MITEL MH88632B, MH88632BT Datasheet

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MH88632B

Central Office Interface Circuit

Preliminary Information

Features

• Supports Loop Star t and Ground Star t protocols

• 2-4 Wire conversion

• Programmable Input Impedance, Network
Balance Impedance and gains

• Three relay drivers

• Line state detection outputs

• 15mA operation allowing long line length
capability

• On-hook reception for Caller Line Identification

• Meets FCC Part 68 Leakage Current
Requirements

Applications

Interface to Central Office telephone line for

• PBX

• Key Telephone System

• Ter minal Equipment

• Digital Loop Carrier

• Wireless Local Loop

ISSUE 3 September 1997

Ordering Information

MH88632B 40 Pin SIL Package
MH88632BT 40 Pin 90˚ Package

0°C to 70°C

Description

The Mitel MH88632B Central Office Interface Circuit
provides a complete analog and signalling link
between audio switching equipment and a subscriber
line. The device is available in a single in line
package for high packing densities or in a 90˚
package for reduced card clearance.

The device is fabricated using thick film hybrid
technology for optimum circuit design and very high
reliability.

RING

TIP

RV FL RL RG TG

Status

Detection

Dummy

Ringer

VCC VEE AGND

Line

Termination

Impedance

Matching

XLA XLB XLC XLD Z1 Z2 Z600 Z900 NS N1 N2 NATT

GRC

Relay Driver Circuit

2-4 Wire

Hybrid

Network
Balance

Figure 1 - Functional Block Diagram

LRCLRDBRCBRDGRD

Receive

Gain

Transmit

Gain

VRLY

RGND

RX

GRX1
GRX0

TX
GTX1

GTX0

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MH88632B Preliminary Information

10
11
12
13
14
15
16
17
18
19
20

1
2
3
4
5
6
7
8
9

N2

Z900

Z1
Z2

TX

RX

GTX0

GTX1
GRX0
GRX1

IC

Z600

NS

TG

RL

RV

FL

RG

VEE

VCC

21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40

TIP

RING

XLA

XLB
XLC
XLD

IC

GRD
IC
IC

RGND
VRLY

LRD

BRD

LRC

BRC

GRC

AGND

NATT

N1

Figure 2 - Pin Connections

Pin Description

Pin # Name Description

1 TIP Tip Lead. Connects to the Tip lead of a telephone line usually via an external protection

circuit.

2 RING Ring Lead. Connects to the Ring lead of a telephone line usually via an external protection

circuit.

3 XLA Loop Relay Contact A. Connects to XLB through relay contacts (K1A) when the relay is

energized.

4 XLB Loop Relay Contact B. Connects to XLA through relay contacts (K1A) when the relay is

energized.

5 XLC Loop Relay Contact C. Connects to XLD through relay contacts (K1B) when the relay is

energized.

6 XLD Loop Relay Contact D. Connects to XLC through relay contacts (K1B) when the relay is

energized.
7ICInternal Connection. No connection should be made to this pin.
8 GRD Ground Ring Lead Relay Drive (Output). Connects to the Ground Ring Lead Relay coil

(K3) and is controlled by GRC.
9ICInternal Connection. No connection should be made to this pin.

10 IC Internal Connection. No connection should be made to this pin.
11 RGND Relay Ground. Return path for relay supply voltage.
12 VRLY Relay Positive Supply Voltage. Normally +5V. Connects to all relay coils and the relay

supply voltage.

13 LRD Loop Relay Drive (Output). Connects to the Loop Relay coil (K1) and is controlled by

LRC.

14 BRD Bias Relay Drive (Output). Connects to the Bias Relay coil (K2) and is controlled by BRC.
15 LRC Loop Relay Control (Input). A logic 1 activates LRD. The Loop Relay (K1) is used for

placing the Line Termination across Tip and Ring.

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Preliminary Information MH88632B

Pin Description (continued)

16 BRC Bias Relay Control (Input). A logic 1 activates BRD. The Bias Relay (K2) is used to

connect Tip and Ring to -48V via bias resistors. This input should be connected to logic 0
when not used.

17 GRC Ground Ring Relay Control (Input). A logic 0 activates GRD. The Ground Ring Lead

Relay (K3) is used to connect Ring to AGND via a bias resistor. This input should be
connected to logic 1 when not used.

18 AGND Analog Ground. 4-Wire Ground. Normally connects to system ground. This pin must be

connected to the system ground in Ground Start applications.

19 NATT Network Balance AT&T Node. Used when setting the Network Balance Impedance to

AT&T compromise network.

