop current
is present.

Forward Loop Detect. In the on-hook status, a logic 0 output indicates that forward loop
battery is present. In the off-hook state, a logic 0 output indicates that forward loop current
is present.

Ringing Voltage Detect (Output). A logic 0 indicates that ringing voltage is across the Tip
and Ring leads.

15, 16,

Internal Connection. No connection should be made to this pin.

Transmit (Output). 4 Wire ground (AGND) referenced analog output.

Receive (Input). 4 Wire ground (AGND) referenced analog input.

SHK

Switch Hook (Output). A logic 0 indicates the presence of forward or reverse battery
when LRC is logic 0 and the presence of forward or reverse loop current when LRC is logic
1.

LRC

Loop Relay Control (Input). A logic 1 activates the Loop Relay Driver output (LRD).

VEE

Negative Power Supply. -5V DC

AGND

Analogue Ground. 4 Wire ground. Normally connected to System Ground.

VCC

Positive Power Supply. +5V DC

VRLY

Relay Supply Voltage. Typically +5V. Connects to the relay supply voltage.

XLB

XLC

TIP

RING

XLD

XLA

LRD

VRLY

VCC

AGND

VEE

LRC

SHK

2
3

4
5

Preliminary Information

MH88636-4

2-289

Functional Description

The MH88636-4 is a Central Office Interface Circuit
(COIC). It is used to correctly terminate a Central
Office 2-Wire telephone line. The device provides a
signalling link and a 2-4 Wire line interface between
the telephone line and subscriber equipment. The
subscriber equipment can include Private Branch
Exchanges (PBX's), Key Telephone Systems,
Terminal Equipment, Digital Loop Carriers and
Wireless Local Loops.

All descriptions assume that the device is connected
as in the application circuit shown in Figure 3.

Isolation Barrier

The MH88636-4 provides an isolation barrier which
is designed to meet FCC Part 68 (November 1987)
Leakage Current Requirements.

External Protection Circuit

An external Protection Circuit assists in preventing
damage to the device and the subscriber equipment,
due to over-voltage conditions. The type of
protection required is dependant upon the
application and regulatorary standards. In Figure 3
the protection is shown in block form. Further details
should be obtained from the specific country's
regulatorary body.

Suitable Markets

The MH88636-4 has a selectable Input Impedance
of 900

or 900

+ 2.16

F. This makes it suitable

primarily for North America or Brazilian markets.

Line Termination

When LRC is at a logic 1,

LRD will sink current

which energizes the Loop Relay (K1), connecting
XLA to XLB and XLC to XLD. This places a line

termination across Tip and Ring. The device can be
considered to be in an off-hook state and DC loop
current will flow. The line termination consists of a
DC resistance and an AC impedance.

When LRC is at a logic 0, the Line Termination is
removed from across Tip and Ring. An external
Dummy Ringer should be permanently connected
across Tip and Ring and under these conditions is
the only load on the line. The device can be
considered to be in an on-hook state and negligible
DC current will flow. The Dummy Ringer is a series
AC load of typically (17k

+330nF) which represents

a mechanical telephone ringer and allows ringing
voltages to be sensed. This load can be considered
negligible when the line has been terminated.

Depending on the Network Protocol being used the
line termination can seize the line for an outgoing
call, terminate an incoming call, or if applied and
disconnected at the correct rate can be used to
generate dial pulse signals.

The DC line termination circuitry provides the line
with an active DC load which is equivalent to a DC
resistance of less than 300

dependant upon the

loop current.

Ringing Equivalent Number

The Ringing Equivalent Number (REN) is application
specific. See the governing regulatory body
specification for details.

AC Input Impedance

The Input Impedance (Zin) is the AC impedance that
the MH88636-4 places across Tip and Ring in order
to terminate the telephone line. It can be set to
either 900

or 900

+ 2.16uF by connecting an

external impedance between XLA and XLD (Zext).

