MH89626C
Functional Description
The MITEL MH89626C OPS SLIC (Off-Premise
Subscriber Line Interface Circuit) provides a
complete interface between an off-premise
telephone line and a digital switching system. All
BORSCHT functions are provided requiring only a
few external components. The input impedance
conforms with Chinese standard requirements.
Preliminary Information
TIP/RING Reversal
For a Direct Inward Dialling (DID) operation, the
MH89626C provides a TIP and RING reversal
function on the hybrid. This built-in line polarity
reversal capability will eliminate the use of an
external bulky mechanical relay and provides fast
and reliable Tip and Ring reversed function. The
serial control stream, SDi, bit 3 at logic low will
reverse the polarity of the Tip and Ring. Refer to
Table 1 for control of the SLIC functions.
Overvoltage Protection
The MH89626C is protected from short term (20ms)
transients (+250V) between TIP and RING, TIp and
Ground, and RING and Ground. However, if the
MH89626C is used in conjunction with MH80626C,
protection sip, it will meet all CCITT K.20
requirements. The applications circuit is shown in
Figure 2.
The MH80626C has two battery feed resistors (50
ohms) and one ringing feed resistor (560 ohms), that
are required to be used with the MH89626C as a
complete line interface. All resistors on the hybrid
are specially designed to withstand high power. The
two battery feed resistors are accurately trimmed to
achieve good longitudinal balance. Two fuses and
current limited resistors (5 ohms) are provided on the
hybrid for lightning and high voltage surge
protection.
Ringing
The ringing insertion circuitry has the capability to
provide ringing voltage to a telephone set by simply
adding an external relay, ring generator. The serial
control stream, SDi, bit 0 at logic low will activate the
Refer to Table 1 for the control of SLIC functions.
Supervision
The loop detection circuit determines whether a low
enough impedance is across Tip and Ring to be
recognized as an off-hook condition. When an
off-hook condition occurs, the SHK and the LED
outputs toggle to a low level. These outputs also
toggle with incoming dial pulses.
During applied ringing, the loop detection circuit
engages a ringing filter. This filter prevents a false
off-hook detection due to the current associated with
the AC ringing voltage as well as current transients
that occur when the ringing voltage is switched in
and out. The Ring trip detection circuitry deactivates
the ring driver after an off-hook condition is detected.
Battery Feed
The MH89626C powers the telephone set with
constant DC loop current for short lines and
automatically reverts to constant voltage for long
lines. If the LCA pin is left open, the constant current
is set at 23 mA. The Constant current can also be
set by adding a resistor connected fro the LCA pin to
V
EE
. The resistance (R) can be calculated as:
147.2 - I
Loop
(0.0001176 X I
Loop
) - 0.002586
Where I
Loop
is the desired constant loop current in
mA, and R is the resistance from pin LCA to pin V
EE
in ohms.
Transmit and Receive Gain
The Transmit Gain (Tip-Ring to Do) is fixed at 0dB.
The Receive Gain (Di to Tip-Ring) is programmable
in -3.5 or -7.0dB, either using software (SDi, bit 2) or
external hardware (GS pin).
R=
R(kΩ)
I
Loop
(mA)
348K
25.0
200K
27.1
80K
34.0
50K
40.2
30K
49.7
2-294
MITEL [ MITEL NETWORKS CORPORATION ]