20 N1 Network Balance Node 1. Used when a Network Balance Impedance which differs from

the Input Impedance is required or when NATT is used.

21 N2 Network Balance Node 2. Used when a Network Balance Impedance which differs from

the Input Impedance is required.

22 Z900 Input Impedance 900Ω Node. Connects to Z1 when selecting an Input Impedance of

900Ω.
23 Z1 Input Impedance Node 1. Used when setting the Input Impedance.
24 Z2 Input Impedance Node 2. Used when a user defined Input Impedance is required.
25 TX Transmit (Output). 4-Wire ground (AGND) referenced analog output.
26 RX Receive (Input). 4-Wire ground (AGND) referenced analog input.
27 GTX0 Transmit Gain Node 0. Connects to GTX1 for 0dB transmit gain.
28 GTX1 Transmit Gain Node 1. Connects to GTX0 for 0dB transmit gain or via a resistor to AGND

for transmit gain programming.
29 GRX0 Receive Gain Node 0. Connects to GRX1 for 0dB receive gain.
30 GRX1 Receive Gain Node 1. Connects to GRX0 for 0dB receive gain or via a resistor to AGND

for receive gain programming.
31 IC Internal Connection. No connection should be made to this pin.
32 Z600 Loop Impedance 600Ω Node. Connects to Z1 when selecting an Input Impedance of

600Ω.
33 NS Network Balance Setting (Input). Used to select the Network Balance impedance.
34 TG Tip Lead Ground Detect (Output). A logic 0 output indicates that the Tip lead is at ground

(AGND) potential.
35 RL Reverse Loop Detect (Output). In the on-hook state, 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.
36 RV Ringing Voltage Detect (Output). A logic low indicates that ringing voltage is across the

Tip and Ring leads.
37 FL Forward Loop Detect (Output). In the on-hook state, 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.
38 RG Ring Lead Ground Detect (Output). A logic 0 output indicates that the Ring lead is at

ground (AGND) potential.
39 VEE Negative Supply Voltage. -5V DC
40 VCC Positive Supply Voltage. +5V DC

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MH88632B Preliminary Information

Functional Description

The MH88632B 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 MH88632B provides an isolation barrier which is
designed to meet FCC Part 68 (November 1987)
Leakage Current Requirements.

External Protection

An external protection circuit may be required to
assist in preventing overvoltage damage to the
device and the subscriber equipment in which it is
incorporated. The type of protection required is
dependant on the application and the regulatory
standards. Please contact the governing regulatory
body and local approvals testing houses for more
assistance.

This protection is shown in block form in Figure 3.

Suitable Markets

The programmability offered by the MH88632B
enhances its suitability for use throughout the
world. However, care should be taken that all
regulatory requirements, e.g. isolation and DC
termination, are being fulfilled for the particular
application in which the device is intended to be
used.

Line Termination

at a logic 0, the Line Ter mination is removed from
across Tip and Ring.

An internal Dummy Ringer is permanently connected
across Tip and Ring which is a series AC load of
(17kΩ+330nF). This 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 between 190Ω and 290Ω dependant on
the loop current.

AC Input Impedance

The Input Impedance (Zin) is the AC impedance that
the MH88632B places across Tip and Ring in order
to terminate the telephone line. It can be user
defined, set to 600Ω or set to 900Ω.

To select a 600Ω Input Impedance, Z1 should be
connected directly to Z600. No connection should
be made to Z2 or Z900.

To select a 900Ω Input Impedance, Z1 should be
connected directly to Z900. No connection should
be made to Z2 or Z600.

In order to user define the Input Impedance an
impedance network should be placed between Z1
and Z2. This should be equivalent to 10 times the
required Input Impedance and must be greater than
100Ω at 3.4kHz. No connection should be made to
Z600 or Z900.

For example, to implement an Input Impedance of
220Ω+(820Ω//115nF) an impedance network of
2200Ω+(8200Ω//11.5nF) should be connected
between Z1 and Z2 as shown below.

When LRC is at a logic 1, LRD is taken to a logic 0
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

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Z1

Z2

2200Ω

8200Ω

11.5nF

Preliminary Information MH88632B

User defined Input Impedances can be used to
satisfy most national requirements. See Table 1.

All connections should be kept as short as possible.

Network Balance Impedance

The MH88632B’s Network Balance Impedance can
be selected to mirror the Input Impedance, to be
AT&T compromise or set to a user defined value.
Thus, the Network Balance Impedance can comply
with most national requirements.

With NS at logic 0, the Network Balance Impedance
is selected to mirror the Input Impedance of the
device. No connection should be made to NATT, N1
and N2.

