Preliminary Information
Functional Description
The device is a Data Access Arrangement (D.A.A.). It
is used to correctly terminate a 2-Wire analog loop. It
provides a signalling link and a 2-4 Wire line
interface between an analog loop and the
subscriber’s data transmission equipment such as
Modems, Facsimiles (Fax’s), Remote Metering and
Electronic Point of Sale equipment (EPOS).
MH88422
Input Impedance
The MH88422 is available in a number of different
variants each of which has its own fixed Tip-Ring AC
input impedance (Zin). Each variant is identified by
the final digit in its part number, as listed below. Also
shown are the countries whose PTT requirements
match these impedances.
MH88422-1
Zin = 220Ω + 820Ω // 120nF
Australia / South Africa / Spain
MH88422BD-1 Zin = 220Ω + 820Ω // 115nF
German BABT ZV5
MH88422-2
North America
Zin = 600Ω
Isolation Barrier
The device provides an isolation barrier implemented
by using optocouplers. This is a reinforced barrier for
an instantaneous power surge of up to 3kV r.m.s., for
example a lightning strike. It also provides full
isolation for a continuous AC voltage level of up to
250V r.m.s.
External Protection Circuit
Should the input voltage from the line exceed that
isolated by the optocoupler, an External Protection
Circuit assists in preventing damage to the device
and the subscriber equipment. See Figure 3.
MH88422-3
Zin = 370Ω + 620Ω // 310nF
UK / New Zealand
Many of these countries now pass equipment
approved to CTR21. The MH88422 will not meet this
specification. See the MH88437 datasheet for a
CTR21 Product.
Line Termination
When Loop Control (LC) is at a logic 0, a line
termination is applied across Tip and Ring. The
device can be considered off-hook and DC loop
current will flow. The line termination consists of both
a DC line termination and an AC input impedance.
When LC is at a logic 1, a Dummy Ringer is applied
across Tip and Ring. The device can be considered
on-hook and negligible DC current will flow. The
dummy ringer is an AC load, which represents a
telephone’s mechanical ringer.
Dummy Ringer
This device supports a dummy ringer option which
can be configured by the inclusion of external
components. Further details relating to component
values and configuration can be obtained from
MSAN-154. For example, Figure 3 shows capacitor
C2 which if set to 1.8µF would meet the New
Zealand dummy ringer requirements.
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 VX, as required by modem/fax
chip sets.
Conversely the device converts the ground
referenced signal input at VR, to a balanced 2-Wire
signal across Tip and Ring.
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 VR, being sent to the line, must
not appear at the output VX. In order to prevent this,
DC Line Termination
When LC is at a logic 0, an active termination is
applied across Tip and Ring, at which time it can be
considered to be in an off-hook state. This is used to
terminate an incoming call, seize the line for an
outgoing call, or if it is applied and disconnected at
the required rate, can be used to generate dial
pulses. This termination resembles approximately
300Ω resistance, which is loop current dependent.
2-15
MITEL [ MITEL NETWORKS CORPORATION ]
