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2-225
Features
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 600
or AT&T compromise balance
network
-1 version: 20Hz Ringing Voltage
-2 version: 50Hz Ringing Voltage
Applications
PBX
Channel bank
Intercom
Key System
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.
T
R
VTR+
VTR-
RXIN-
RXIN+
LPGND RINGND
RC1
RD1
RGND
CD
RD2
RC2
VCC+
RG FC RV RC TG
QB QE XA XB XC
VDD VEE
RXOUT
TXIN+
TXIN-
NETBAL
GSX
VX
VR
GSR
VCC- AGND
Signal Detection
Circuitry
Active
Termination
Ring
Ground
Network
Balance
Relay
Driver 1
Relay
Driver 2
Audio Section
Circuitry
(2-4 Wire)
ISSUE 4
April 1995
Ordering Information
MH88631-1 40 Pin SIL Package
MH88631-2 40 Pin SIL Package
0
C to 70
C
Figure 1 - Functional Block Diagram
MH88631
Central Office Interface (LS/GS)
Preliminary Information
2-226
MH88631
Preliminary Information
Figure 2 - Pin Connections
Pin Description
Pin #
Name
Description
1
LPGND
Loop Ground is the system ground reference with respect to -48V.
2
VDD
Positive Power Supply (+5V).
3
RINGND
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 (390
). This is a signal to the C.O.
that the interface is seizing the line.
4
T
Tip (Input). Normally connects to the "Tip" lead of the C.O.
5
R
Ring (Input). Normally connects to the "Ring" lead of the C.O.
6
VTR-
Connects to the TXIN+ (Pin 29) via an external capacitor (C1).
7
VTR+
Connects to the collector of an external transistor (Q1) and TXIN- (Pin 27) via an external
capacitor (C4)
8
XC
External relay contact (K1) connection from XB (Pin 21), activates by loop seize control
input (RC1).
9
XA
External relay contact (K1) connection from VTR+ (Pin 7), activated by loop seize control
input (RC1).
10
QE
Connects to the emitter of an external transistor (Q1).
11
AGND
Analog Ground.
12
RXIN+
Differential Receive (Input). Connects to analog ground via an external capacitor (C2).
13
RXIN-
Differential Receive (Input). Connects to RXOUT (Pin 24) via an external capacitor (C3).
14
VEE
Negative Power Supply. (-5V).
15
TG
Tip Lead Ground Detect (Output). Active low.
16
RC
Reverse Loop Current Detect (Output). Active low.
17
RV
Ringing Voltage Detect (Output). Active low.
18
FC
Forward Loop Current Detect (Output). Active low
19
RG
Ring Lead detect (Output). Active low.
20
QB
Connects to the base of an external transistor (Q1).
XB
VEE
VDD
RXOUT
VCC+
GSR
TXIN-
VCC-
TXIN+
AGND
VR
VX
RGND
GSX
RC1
NETBAL
RD1
CD
RD2
RC2
LPGND
VDD
RINGND
T
R
VTR-
VTR+
XC
XA
QE
AGND
RXIN+
RXIN-
VEE
TG
RC
RV
FC
RG
QB
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
2-227
Preliminary Information
MH88631
21
XB
External relay contact (K1) connection from XC (Pin 8), activated by loop seize control input
(RC1).
22
VEE
Negative Power Supply. (-5V).
23
VDD
Positive Power Supply. (+5V)
24
RXOUT
Audio receive signal (output) tothe RXIN- (Pin 13), via an external decoupling capacitor
(C3).
25
VCC+
Positive analog Power Supply Voltage. (+12V).
26
GSR
Gain Setting Receive (Input). Normally used to set the receive gain with an external
resistor connected to RXOUT (Pin 24).
27
TXIN-
Transmit (Input). Differential audio signal from VTR- (Pin 6), via an external capacitor
(C4).
28
VCC-
Negative Analog Power Supply Voltage. (-12V).
29
TXIN+
Transmit (Input). Differential audio signal from VTR- (Pin 6), via an external decoupling
capacitor (C1).
30
AGND
Analog Ground.
31
VR
Audio Receive (Input). The four-wire audio signal input to the interface.
