®
MH88631
Central Office Interface (LS/GS)
Preliminary Information
Features
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Transformerless 2-4 wire conversion
Line state detection outputs
- forward current
- reverse current
- ring ground
- tip ground
- ringing voltage
Programmable audio transmit and receive gain
Loop start or ground start termination
Selectable 600Ω or AT&T compromise balance
network
-1 version: 20Hz Ringing Voltage
-2 version: 50Hz Ringing Voltage
ISSUE 4
April 1995
Ordering Information
MH88631-1
MH88631-2
40 Pin SIL Package
40 Pin SIL Package
0°C to 70°C
Description
The Mitel MH88631-1, 2 Central Office Trunk
Interface circuit provides a complete audio and
signalling link between audio switching equipment
and a central office. The loop seize circuitry is
controlled by an external input to provide either a
loop start of ground start termination. The device is
fabricated using thick film ceramic technology to
achieve high density circuit design.
Applications
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•
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PBX
Channel bank
Intercom
Key System
RG FC RV RC TG
LPGND RINGND
VCC+ VCC- AGND
T
R
Signal Detection
Circuitry
Ring
Ground
Relay
Driver 1
Relay
Driver 2
RC1
RD1
RGND
CD
RD2
RC2
NETBAL
Network
Balance
VTR+
VTR-
RXIN-
RXIN+
Active
Termination
Audio Section
Circuitry
(2-4 Wire)
GSX
VX
VR
GSR
QB QE XA XB XC
VDD VEE
RXOUT
TXIN+ TXIN-
Figure 1 - Functional Block Diagram
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MH88631
LPGND
VDD
RINGND
T
R
VTR-
VTR+
XC
XA
QE
AGND
RXIN+
RXIN-
VEE
TG
RC
RV
FC
RG
QB
Preliminary Information
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
XB
VEE
VDD
RXOUT
VCC+
GSR
TXIN-
VCC-
TXIN+
AGND
VR
VX
RGND
GSX
RC1
NETBAL
RD1
CD
RD2
RC2
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
Name
LPGND
VDD
RINGND
Description
Loop Ground
is the system ground reference with respect to -48V.
Positive Power Supply
(+5V).
Ring Ground (Input).
For Ground Start Trunk only a logic low input will enable the trunk
circuit to ground the Ring lead through a low resistance (390Ω). This is a signal to the C.O.
that the interface is seizing the line.
Tip (Input).
Normally connects to the “Tip” lead of the C.O.
Ring (Input).
Normally connects to the “Ring” lead of the C.O.
Connects to the TXIN+ (Pin 29) via an external capacitor (C1).
Connects to the collector of an external transistor (Q1) and TXIN- (Pin 27) via an external
capacitor (C4)
External relay contact (K1) connection from XB (Pin 21), activates by loop seize control
input (RC1).
External relay contact (K1) connection from VTR+ (Pin 7), activated by loop seize control
input (RC1).
Connects to the emitter of an external transistor (Q1).
Analog Ground.
Differential Receive (Input).
Connects to analog ground via an external capacitor (C2).
Differential Receive (Input).
Connects to RXOUT (Pin 24) via an external capacitor (C3).
Negative Power Supply.
(-5V).
Tip Lead Ground Detect (Output).
Active low.
Reverse Loop Current Detect (Output).
Active low.
Ringing Voltage Detect (Output).
Active low.
Forward Loop Current Detect (Output).
Active low
Ring Lead detect (Output).
Active low.
Connects to the base of an external transistor (Q1).
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
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T
R
VTR-
VTR+
XC
XA
QE
AGND
RXIN+
RXIN-
VEE
TG
RC
RV
FC
RG
QB
Preliminary Information
Pin Description (Continued)
Pin #
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
MH88631
Description
Name
XB
VEE
VDD
RXOUT
VCC+
GSR
TXIN-
VCC-
TXIN+
AGND
VR
VX
RGND
GSX
RC1
NETBAL
RD1
CD
RD2
RC2
External relay contact (K1) connection from XC (Pin 8), activated by loop seize control input
(RC1).
Negative Power Supply.
(-5V).
Positive Power Supply.
(+5V)
Audio receive signal (output) tothe RXIN- (Pin 13), via an external decoupling capacitor
(C3).
Positive analog Power Supply Voltage.
(+12V).
Gain Setting Receive (Input).
Normally used to set the receive gain with an external
resistor connected to RXOUT (Pin 24).
Transmit (Input).
Differential audio signal from VTR- (Pin 6), via an external capacitor
(C4).
