®
MH88400
Line Interface Circuit
Preliminary Information
Features
•
•
•
•
•
•
•
•
•
•
FAX and Modem interface
Suitable for V.29 FAX & V.22 bis Data
Allows caller identification
Variants match German, UK, 600
Ω
& 900Ω
network impedances
Isolation circuitry conforms to international PTT
requirements
Transformerless 2-4 Wire conversion
Loop start operation
Pulse and DTMF operation
Accommodates external monitor phone
Line state detection outputs:
-loop current/ringing outputs
-monitor phone switch hook
Single +5V operation
ISSUE 1
April 1995
Ordering Information
MH88400-1 14 Pin DIL Package
MH88400-2 14 Pin DIL Package
MH88400-3 14 Pin DIL Package
0°C to 70°C
Description
The Mitel MH88400 Line Interface Circuit provides a
complete audio and signalling link between audio
equipment and central office. The functions provided
by the MH88400 include 2-4 Wire conversion, loop
seizure, external monitor phone switch hook status
and ringing voltage and loop current detection. The
device is fabricated as a thick film hybrid which
incorporates various technologies for optimum circuit
design, high voltage isolation and very high
reliability.
Variants are provided to meet German (-1) and UK
(-3) and general 600Ω (-2) line impedance (see
section on Line Impedances).
Isolation Barrier
Power
Supply
•
Applications
Interface to Central Office for:
• DAA
• Modem
• FAX
• Answering Machine
• Terminal Equipment
Transformer
Isolation
VDD
AGND
TIP
RING
Active
Termination
Opto-
Isolation
Opto-
Isolation
Audio
Buffer
Logic Input
Buffer
VR
LC
TXIN
TF
RLS
Audio Input
Buffer
Input
Buffer
Opto-
Isolation
Opto-
Isolation
Audio
Buffer
Ring & Loop
Buffer
Switch Hook
Buffer
VX
RVLC
PHS
Opto-
Isolation
SHK
NETWORK CONNECTIONS
USER CONNECTIONS
Figure 1 - Functional Block Diagram
2-11
MH88400
Preliminary Information
VDD
AGND
LC
RVLC
SHK
VX
VR
1
2
3
4
5
6
7
14
13
12
11
10
9
8
TIP
RLS
PHS
TF
TXIN
RING
NC
Figure 2 - Pin Connections
Pin Description
Pin #
1
2
3
4
Name
VDD
AGND
LC
RVLC
Description
Positive Power Supply Voltage.
+5V.
Analog Ground.
4-Wire Ground. Normally connected to System Ground,
Loop Control (Input).
A logic low activates internal circuitry which provides a dc
termination across Tip and Ring. Used for seizing the line and dial pulsing
Ring Voltage and Current Detect (Output).
A logic low indicates that loop current is
detected. The loop current can be due to the external monitor phone or the MH88400 in
the off- hook mode. The RVLC outputs pulses when the external monitor phone is dial
pulsing or when the MH88400 is dial pulses via the LC input. In addition, when the
MH88400 is in the on-hook mode, a pulsing output indicates that ringing voltage is
across the Tip and Ring leads; the pulsing outputs frequency is twice the ringing
frequency.
Switch Hook Detect (Output).
This is an optional output which can be used with the
PHS input and an external phone. When loop current flows from PHS to TIP, SHK goes
to logic low.
Transmit (Output).
4-Wire ground(AGND) referenced audio output, biased at 2.5V.
Outputs for both off-hook and on-hook.
Receive (Input).
4-Wire ground (AGND) referenced audio input, biased at 2.5V. LC
must be activated low and loop current must be flowing.
No Connection.
This pin is not connected internally
Ring Lead.
Connects to the “Ring” lead of the central office through a relay contact. The
central office “Tip” and “Ring” leads may be interchanged
Transmit (Input).
Connects to the “Ring” lead of the central office through a coupling
capacitor.
Tip Feed.
Connects to the “Tip” lead of the central office through an internal resistor and
an optional external resistor.
Monitor Phone Sense (Input).
This is an optional input which can be used with the
SHK output and an external phone. When loop current flows from PHS to TIP, SHK goes
to logic low.
Ring Loop Sense (Input).
Sense node for ringing voltage detector and the loop current
detector.
Tip Lead.
Connects to the “Tip” lead of the central office through an optional relay
contact. The central office “Tip” and “Ring” leads may be interchanged.