To select a 900

Input Impedance, 1000

must be

connected across XLA and XLD.

LRD

Loop Relay Drive (Output). Connects to the Loop Relay coil. When LRC is at a logic 1 an
open collector output at LRD sinks current and energizes the relay.

XLA

Loop Relay Contact A. Connects to XLB through the Loop Relay (K1) contacts when the
relay is activated.

XLD

Loop Relay Contact D. Connects to XLC through the Loop Relay (K1) contacts when the
relay is activated.

Pin Description (continued)

MH88636-4

Preliminary Information

2-290

To select a 900

+ 2.16

F Input Impedance, 1000

+ 360

//2.2uF must be connected across XLA and

XLD.

All connections should be kept as short as possible.

Network Balance Impedance

The MH88636-4's Network Balance Impedance has
been optimised for either of the two Input
Impedances.

2-4 Wire Conversion

The device converts the balanced 2-Wire input,
presented by the line at Tip and Ring, to a ground
referenced signal at TX. This circuit operates with or
without loop current; signal reception with no loop
current is required for on-hook reception enabling the
detection of Caller Line Identification (CLI) signals.

Conversely the device converts the ground
referenced signal input at RX, to a balanced 2-Wire
signal across Tip and Ring.

The 4-Wire side (TX and RX) can be interfaced to a
filter/codec, such as the Mitel MT896X, for use in
digital voice switched systems

During full duplex transmission, the signal at Tip and
Ring consists of both the signal from the device to
the line and the signal from the line to the device.
The signal input at RX, being sent to the line, must
not appear at the output TX. In order to prevent this,
the device has an internal cancellation circuit. The
measure of attenuation is Transhybrid Loss (THL).

Transmit and Receive Gain

The Transmit Gain of the device is the gain from the
balanced signal across Tip and Ring to the ground
referenced signal at TX. It is set at 0dB.

The Receive Gain of the device is the gain from the
ground referenced signal at RX to the balanced
signal across Tip and Ring. It is set at -2dB.

Supervision Features

Line Status Detection Outputs

The MH88636-4 supervisory circuitry provides the
signalling status outputs which are monitored by the

system controller. The supervisory circuitry is
capable of detecting: Ringing Voltage; Forward and
Reverse loop battery; Forward and Reverse loop
current; and Switch Hook.

Ringing Voltage Detect Output (RV)

The RV output provides a logic 0 when ringing
voltage is detected across Tip and Ring. It toggles at
the ringing frequency, typically going low 50ms after
the ringing voltage is applied and remains low for 50
ms after ringing voltage is removed.

Forward Loop and Reverse Loop Detect
Outputs (FL & RL)

The FL output provides a logic 0 when either forward
loop battery or forward loop current is detected, that
is the Ring pin voltage is negative with respect to Tip
pin voltage.

The RL output provides a logic 0 when either reverse
loop battery or reverse loop current is detected, that
is the Tip pin voltage is negative with respect to Ring
pin voltage.

The Switch Hook Detect (SHK)

The SHK output is active if either forward loop or
reverse loop current is detected, or if forward or
reverse battery voltage is detected.

Control InputS

The MH88636-4 accepts a control signal from the
system controller at the Loop Relay Control input
(LRC). This energises the relay drive output Loop
Relay Drive (LRD). The output is active low and has
an internal clamp diode to VRLY.

The intended use of this relay driver is to add and
remove the Line Termination from across Tip and
Ring, as shown in Figure 3.
If this Control input and the Supervisory Features are
used as indicated in Figure 3, Loop-Start Signalling
can be implemented.

Mechanical Data

See Figure 9 and 10 for details of the mechanical
specification.

Document Outline

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Document Outline

ÐÐ»ÐµÐºÑ‚Ñ€Ð¾Ð½Ð½Ñ‹Ð¹ ÐºÐ¾Ð¼Ð¿Ð¾Ð½ÐµÐ½Ñ‚: MH88636AD-4I