Negative Analog Power Supply Voltage.
(-12V).
Transmit (Input).
Differential audio signal from VTR- (Pin 6), via an external decoupling
capacitor (C1).
Analog Ground.
Audio Receive (Input).
The four-wire audio signal input to the interface.
Audio Transmit (Output).
The four-wire audio signal output from the interface.
Relay Ground.
Gain Setting Transmit (Input).
Normally used to set the transmit gain with an external
resistor connected to VX (Pin 32).
Relay Control 1 (Input).
A logic high will activate the relay (K1) to provide loop seize
across Tip and Ring.
Network Balance (Input).
When there is no connection (open), the network is balanced at
600Ω. When the pin is grounded, the network is balanced at AT&T compromise.
Relay Driver 1 (Output).
Open collector sinks current when RC1 is high. Diode clamp
protected.
Clamping Diode.
Normally connects to the positive supply voltage.
Relay Driver 2 (Output).
Open collector sinks current when RC2 is high. Diode clamp
protected.
Relay Driver 2 (Input).
A logic high will activate the relay (K2) to provide proper biasing to
the Tip and Ring. This input control is used for Ground Start Trunk only.
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MH88631
Functional Description
The MH88631-1, 2, is a Central Office Loop Start
and/or Ground start analog Trunk interface circuit
providing a complete audio and signalling link to the
Central Office.
The trunk interface circuit performs transformerless
2 to 4 wire conversion, between the 2-wire telephone
loop and the 4 wire transmit and receive pairs of a
voice switching system. The 4-wire connection can
be interfaced to a filter/ codec, such as the MT896X,
for use in a digital voice switched system.
Voiceband analog signals, coming for the C.O.,
applied differentially across Tip and Ring, pass
through a bridge rectifier and appear at VTR+ and
VTR- where they are actively terminated. Refer to
Fig. 3 - Application Circuit. External capacitors C1
and C4 couple the incoming signals into the
balanced input of the Transmit audio circuitry via
TXIN+ and TXIN-. The transmit gain is adjusted by
feedback resistor R4. For best performance R4
should be physically located as close as possible to
the GSX pin. The gain adjusted transmit signal
appears at the Vx output pin.
Relay K1 is the loop seize which applies active line
termination and also provides biasing of the current
modulator circuitry. Activating and deactivating K1
provides dial pulsing. Q1 is the current modulator
output transistor, referred to as the loop driver
transistor.
Outgoing analog signals from the system are
provided to the interface at the VR input where they
enter an amplifier section with the gain controlled by
the selection of the Rx gain feedback resistor R3.
For best performance, R3 should be physically
located as close as possible to the GSR pin. The
output of the amplifier, RXOUT, is coupled to RXIN-,
the current modulator circuitry, via C3. The balanced
input to the current modulator is completed with the
connection of C2 from RXIN+ to ground.
Transmission to the C.O. is accomplished by
modulating the loop with the outgoing analog
signals. To ensure that the transmitted signals are
not coupled to the receive circuitry, transhybrid loss
is maximised. The impedance matching, performed
by the balance network reduces power loss and
signal reflections. The network balance input,
NETBAL, of the interface’s 2-wire to 4-wire convertor
circuitry provides selection of a 600Ω balance, used
when feeding channel banks or when performing
external tests on the trunk circuit, or the AT&T
compromise. When the NETBAL input pin is
grounded, the interface balance against the AT&T
Preliminary Information
compromise network consisting of 350Ω plus 1kΩ
shunted by a 0.21µF capacitor. This is typical of
North American C.O. connections.
The Tip and Ring also provide the balanced input to
the signal detection circuitry which generates the
signalling status outputs TG, RC, RV, FC and RG.
For Ground Start signalling, relay K2 and resistor R1
and R2, are required. Activation of K2 is controlled
by the relay control logic input signal, RC2. In the
idle state, K2 is closed connecting the -48 VDC
supply Tip and Ring through biasing resistors R1 and
R2. Upon detection of TG or RG, the system then
pulls RC1 low, closing K1, then pulls RC2 low which
opens K2 to remove the -48 volts supply from Tip
and Ring.
In the Ground Start signalling environment, initiating
a call to the C.O. is performed by the following
sequence of events. The system provides a logic low
on the ring ground input pin of the interface. This
activates the circuitry which grounds the ring lead
through a current limiting resistance. The C.O.
recognizes the ground condition and connects the tip
lead to ground. The interface senses this condition
and the tip lead ground detect output switches to a
logic low. The system then applies active line
termination by closing K1 using RC1 and opens K2
using RC2. A call from the C.O. can be performed
similarly. The C.O. can signal to the interface by
pulling either Tip or Ring to ground potential, or by
applying ringing voltage to the Ring lead.
