Preliminary Information
Functional Description
MH88500IN
Line Feed/Ring Feed Circuit
Speech Circuit
The speech circuit converts the bidirectional TIP and
RING line pair to a bidirectional single ended junctor
line. Figure 3 illustrates a typical connection between
two SLIC’s through two crosspoint switches. This
configuration gives optimum transhybrid loss as
seen from Figure 4 given that the output impedance
of the Junctor line is 604Ω.
The MUTE input mutes signals coming from TIP and
RING to the junctor line while allowing the signal
from the junctor to the tip-ring pair to be transmitted.
The line feed circuit provides loop current and the
ability to apply ringing onto TIP and RING. The
impedance from RING FEED to GND is 600Ω which
gives the loop current as:
IL = Voltage at RING FEED pin
Telephone Impedance + 600
Amps
The positive supply for the line feed circuit is V
A1
through the loop current is determined from RING
FEED and GND.
Loop Detection
The loop detection circuit determines whether a low
enough impedances is across TIP and RING to be
recognized as an off-hook condition.
(Threshold impedance = 5.4kΩ with no adjustment)
This threshold level can be adjusted by the use of
external resistors as shown in Figure 6. OFHK has
low output drive capability so it may drive CMOS
operating with different power supplies.
Relay Drive Circuit
The relay drive circuit switches ringing onto RING
FEED (Fig.7). The diode is present to suppress
voltage transients during relay switches caused by
the inductive coils of the relay. Ringing voltage
includes AC ringing (90V typically) an dDC line feed
voltage (-24V typically).
MH88500IN
1
TIP
TIP
JUNC
RING
3
RING
CROSSPOINT
SWITCH
16
75Ω
MH88500IN
1
TIP
TIP
JUNC
RING
3
RING
CROSSPOINT
SWITCH
16
75Ω
Figure 3 - SLIC Crosspoint Switch Connection
2-45
MITEL [ MITEL NETWORKS CORPORATION ]
