®
MH88632B
Central Office Interface Circuit
Preliminary Information
Features
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Supports Loop Start and Ground Start 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
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.
Applications
Interface to Central Office telephone line for
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PBX
Key Telephone System
Terminal Equipment
Digital Loop Carrier
Wireless Local Loop
RV
FL
RL RG TG
VCC
VEE
AGND
GRD GRC BRD BRC LRD LRC
Status
Detection
VRLY
Relay Driver Circuit
RGND
RING
Line
Termination
2-4 Wire
Hybrid
Receive
Gain
Dummy
Ringer
RX
GRX1
GRX0
Transmit
Gain
TIP
Network
Balance
TX
GTX1
GTX0
Impedance
Matching
XLA XLB XLC XLD
Z1 Z2 Z600 Z900
NS N1 N2 NATT
Figure 1 - Functional Block Diagram
2-239
MH88632B
Preliminary Information
TIP
RING
XLA
XLB
XLC
XLD
IC
GRD
IC
IC
RGND
VRLY
LRD
BRD
LRC
BRC
GRC
AGND
NATT
N1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
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
Figure 2 - Pin Connections
Pin Description
Pin #
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Name
TIP
RING
XLA
XLB
XLC
XLD
IC
GRD
IC
IC
RGND
VRLY
LRD
BRD
LRC
Description
Tip Lead.
Connects to the Tip lead of a telephone line usually via an external protection
circuit.
Ring Lead.
Connects to the Ring lead of a telephone line usually via an external protection
circuit.
Loop Relay Contact A.
Connects to XLB through relay contacts (K1A) when the relay is
energized.
Loop Relay Contact B.
Connects to XLA through relay contacts (K1A) when the relay is
energized.
Loop Relay Contact C.
Connects to XLD through relay contacts (K1B) when the relay is
energized.
Loop Relay Contact D.
Connects to XLC through relay contacts (K1B) when the relay is
energized.
Internal Connection.
No connection should be made to this pin.
Ground Ring Lead Relay Drive (Output).
Connects to the Ground Ring Lead Relay coil
(K3) and is controlled by GRC.
Internal Connection.
No connection should be made to this pin.
Internal Connection.
No connection should be made to this pin.
Relay Ground.
Return path for relay supply voltage.
Relay Positive Supply Voltage.
Normally +5V. Connects to all relay coils and the relay
supply voltage.
Loop Relay Drive (Output).
Connects to the Loop Relay coil (K1) and is controlled by
LRC.
Bias Relay Drive (Output).
Connects to the Bias Relay coil (K2) and is controlled by BRC.
Loop Relay Control (Input).
A logic 1 activates LRD. The Loop Relay (K1) is used for
placing the Line Termination across Tip and Ring.
2-240
Preliminary Information
Pin Description (continued)
16
BRC
MH88632B
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.
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.
Analog Ground.
4-Wire Ground. Normally connects to system ground. This pin must be
connected to the system ground in Ground Start applications.
Network Balance AT&T Node.
Used when setting the Network Balance Impedance to
AT&T compromise network.
Network Balance Node 1.
Used when a Network Balance Impedance which differs from
the Input Impedance is required or when NATT is used.
Network Balance Node 2.
Used when a Network Balance Impedance which differs from
the Input Impedance is required.
Input Impedance 900Ω Node.
Connects to Z1 when selecting an Input Impedance of
900Ω.
Input Impedance Node 1.
Used when setting the Input Impedance.
Input Impedance Node 2.
Used when a user defined Input Impedance is required.
Transmit (Output).
4-Wire ground (AGND) referenced analog output.
Receive (Input).
4-Wire ground (AGND) referenced analog input.
Transmit Gain Node 0.
Connects to GTX1 for 0dB transmit gain.
Transmit Gain Node 1.
Connects to GTX0 for 0dB transmit gain or via a resistor to AGND
for transmit gain programming.
Receive Gain Node 0.
Connects to GRX1 for 0dB receive gain.
Receive Gain Node 1.
Connects to GRX0 for 0dB receive gain or via a resistor to AGND
for receive gain programming.
Internal Connection.
No connection should be made to this pin.
Loop Impedance 600Ω Node.
Connects to Z1 when selecting an Input Impedance of
600Ω.
Network Balance Setting (Input).
Used to select the Network Balance impedance.
Tip Lead Ground Detect (Output).
A logic 0 output indicates that the Tip lead is at ground
(AGND) potential.
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.
Ringing Voltage Detect (Output).
A logic low indicates that ringing voltage is across the
Tip and Ring leads.
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.
Ring Lead Ground Detect (Output).
A logic 0 output indicates that the Ring lead is at
ground (AGND) potential.
Negative Supply Voltage.
-5V DC
Positive Supply Voltage.
+5V DC
17
GRC
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
AGND
NATT
N1
N2
Z900
Z1
Z2
TX
RX
GTX0
GTX1
GRX0
GRX1
IC
Z600
NS
TG
RL
36
37
RV
FL
38
39
40
RG
VEE
VCC
2-241
MH88632B
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.
Preliminary Information
at a logic 0, the Line Termination 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.
Isolation Barrier
The MH88632B provides an isolation barrier which is
designed to meet FCC Part 68 (November 1987)
Leakage Current Requirements.
AC Input Impedance
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.
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.
2200Ω
Z1
8200Ω
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
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
Z2
11.5nF
2-242
Preliminary Information
User defined Input Impedances can be used to
satisfy most national requirements. See Table 1.
All connections should be kept as short as possible.
MH88632B
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).
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.
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.
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 (R
TX
) between
GTX1 and AGND. The value of resistor is selected
using the following formulae.
R
TX
=
5000
10
(-GTX/20)
- 0.5
GTX = - 20 log(0.5+5000)
R
TX
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 (R
RX
) between
GRX1 and AGND. The value of resistor is selected
using the following formulae.
R
RX
=
5000
10
(-GRX/20)
- 0.5
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.
GRX = -20 log(0.5+5000)
R
RX
For the correct programming of Transmit and
Receive Gains the selected Input Impedance must
match the specified telephone line characteristic
impedance.
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