Signal Detection Circuitry
The signal detection circuitry provides the signalling
status outputs. The system, controlling the
interfaces, monitors these active low logic outputs.
RV is the Ringing Voltage detect outputs. When the
C.O. applies ringing voltage to the termination, the
trunk interface provides a 50ms debounced output at
RV during the ringing burst period. This output will
remain low for approximately 50ms after C.O.
removes the ringing Voltage. Ringing voltage above
40 VRMS at 20Hz will be detected.
TG is the Tip lead Ground detect output and RG is
the Ring lead Ground detect output. The TG and RG
outputs provides a means of determining call original
or other hand shaking functions. The high
impedance detection circuitry of the interface will
detect both Tip and Ring Ground voltages above
approximately -15.3V of true ground.
RC is the Reverse Loop Current detect output and
FC is the Forward Loop Current detect output.
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Preliminary Information
The RC and FC outputs of the interfaces are used to
determine the polarity of the Tip and Ring pair which
the C.O. uses for signalling during the active (off-hook)
state of the interface. When the loop is closed by the
interface, the trunk is in the normal or unreversed
state. Some C.O.’s may reverse the polarity of Tip and
Ring, to indicate the talking state. The interface will
detect this condition and RC will output a low level.
MH88631
and Ring inputs of the interface, following by metal
oxide varistors from Tip and Ground, Ring to Ground
and Tip to Ring.
Components List (for Figure 3)
R1*, R2*
R4
= 30.9kΩ, ± 1%
1
/
4
W
= Transmit Gain Adjust Resistor
= 301.5 E3x Gain VX - 100 E3
VTR
Typical Value = 200kΩ, ± 1%
1
/
4
W
= Receive Gain Adjust Resistor
= 523.2 E3 x Gain VTR
V
R
External Circuitry Requirements
R3
The loop seize circuit is completed with the addition
of external components Q1, C5, R5 and K1.
K1, a DPST reed relay, is activated by relay control
signal RC1. When the loop seizure is required, K1 is
closed and the interface applies active termination
across Tip and Ring. The relay should have a 0.5
amp contact capability and 12 VDC operation with a
typical 500Ω coil resistance. To prevent back EMF
from damaging the relay drive transistor, (caused by
the collapsing field of the inductive coil of the relay) a
snubbing diode is provided on the hybrid and
therefore not required externally. C5 and R5 provide
relay contact noise filtering and transient noise
suppression necessary for clamping inductive spikes
created when the loop is closed during line seizure
or dial pulsing.
Q1 provides current drive for the active termination,
controlling the loop current flow of the current
modulator circuitry. Selection of a suitable transistor
for Q1 is made based on worst case conditions
which include fault conditions. A 350 volt, or higher,
rating for Q1 is necessary to meet high voltage
requirements. The Tip and Ring input protection
varistors limit any high voltage spikes to
approximately 300 volts. Under worst case
conditions Q1 must be able to handle close to
100mA of collector current and dissipate two watts
continuously. During pulse dialling, current spikes
are generated due to the inductive nature of the loop.
A 0.5 amp continuous connector current rating is
therefore recommended to provide a safe margin.
K2, R1 and R2 are required only for Ground Start
applications. K2 is the same type of relay as K1 and
is activated by RC2. Once again the snubbing diode
is provided on the hybrid. R1 and R2 provide the
-48V biasing the signalling (on-hook) state.
R6 and C6 constitute the Dummy Ringer required for
the LS/GS trunk. These components are also part of
the 600Ω input impedance.
Protection circuitry on the Tip and Ring inputs may
be required depending upon the trunk interface
application. For maximum protection it is
recommended to place fuses in series with the Tip
Typical Value = 505kΩ, ± 1%,
1
/
4
W
R5
= 510Ω, ± 5%,
1
/
4
W
R6
= 10kΩ, ± 5%,
1
/
4
W
C1, C2, C3, C4= 0.22µF, ± 10%, 200V
C5
= 0.1µF, ± 5%, 250V
C6
= 1.0µF, ± 5%, 250V
Q1
= 2N5657, NPN 350V, 0.5A, 20W
K1, K2
= 2A Reed Relay, E/M 12V
2 Form C Dip
*Note: required for Ground Start applications, not
required for Loop start applications.
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