32
VX
Audio Transmit (Output). The four-wire audio signal output from the interface.
33
RGND
Relay Ground.
34
GSX
Gain Setting Transmit (Input). Normally used to set the transmit gain with an external
resistor connected to VX (Pin 32).
35
RC1
Relay Control 1 (Input). A logic high will activate the relay (K1) to provide loop seize
across Tip and Ring.
36
NETBAL
Network Balance (Input). When there is no connection (open), the network is balanced at
600
. When the pin is grounded, the network is balanced at AT&T compromise.
37
RD1
Relay Driver 1 (Output). Open collector sinks current when RC1 is high. Diode clamp
protected.
38
CD
Clamping Diode. Normally connects to the positive supply voltage.
39
RD2
Relay Driver 2 (Output). Open collector sinks current when RC2 is high. Diode clamp
protected.
40
RC2
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.
Pin Description (Continued)
Pin #
Name
Description
2-228
MH88631
Preliminary Information
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 600
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
compromise network consisting of 350
plus 1k
shunted by a 0.21
F 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.
Preliminary Information
MH88631
2-229
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.
External Circuitry Requirements
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 500
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 600
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
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*
= 30.9k
, 1%
1
/
4
W
R4
= Transmit Gain Adjust Resistor
= 301.5 E3x Gain VX - 100 E3
VTR
Typical Value = 200k
, 1%
1
/
4
W
R3
= Receive Gain Adjust Resistor
= 523.2 E3 x Gain VTR
V
R
Typical Value = 505k
, 1%,
1
/
4
W
R5
= 510
, 5%,
1
/
4
W
R6
= 10k
, 5%,
1
/
4
W
C1, C2, C3, C4= 0.22
F, 10%, 200V
C5
= 0.1
F, 5%, 250V
C6
= 1.0
F, 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.
2-230
MH88631
Preliminary Information
Absolute Maximum Ratings
*
* Exceeding these values may cause permanent damage. Functional operation under these conditions is not implied.
Recommended Operating Conditions
* Typical figures are at 25
C with nominal
+
5V supplies for design aid only.
DC Electrical Characteristics
DC Electrical Characteristics are over recommended operating conditions unless otherwise stated.
* Typical figures are at 25
C with nominal
+
5V supplies and are for design aid only.
Control Inputs State Table
Parameter
Sym
Min
Max
Units
1
DC Supply Voltage
V
DD -
GNDD
V
EE -
GNDD
V
CC +
-GNDA
V
CC
- GNDA
-0.3
-0.6
+18.0
+6.0
+0.3
+18.0
V
V
V
V
2
Storage Temperature
T
STG
-55
+125
C
Parameter
Sym
Typ*
Min
Max
Units
Comments
1
Operating Supply Voltage
V
DD
V
EE
V
CC+
V
CC
V
Bat
4.75
-5.25
11.4
-11.4
5.0
-5.0
12.0
12.0
-48
5.25
-4.75
12.6
-12.6
V
V
V
V
V
2
Operating Current
I
DD
I
DD
I
DD
I
DD
6.0
-6.0
8.0
-8.0
mA
mA
mA
mA
RINGND high
3
Power Consumption
P
C
265
mW
4
Operating Temperature
T
OP
0
70
C
Characteristics
Sym
Min
Typ*
Max
Units
Test Conditions
1
Input High Voltage RC1, RC2
RINGND
V
IH
2.7
4.5
V
V
2
Input High Current RC1, RC2
RINGND
I
IH
2.5
5.0
-100
mA
A
3
Input Low Voltage RC1, RC2
RINGND
V
IL
0.7
V
4
Input Low Current RC1, RC2
RINGND
I
IL
1.0
1.1
A
mA
Parameter
Active
Idle
1
RC1
Logic high
Logic Low
2
RC2
Logic High
Logic Low
3
RINGND
Logic Low
Logic High
4
NETBAL
AT&T compromise network
(350
+ 1k
/
0.2
F)
600
network
AGND
Open (no connection)
2-231
Preliminary Information
MH88631
AC Electrical Characteristics
* Typical figure are at 25C with nominal 5V supplies and are for design aid only
Note 1: Input 0dBm at VTR, or input 0.0775Vrms at VR. TR=600
Termination.