5
SHK
6
7
8
9
10
11
12
VX
VR
NC
RING
TXIN
TF
PHS
13
14
RLS
TIP
2-12
Preliminary Information
Functional Description
The MH88400 Line Interface Circuit is a COIC
(Central Office Interface Circuit) used to interface
FAX’s Modems or user defined equipment to
Central Office 2-Wire Analog Trunks.
MH88400
In addition, when the MH88400 is in the on-hook
mode, a pulsing output indicates that ringing voltage
is across the tip and ring leads; the pulsing output
frequency is twice the ringing frequency. See
Figure 3.
Ringing frequency may require external validation.
An RC monostable is usually satisfactory for this
purpose or this may also be achieved using software
applications.
Opto-Isolation
The isolation barrier is designed to meet regulatory
requirements for a reinforced barrier of 3kVac. It
provides full isolation of mains voltages up to 250V
RMS and all telecom voltages. In order that this
barrier is not bypassed a creepage/clearance
distance of 6.4mm minimum must be maintained
between wiring, pcb tracking, etc., connected to
external circuitry on either side of the barrier. To
make this requirement simpler, pins on the
MH88400 connected to opposite sides of the
barrier are on opposite sides of the package.
2-4 Wire Conversion
The 2-4 Wire conversion circuit converts the
balanced full duplex signal at Tip and Ring of the
central office line into a transmit ground referenced
signal at VX (Transmit) of the MH88400. It also
converts the receive ground referenced signal at VR
(Receive) of the MH88400 into a balanced transmit
signal at Tip and Ring of the central office line.
In full duplex transmission, the Tip-Ring signal
consists of an audio signal from the central office as
well as an audio signal due to the VR input.
Consequently, both of these signals will appear at the
VX output. The degree to which the 2-4 wire
conversion circuit minimizes the contribution of the
VR signal at the VX output is specified as transhybrid
loss (THL).
A simple THL cancellation circuit as shown in figure 7
can be used for certain applications to give the
required VX/VR signal separation. The MH88400 is
then suitable to drive a COMBO 2 CODEC or a VLSI
MODEM device.
External Protection Circuit
To meet regulatory high voltage requirement, an
external protection circuit is required. The
protection circuit shown in Figure 3 (Clamp Diode
D1) is recommended.
DC Loop Termination
The DC loop termination circuitry provides the loop
with an active DC load termination when a logic low
is applied to the LC (Loop Control) input. The
termination is simpler to a resistance of
approximately 300Ω (loop current dependant).
Internal optically isolated circuitry is used to switch
the termination in and out the loop. This is used for
both seizing the line as well as generating dial
pulses.
Line Impedance
The MH88400 is suitable to drive a COMBO 2
CODEC or a VLSI modem device. The MH88400
provides a fixed Tip-Ring impedance which conforms
to the following PTT requirements.
MH88400-1 Zin = 200 + 820 // 115nF
Germany FTZ
MH88400-2 600Ω
MH88400-3 Zin = 370 + 620 // 310nF
UK BSI
Supervision Features
The supervision circuitry is capable of detecting
ringing voltage and loop current as well as the
status of an optional external monitor phone. The
RVLC (Ring Voltage Loop Current Detect) output
provides a logic low when loop current due to the
external monitor phone or due to the MH88400
being in the off-hook mode is detected.
The RVLC outputs pulses when the external
monitor phone is dial pulsing or when the MH88400
is dial pulsing via the LC mode.
2-13
MH88400
Parameter
1
2
3
4
5
DC Supply Voltage
Storage Temperature
DC Loop Voltage
Ringing Voltage
Loop Current
Symbol
V
DD
T
S
V
BAT
V
R
I
Loop
Preliminary Information
.
Absolute Maximum Ratings* -
All voltages are with respect to AGND unless otherwise specified.
Min
-0.3
-55
-100
-
-
Max
6
+80
+100
120
90
Units
V
°C
V
V
RMS
mA
* Exceeding these values may cause permanent damage. Functional operation under these conditions is not implied.
Recommended Operating Conditions
Parameters
1
2
3
DC Supply Voltages
Operating Temperature
Ringing Voltage
Sym
V
DD
T
OP
V
R
Min
4.5
0
75
Typ
‡
5.0
Max
5.5
70
Units
V
°C
V
RMS
V
BAT
=-48V
Test Conditions
‡ Typical figures are at 25°C with nominal +5V supplies and are for design aid only.