To select a Network Balance Impedance equal to
AT&T Compromise (i.e. 350Ω+(1kΩ//210nF) ), NS
should be set to a logic 1 and a direct connection
made between NATT and N1. No connection should
be made to N2.

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).

Programmable Transmit and Receive Gain

The Transmit Gain (GTX) of the MH88632B is the
gain from the balanced signal across Tip and Ring to
the ground referenced signal at TX. It is
programmed by making a connection to GTX1. A
direct connection from GTX1 to GTX0 selects a gain
of 0dB. A direct connection from GTX1 to AGND
selects a gain of +6dB. Other gains can be
programmed by connecting a resistor (RTX) between
GTX1 and AGND. The value of resistor is selected
using the following formulae.

To set a user defined Network Balance Impedance
NS is set to a logic 1. An impedance network which
is 10 times the required Network Balance Impedance
must be placed between N1 and AGND. Another
impedance network must be placed between N1 and
N2 which is 10 times the selected input impedance
of the device.

For example, to implement a Network Balance
Impedance of 220Ω+(820Ω//115nF), an impedance
network of 2200Ω+(8200Ω//11.5nF) must be
connected between N1 and AGND. An impedance
network equal to 10 times the selected Input
Impedance must be connected between N1 and N2.
See Table 2.

All connections should be kept as short as possible.

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 signals.

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

RTX = 5000
10

GTX = - 20 log(0.5+5000)
R

The Receive Gain (GRX) of the MH88632B is the
gain from the ground referenced signal at RX to the
balanced signal across Tip and Ring. It is
programmed by making a connection to GRX1. A
direct connection from GRX1 to GRX0 selects a gain
of 0dB. A direct connection from GRX1 to AGND
selects a gain of +6dB. Other gains can be
programmed by connecting a resistor (RRX) between
GRX1 and AGND. The value of resistor is selected
using the following formulae.

RRX = 5000
10

GRX = -20 log(0.5+5000)
R

For the correct programming of Transmit and
Receive Gains the selected Input Impedance must
match the specified telephone line characteristic
impedance.

(-GTX/20)

(-GRX/20)

- 0.5

TX

- 0.5

RX

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MH88632B Preliminary Information

Both Gains are programmable in the range -12dB to
+6dB. This wide range is capable of accommodating
most system loss plans. See Tables 3 and 4.

Caller Line Identification

Caller Line Identification (CLI) provides the called
party with the calling party telephone number. The
Central Office will utilise the voice path of a regular
loop-start telephone line when the MH88632B is in
the on-hook state. The CLI information is typically a
Frequency Shift Keyed (FSK) data signal which is
output at TX.

Supervisory Features

Line Status Detection Outputs

The MH88632B 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; grounded tip lead; and grounded ring lead.

If these Supervisory Features and the Control
Features are used as indicated in Figure 3 they can
implement common Network Protocols such as
Loop-Start Signalling and Ground-Star t Signalling.

1. Ringing Voltage Detect Output (RV)

3. Tip Ground and Ring Ground Detect Outputs
(TG & RG)

The TG output provides a logic 0 when the Tip pin is
at ground (AGND) potential.

The RG output provides a logic 0 when the Ring pin
is at ground (AGND) potential.

Control Inputs

The MH88632B accepts control signals from the
system controller at the inputs Loop Relay Control
(LRC), Bias Relay Control (BRC) and Ground Ring
Relay Control (GRC). These energize the relay drive
outputs Loop Relay Drive (LRD), Bias Relay Drive
(BRD) and Ground Ring Relay Drive (GRD)
respectively. Each output is active low and has an
internal clamp diode to VRLY.

The intended use of each of these relay drivers is
shown in Figure 3. LRC is being used to add and
remove the Line Termination from across Tip and
Ring. BRC is used to connect Tip and Ring to -48V
via external bias resistors. GRC is controlling the
connection of Ring to AGND via an external bias
resistor.

If these Control Features and the Supervisory
Features are used as intended they can be used to
implement common Network Protocols such as
Loop-Start Signalling and Ground-Star t Signalling.

The RV output provides a logic 0 when ringing
voltage is detected across Tip and Ring. This
detector includes a filter which ensures that the
output toggles at the ringing cadence and not at the
ringing frequency. Typically this output switches to a
logic 0 after 50ms of applied ringing voltage and
remains at a logic 0 for 50ms after ringing voltage is
removed.

2. 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.

Mechanical Information

See Figure 9 for mechanical specifications for the
MH88632B and Figure 10 for mechanical
specifications for the MH88632BT.

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