Preliminary Information
Functional Description
The BORSH Functions
The MH88510/11 performs all of the Borsh functions
of Battery Feed, Overvoltage Protection, Ringing,
Supervision and Hybrid (2-2 Wire).
MH88510/11
switches ringing voltage on to the line via the
external ring relay. The SLIC provides two internal
300W battery feed resistors through which the
ringing current will flow. A clamp diode is included
which suppresses voltage transients during relay
switching caused by the relay coil. In addition, the
circuit prevents connection of the ringing source
during off-hook conditions. See figure 5 for typical
application.
Battery Feed
Hybrid
The MH88510/11 powers the telephone set with
constant DC loop current for shortlines and
automatically reverts to constant voltage for long
lines. Since the Tip drive is powered down from the
V
DD
supply, loop current flows through both the V
BAT
supply and the V
DD
supply.
The 2-2 Wire hybrid circuit converts the incoming
balanced signal at Tip and Ring of the telephone line
into a ground referenced output signal at JUNC of
the SLIC, and converts the ground referenced input
signal at JUNC of the SLIC into a non-balanced
output signal at Tip and Ring of the telephone line.
Overvoltage Protection
Line Impedance
The MH88510/11 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 Figures 5 and
6 is all that is required.
The MH88510/11’s Tip-Ring(Zin) impedance is fixed
at 600Ω. For correct SLIC impedance, JUNC must
be appropriately terminated. See
AC Electrical
Characteristics.
Supervision
The loop detection circuit determines whether a low
enough impedance is across Tip and Ring to be
recognised as an Off-Hook condition. When an off-
hook condition occurs, the SHK and LED outputs
toggle to a logic low level. These outputs also toggle
during incoming dial pulses.
Ringing
The ringing insertion circuitry has the capability to
provide ringing voltage to the telephone set by
simply adding an external relay, ring generator and a
200Ω limiting resistor. The internal relay driver
MH88510/11
SLIC 1
1
TIP
TIP
JUNC
RING
3
RING
CROSSPOINT
SWITCH
16
75Ω
MH88510/11
SLIC 2
1
TIP
TIP
JUNC
RING
3
RING
CROSSPOINT
SWITCH
16
75Ω
Figure 3 - SLIC Crosspoint Switch Connection
2-55
MITEL [ MITEL NETWORKS CORPORATION ]