Loop Electrical Characteristics*
Characteristics
1
2
3
4
5
6
Ringing Voltage
Ringing Frequency
On-Hook 2-wire Impedance
Operating Loop Current
Operating Loop Resistance
Off-Hook DC
Resistance
-1 & -2 Variants
-3 Variant
7
8
9
10
Leakage Current (2-Wire to AGND)
Leakage Current on Hook
(Tip to Ring)
DC Resistance during dialling
-1 variant
Dial Pulse Distortion
ON
Off
0
9
200
220
+4
0
+8
+2
ms
ms
310
160
350
300
250
400
10
No Detect
Detect
Sym
VR
27
15
40k
80
4000
480
480
300
450
10
10
68
Min
Typ
‡
Max
20
Units
V
RMS
V
RMS
Hz
Ω
mA
Ω
Ω
Ω
Ω
Ω
µA
µA
Ω
V
BAT
=-48V, I
Loop
=10mA
I
Loop
=20mA
I
Loop
=40mA
I
Loop
=40mA
I
Loop
=20mA
100V
DC
V
BAT
=50V
I
Loop
=20-40mA
1kHz
Test Conditions
* Loop Electrical Characteristics are over recommended operating conditions unless otherwise stated.
‡ Typical figures are at 25°C and are for design aid only.
Note: All of the above characteristics use a test circuit as per Figure 3.
2-14
Preliminary Information
DC Electrical Characteristics
†
Characteristics
1
2
3
4
5
RVLC
SHK
MH88400
Sym
I
DD
PC
V
OL
V
OH
V
IL
V
IH
I
IH
I
IL
3.8
0.9
3.5
0.6
1
Min
Typ
‡
15
75
0.4
Max
Units
mA
mW
V
V
V
V
mΑ
µA
Test Conditions
V
DD
= 5.0V, I
Loop
=
40mA
V
DD
= -5.0V,I
Loop
=
40mA
I
OL
= 1.0mA
I
OH
= 1.0mA
Supply Current
Power Consumption
Low Level Output Voltage
High level Output Voltage
Low Level Input Voltage
High level Input Voltage
High Level Input Current
Low Level Input Current
V
IH
= 5.0V
V
IL
= 0.0V
† 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.
Note: See figure 3a and 3b.
AC Electrical Characteristics
†
- MH88400 All Variants
Characteristics
1
2
3
4
5
6
7
Input Impedance VR
Output Impedance at VX
Transmit Gain (2-Wire to VX)
-0.4
Frequency Response Gain
(relative to Gain @ 1kHz)
Receive Gain (VR to 2-wire)
3.5
Frequency Response Gain
(relative to Gain @ 1kHz)
Signal Output Overload Level
-3.0
0.0
+2.0
-3.0
0.0
+2.0
THD
at 2-wire
at VX
9
Power Supply Reject Ratio
at 2-wire
at VX
PSRR
35
10
dB
dB
0.9
1.2
%
%
dBm
dBm
dBm
dBm
dBm
dBm
0
0
dB
dB
-2.4
-3.0
dB
dB
Sym
Min
Typ
‡
47
5
Max
Units
kΩ
Ω
dB
Input 0.5V at 1kHz off-
hook
300 Hz
3400 Hz
Test Conditions
dB
Input 0.5V at 1kHz
300 Hz
3400 Hz
THD <5% @ 1kHz
I
Loop
=10-40mA
V
DD
=4.5V
V
DD
=5.0V
V
DD
=5.5V
V
DD
=4.5V
V
DD
=5.0V
V
DD
=5.5V
Input 0.5V at 1kHz
DC loop = 1000Ω
V
DD
=5.0V
Ripple 0.1V,1kHz on
V
DD
at 2-Wire
at Vx
8
Total Harmonic Distortion
† AC Electrical Characteristics are over Recommended Operating Conditions unless otherwise stated.
‡ Typical figures are at 25°C and are for design aid only.
Note 1: All of the above characteristics use a test circuit as per Figure 3.
Note 2: All of the above test conditions use a test source impedance which matches the device’s impedance.
Note 3: dBm is referenced to 600Ω unless otherwise stated.
Note 4: THD is measured with a “Weight” filter.
2-15
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