DC Electrical Characteristics
* Typical figure are at 25C with nominal 5V supplies and are for design aid only
Characteristics
Sym
Min
Typ*
Max
Units
Test Conditions
1
Ringing Voltage
VR
40
90
130
V
rms
2
Ringing Frequency -1 version
-2 version
20
50
Hz
Hz
Voltage range 40
-120Vrms
Voltage range 35
-120Vrms
3
Operating Loop Current
IL
18
70
mA
4
Off-Hook DC Resistance
RT
300
@18mA
5
Operating Loop Resistance
RL
2300
@18mA
6
On-Hook Leakage Current
3
RINGND = 5.0Vdc
7
Ring Ground Sink Current
IRG
100
m
-48VDC with 200
in
series on Ring lead
8
Tip and Ring AC Impedance
600
with 1.0k
+ 1.0
F in
parallel with Tip and
Ring
9
Longitudinal Balance
metallic to longitudinal
longitudinal to metallic
60
40
58
53
dB
dB
dB
dB
200-1000Hz
1000-4000Hz
200-1020Hz
1020-3020Hz
10
Return Loss Trunk to Line
20
26
30
dB
dB
dB
200-500Hz
500-1000Hz
1000-340Hz
11
Transhybrid Loss (single frequency)
into 600
THL
18.5
34
30
dB
dB
dB
200Hz
1000Hz
3000Hz
12
Transhybrid Loss (single frequency)
into AT&T Compromise
THL
18
20
dB
dB
200-1000Hz
1000-4000Hz
13
Frequency response (Output relative
to 1kHz, VTR/VR and VX/VTR)
-0.15
-0.10
-0.10
-0.15
0.05
0.05
0.05
0.05
dB
dB
dB
dB
200Hz
300Hz
3000Hz
3400Hz
See Note 1
14
Idle channel noise
8
dBrnco
C-Message
15
Power Supply rejection ratio
PSR
R
40
dB
16
Analog signal overload level
(adjustable gain)
6
dBm
1kHz,0dBm
= 0.775Vrms into 600
Characteristics
Sym
Min
Typ*
Max
Units
Test Conditions
1
Output High Voltage
(TG, RC, RV, FC, RG)
4.75
V
No Load on output
2
Output High Current
(TG, RC, RV, FC, RG)
0.17
mA
V
OH
=-2.7Vdc
3
Output Low Voltage
(TG, RC, RV, FC, RG)
-0.30
V
No Load on output
4
Output Low Sink Current
(TG, RC, RV, FC, RG)
-0.40
mA
V
OL
=-0.4Vdc
2-232
MH88631
Preliminary Information
Loop Current Setting
Figure 3 - Application Circuit
MH88631-1, -2
PROTECTION
LIGHTNING
V
EE
V
DD
TIP
RING
T
R
AGND
LPGND
RGND
RC2
RD2
CD
RD1
RC1
VTR-
TXIN+
RXIN+
RXIN-
RXOUT
GSR
XB
XC
XA
TXIN
VTR+
QB
QE
NETBAL
VR
VX
GSX
V
CC-
V
CC+
RC
RV
FC
RG
TG
RINGND
DUMMY RINGER
-48V BATTERY
-48V RETURN
R6
C6
C1
C2
C3
C4
C5
R1
R2
11,30
K2
K1
R4
Q1
R5
R3
+5V
-5V
+12V -12V
+12V
4
5
1
33
40
39
38
37
35
16
17
18
19
15
2,23 22,14
25
28
34
32
31
3
36
10
20
7
27
9
8
21
26
24
13
12
29
6
K1A
K1B
K2A
K2B
2-233
Preliminary Information
MH88631
Figure 4 - Mechanical Data
4.00 + 0.03
(102 + 1.0)
0.60 Max
(15.25 Max)
0.14 Max
(3.6 Max)
0.010 + 0.002
(0.25 + 0.05)
0.140 Max
(3.6 Max)
Side View
*
0.05 + 0.01
(1.3 + 0.5)
*
*
*
0.05 + 0.02
(1.3 + 0.05)
0.020 + 0.05
(0.51 + 0.13)
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