MH89625C
Functional Description
The Mitel MH89625C OPS SLIC (Off-Premise
Subscriber Line Interface Circuit) provides a
complete interface between an off-premise
telephone line and an digital switching system. All
BORSCHT functions are provided requiring only a
few external components. The input impedance
conforms with Chinese standard requirements.
All functions of the SLIC are controlled by the system
Drive (SD) outputs of the internal Mitel A-Law Codec
MT8967. The SD outputs are controlled by the serial
data input stream at CSTi.
Preliminary Information
Ringing
The ringing insertion circuit has the capability to
provide ringing voltage to a telephone set by simply
adding an external relay, ring generator and a
transient protector. The internal relay driver switches
ringing voltage onto the line via the external ring
relay. A clamp diode is included which suppresses
voltage transients during relay switching caused by
the relay coil. The serial data input at CSTi controls
the internal Codec’s SDo output which activates the
ring driver. Refer to Table 1 for control of SLIC
functions.
Supervision
The loop detection circuit determines whether a low
enough resistance is across Tip and Ring to be
recognized as an off-hook condition. When an
off-hook condition occurs, the SHK and LED (the
LED output can drive an LED directly) outputs toggle
to a low level. These outputs also toggle with
incoming dial pulses.
During applied ringing (ring relay driver activated),
the loop detection circuit engages a ringing filter.
This filter prevents 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 relay driver after an
off-hook condition is detected.
The BORSCHT Functions
The MH89625C performs all of the BORSCHT
functions of Battery Feed, Overvoltage Protection,
Ringing, Supervision, Codec, Hybrid and Test.
Battery Feed
The MH89625C powers the telephone set with
constant DC loop current for short lines and
automatically reverts to constant voltage for long
lines. The constant loop current is a function of the
resistance connected from the LCA pin to V
EE.
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.
R (kΩ)
R=
Codec
The Codec function of the SLIC is implemented
using the Mitel MT8967 A-Law Codec. This device
provides the conversion interface between the
voiceband analog signals of a telephone subscriber
loop and the digital signals required in a digital PCM
(pulse code modulation) system. Eight-bit PCM
encoded digital data enters and leaves the chip
serially on DSTi and DSTo pins, respectively.
For detailed information on the CODEC portion of
the MH89625C, refer to the MT8967 integrated PCM
Filer/Codec data sheet (Microelectronics Digital
Communications Handbook, Mitel Semiconductor
Issue 9).
open
348k
200k
80k
34.0
50k
40.2
30k
49.7
I
Loop
(mA)
22.0
25.0
27.1
Overvoltage Protection
The MH89625C is protected from short term (20ms)
transients (+250V) between TIP and RING, TIP and
ground, and RING and ground. However, additional
protection circuitry may be needed depending on the
requirements which must be met. Normally simple
external shunt protection as shown in Figure 4 is all
that is required.
2-272
MITEL [ MITEL NETWORKS CORPORATION ]
