Advance Information
Pin Description (continued)
18
19
20
21
AGND
LRC
VRLY
LRD
MH88634B
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
MITEL [ MITEL NETWORKS CORPORATION ]
