MH88634-2
Central Office Interface Circuit
Advance Information
Features
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Loop Start Trunk Interface
600Ω
& 900Ω
Input Impedance
2-4 Wire Conversion
Line state Detection Outputs:
- Forward Loop
- Reverse Loop
- Ringing Voltage
- Switch Hook
One Relay Driver
On-Hook Reception
Small footprint area
Meets FCC Part 68 Leakage Current
Requirements
DS5061
ISSUE 6
January 1999
Ordering Information
MH88634BV-2
21 Pin SIL Package
MH88634BT-2
21 Pin 90° L/F Package
MH88634BV-4
21 Pin SIL Package
0°C to 70°C
Description
The Mitel MH88634-2 Central Office Interface Circuit
trunk provides a complete analogue and signalling
link between audio switching equipment and a
telephone Line. The device is available in a single in
line package for high packing densities or with a lead
frame formed at 90° for low clearance applications.
The device is fabricated as a thick film hybrid
incorporating various technologies for optimum
circuit design and very high reliability.
This part supercedes the MH88634-2 and is pin for pin
compatible.
We advise that the B rev parts are fully tested.
However, we do not expect to see changes between
MH88634-2 and MH88634BV-2.
The component design has been changed to
improve the general performance of the part. It is
also now capable of operating at a 24V battery.
Applications
Interface to Central Office for:
• PABX
• Key Telephone Systems
• Channel Bank
• Voice Mail
• Terminal Equipment
• Digital Loop Carrier
• Optical Multiplexer
RV
FL
RL
SHK
XLA XLB XLC XLD
VCC
VEE
AGND
Status
Detection
TIP
Dummy
Ringer
RING
LRC
LRD
Loop Relay
Driver
Line
Termination
2 - 4 Wire
Hybrid
Receive
Gain
RX
Transmit
Gain
TX
Impedance
Matching
Network
Balance
VRLY
Figure 1 - Functional Block Diagram
2-265
MH88634-2
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
21
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
Advance Information
TIP
RING
XLA
XLD
XLB
XLC
IC
IC
IC
SHK
RX
VEE
TX
RV
FL
RL
VCC
AGND
LRC
VRLY
LRD
Figure 2 - Pin Connections
Pin Description
Pin #
1
2
3
4
5
6
7-9
10
Name
TIP
RING
XLA
XLD
XLB
XLC
IC
SHK
Description
Tip Lead.
Connects to the "Tip" lead of a Telephone Line.
Ring Lead.
Connects to the "Ring" lead of a Telephone Line.
Loop Relay Contact A.
Connects to XLB through the Loop relay (K1) contacts when the
relay is activated.
Loop Relay Contact D.
Connects to XLC through the loop relay (K1) contacts, when the
relay is activated.
Loop Relay Contact B.
Connects to XLA through the loop relay (K1) contacts, when the
relay is activated.
Loop Relay Contact C.
Connects to XLD through the loop relay (K1) contacts, when the
relay is activated.
Internal Connection.
No connection should be made to this pin.
Switch Hook (Output).
A logic 0 indicates the presence of forward or reverse battery
voltage when LRC is logic 0 and the presence of forward or reverse loop current when LRC
is logic 1.
Receive (Input).
4-Wire ground (AGND) referenced analog input.
Negative Supply Voltage.
-5V DC
Transmit (Output).
4-Wire ground (AGND) referenced analog output.
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 indicates that forward loop
current is present.
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.
Positive Supply Voltage.
+5V DC
11
12
13
14
15
RX
VEE
TX
RV
FL
16
RL
17
VCC
2-266
Advance Information
Pin Description (continued)
18
19
20
21
AGND
LRC
VRLY
LRD
MH88634-2
Analog Ground.
4-wire ground (AGND). Normally connected to system ground.
Loop Relay Control (Input).
A logic 1 activates the Loop Relay Driver output (LRD).
Relay Positive Supply Voltage.
Typically +5V. Connects to the relay supply voltage.
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.
Functional Description
The MH88634 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 (PBXs), 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.
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 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 terminate an incoming call,
seize the line for an outgoing 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 termination which is
equivalent to a DC resistance of 280Ω at 20mA
Isolation Barrier
The MH88634 provides an isolation barrier which is
designed to meet FCC Part 68 (November 1987)
Leakage Current Requirements.
External Protection Circuit
An external Protection Circuit Device assists in
preventing damage to the device and the
subscriber’s equipment, due to over-voltage
conditions. The type of protection required is
dependant upon the application and regulatorary
standards. Further details should be obtained from
the specific country’s regulatorary body. Typically
you will need lightening protection supplied by
resettable fuses or PTC
TM
and mains crossover
protection via a foldover diode.
Ringing Equivalent Number
The Ringing Equivalent Number (REN) is application
specific.
See the governing regulatory body
specification for details.
Suitable Markets
The MH88634BV-2/BT-2 has fixed 600Ω line and
network balance impedance for use in North
America and Asia.
The MH88634BV-4 has a fixed 900R impedance and
is also suitable for some applications in North and
South America
Input Impedance
The input impedance (Zin) is the AC impedance that
the MH88634 places across Tip and Ring to
terminate the Telephone line. This is fixed at 600Ω
on the -2 variant and 900R on the -4.
2-267
MH88634-2
Network Balance Impedance
The
MH88634BV-2/BT-2’s
Impedance is fixed at 600Ω.
Network
Balance
Advance Information
Ringing Voltage Detect Output (RV)
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.
The threshold is different on the two variants. The
900R part has been designed to meet the ring
sensitivity requirements of TR57, 30 not detecting
ringin below 35Vrms.
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 more negative than the 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 more negative than the Ring
pin voltage.
Switch Hook (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.
The MH88634BV-4 Network Balance Impedance is
fixed at 900R.
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.
The MH88634 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.
Control Input
Supervision Features
Line Status Detection Outputs
The MH88634 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.
Mechanical Data
See Figure 9
specification.
for
details
of
the
mechanical
2-268
Advance Information
MH88634-2
MH88634
+5V
1
Tip
Protection
Circuit
Ring
+5V
TIP
VCC
17
C1
2
RING
13
TX
K1
21
20
19
K1
LRD
RX
VRLY
LRC
RV
5
3
K1
XLB
XLA
FL
RL
SHK
Analog Out
11
Analog In
Loop Relay Control
14
15
16
10
Ringing Detect
Forward Loop
Reverse Loop
Switch Hook
6 XLC
4
XLD
AGND
VEE
18
C2 12
NOTES:
1) K1 Electro Mechanical 2 Form A
2) C1 and C2 are decoupling capacitors
-5V
Figure 3 - Typical LS Application Circuit
2-269
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